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RESEARCH METHODS
IN
ECOLOGY

BY

FREDERIC EDWARD CLEMENTS, Ph.D.

ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY IN THE UNIVERSITY OF NEBRASKA

ILLUSTRATED

LINCOLN, NEBRASKA

The University Publishing Company

1905

Copyright, 1905

By FREDERIC E. CLEMENTS AND IRVING S. CUTTER

All rights reserved

Press of Jacob North & Company

Lincoln, Nebraska

PREFACE

The present volume is intended as a handbook for investigators and for advanced students of ecology, and not as a text-book of the subject. An elementary text-book covering the same field, but adapted to the needs of undergraduate students, is in preparation. The handbook is essentially an account of the methods used by the author in his studies of the last eight years, during which a serious attempt has been made to discover and to correlate the fundamental points of view in the vast field of vegetation. No endeavor is made to treat any portion of the subject exhaustively, since a discussion of general methods and general principles is of much greater value in the present condition of ecology. The somewhat unequal treatment given the different subjects is due to the fact that it has been found possible to develop some of these more rapidly than others. Finally, it must be constantly kept in mind that ecology is still in a very plastic condition, and in consequence, methods, fundamental principles, and matters of nomenclature and terminology must be approached without prejudice in order that the best possible development of this field may be attained.

Grateful acknowledgment for criticisms and suggestions is made to Professor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound, who have read the text. The author is under especial obligations to Doctor Edith S. Clements for the drawings of leaf types, as well as for reading and criticising the manuscript. Professor Goodwin D. Swezey, Professor of Astronomy in the University of Nebraska, has kindly furnished much material for the determination of the sun’s altitude, and consequent light intensities, and has read the section devoted to light. Mr. George A. Loveland, Director of the Nebraska Section of the U. S. Weather Bureau, has contributed many helpful suggestions to the discussion of meteorological instruments. To Nella Schlesinger, Alice Venters, and George L. Fawcett, advanced students in experimental ecology, the author is indebted for many experiments which have been used in the discussion of adjustment and adaptation.

Acknowledgment is also made to the following for various cuts: Henry J. Green, Brooklyn, New York; Julien P. Friez, Baltimore, Maryland; C. H. Stoelting Co., Chicago, Illinois; Draper Manufacturing Co., New York city; Gundlach-Manhattan Optical Co., Rochester, New York; Rochester Optical Co., Rochester, New York; Bausch and Lomb Optical Co., Rochester, New York.

FREDERIC EDWARD CLEMENTS.

The University of Nebraska,

May, 1905.

CONTENTS

RESEARCH METHODS IN ECOLOGY

CHAPTER I. THE FOUNDATION OF ECOLOGY
The Need of a System

1. The scope of ecology. The clue to the field of ecology is found in the Greek word, οἲκος, home. The point of view in the following treatise is constantly that which is inherent in the term itself. Ecology is therefore considered the dominant theme in the study of plants, indeed, as the central and vital part of botany. This statement may at first appear startling, if not unfounded, but mature reflection will show that all the questions of botanical science lead sooner or later to the two ultimate facts: plant and habitat. The essential truth of this has been much obscured by detached methods of study in physiology, morphology, and histology, which are too often treated as independent fields. These have suffered incomplete and unsymmetric development in consequence of extreme specialistic tendencies. Analytic methods have dominated research to the exclusion of synthetic ones, which, in a greatly diversified field, must be final, if botanical knowledge is something to be systematized and not merely catalogued. Physiology in particular has suffered at the hands of detached specialists. Originally conceived as an inquiry into the origin and nature of plants, it has been developed strictly as a study of plant activities. It all but ignores the physical factors that control function, and the organs and tissues that reflect it.

2. Ecology and physiology. There can be little question in regard to the essential identity of physiology and ecology. This is evident when it is clearly seen that the present difference between the two fields is superficial. Ecology has been largely the descriptive study of vegetation; physiology has concerned itself with function; but, when carefully analyzed, both are seen to rest upon the same foundation. In each, the development is incomplete: ecology has so far been merely superficial, physiology too highly specialized. The one is chaotic and unsystematized, the other too often a minute study of function under abnormal circumstances. The greatest need of the former is the introduction of method and system, of the latter, a broadening of scope and new objectives. The growing recognition of the identity of the two makes it desirable to anticipate their final merging, and to formulate a system that will combine the good in each, and at the same time eliminate superficial and extreme tendencies. In this connection, it becomes necessary to point out to ecologist and physiologist alike that, while they have been working on the confines of the same great field, each must familiarize himself with the work and methods of the other, before his preparation is complete. Both must broaden their horizons, and rearrange their views. The ecologist is sadly in need of the more intimate and exact methods of the physiologist: the latter must take his experiments into the field, and must recognize more fully that function is but the middleman between habitat and plant. It seems probable that the final name for the whole field will be physiology, although the term ecology has distinct advantages of brevity and of meaning. In this event, however, it should be clearly recognized that, although the name remains the same, the field has become greatly broadened by new viewpoints and new methods.

HISTORICAL DEVELOPMENT

3. Geographical distribution. The systematic analysis of the great field of ecology is essential to its proper development in the future. A glance at its history shows that, while a number of essential points of attack have been discovered, only one or two of these have been organized, and that there is still an almost entire lack of correlation and coordination between these. The earliest and simplest development of the subject was concerned with the distribution of plants. This was at first merely an offshoot of taxonomy, and, in spite of the work of Humboldt and Schouw, has persisted in much of its primitive form to the present time, where it is represented by innumerable lists and catalogues. Geographical distribution was grounded upon the species, a fact which early caused it to become stereotyped as a statistical study of little value. This tendency was emphasized by the general practice of determining distribution for more or less artificial areas, and of instituting comparisons between regions or continents too little known or too widely remote. The fixed character of the subject is conclusively shown by the fact that it still persists in almost the original form more than a half century after Grisebach pointed out that the formation was the real unit of vegetation, and hence of distribution.

4. The plant formation. The corner-stone of ecology was laid by Grisebach in 1838 by his recognition of the plant formation as the fundamental feature of vegetation. Earlier writers, notably Linné (1737, 1751), Biberg (1749), and Hedenberg (1754), had perceived this relation more or less clearly, but failed to reduce it to a definite guiding principle. This was a natural result of the dominance of descriptive botany in the 18th century, by virtue of which all other lines of botanical inquiry languished. This tendency had spent itself to a certain degree by the opening of the 19th century, and both plant distribution and plant physiology began to take form. The stimulus given the former by Humboldt (1807) turned the attention of botanists more critically to the study of vegetation as a field in itself, and the growing feeling for structure in the latter led to Grisebach’s concept of the formation, which he defined as follows: “I would term a group of plants which bears a definite physiognomic character, such as a meadow, a forest, etc., a phytogeographic formation. The latter may be characterized by a single social species, by a complex of dominant species belonging to one family, or, finally, it may show an aggregate of species, which, though of various taxonomic character, have a common peculiarity; thus, the alpine meadows consist almost exclusively of perennial herbs.” The acceptance of the formation as the unit of vegetation took place slowly, but as a result of the work of Kerner (1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), and Warming (1889), this point of view came to be more and more prevalent. It was not, however, until the appearance of three works of great importance, Warming (1895), Drude (1896), and Schimper (1898), that the concept of the formation became generally predominant. With the growing recognition of the formation during the last decade has appeared the inevitable tendency to stereotype the subject of ecology in this stage. The present need, in consequence, is to show very clearly that the idea of the formation is a fundamental, and not an ultimate one, and that the proper superstructure of ecology is yet to be reared upon this as the foundation.

5. Plant succession. The fact that formations arise and disappear was perceived by Biberg as early as 1749, but it received slight attention until Steenstrup’s study of the succession in the forests of Zealand (1844 prox.). In the development of formations, as well as in their recognition, nearly all workers have confined themselves to the investigation of particular changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming (1890), and others have added much to our detailed knowledge of formational development. Notwithstanding the lapse of more than a half century, the study of plant successions is by no means a general practice among ecologists. This is a ready explanation of the fact that the vast field has so far yielded but few generalizations. Warming (1895) was the first to compile the few general principles of development clearly indicated up to this time. The first critical attempt to systematize the investigation of succession was made by Clements (1904), though this can be considered as little more than a beginning on account of the small number of successions so far studied. Future progress in this field will be conditioned not only by the more frequent study of developmental problems by working ecologists, but also, and most especially, by the application of known principles of succession, and by the working out of new ones.

6. Ecological phytogeography. Until recent years, the almost universal tendency was to give attention to formations from the standpoint of vegetation alone. While the habitat was touched here and there by isolated workers, and plant functions were being studied intensively by physiologists, both were practically ignored by ecologists as a class. The appearance of Warming’s Lehrbuch der oecologischen Pflanzengeographie (1896) and of Schimper’s Pflanzengeographie auf physiologischer Grundlage (1898) remedied this condition in a measure by a general discussion of the habitat, and by emphasizing the importance of the ecological or physiological point of view. Despite their frank recognition of the unique value of the habitat, the major part of both books was necessarily given to what may be termed the general description of formations. For this reason, and for others arising out of an almost complete dearth of methods of investigation, ecology is still almost entirely a floristic study in practice, although there is a universal recognition of the much greater value of the viewpoint which rests upon the relation between the formation and its habitat.

7. Experimental ecology. Properly speaking, the experimental study of ecology dates from Bonnier[^[1]] (1890, 1895), though it is well understood that experimental adjustment of plants to certain physical factors had been the subject of investigation before this time. The chief merit of Bonnier’s work, however, lies in the fact that it was done out of doors, under natural conditions, and for these reasons it should be regarded as the real beginning of this subject. Bonnier’s experiments were made for the purpose of determining the effect of altitude. Culture plots of certain species were located in the Alps and the Pyrenees, and the results were compared with control cultures made in the lowlands about Paris. In 1894 he also made a comparative study of certain polydemic species common to the arctic islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods are fundamental to field experiment, but the results are inconclusive, inasmuch as altitude is a complex of factors. As no careful study was made of the latter, it was manifestly impossible to refer changes and differences of structure to the definite cause. In a paper that has just appeared, E. S. Clements (1905) has applied the method of polydemic comparison to nearly a hundred species of the Rocky mountains. In this work, the all-important advance has been made of determining accurately the decisive differences between the two or more habitats of the same species in terms of direct factors, water-content, humidity, and light. In his own investigations of Colorado mountain vegetation, the author has applied the method of field cultures by planting seeds of somewhat plastic species in habitats of measured value, and has thought to initiate a new line of research by applying experimental methods to the study of vegetation as an organism. In connection with this, there has also been developed a method of control experiment in the plant house under definitely measured differences of water and light.

8. Ecology of the habitat. Since the time of Humboldt, there have been desultory attempts to determine the physical factors of habitats with some degree of accuracy. The first real achievement in this line was in the measurement of light values by Wiesner in 1896. In 1898 the writer first began to study the structure of habitats by the determination of water-content, light, humidity, temperature, wind, etc., by means of instruments. These methods were used by one of his pupils, Thornber (1901), in the study of a particular formation, and by another, Hedgcock (1902), in a critical investigation of water-content. Two years later, similar methods of measuring physical factors were put into operation in connection with experimental evolution under control in the plant house. E. S. Clements (1905), as already indicated, has made the use of factor instruments the foundation of a detailed study of polydemic species, i. e., those which grow in two or more habitats, and which are, indeed, the most perfect of all experiments in the production of new forms. In a volume in preparation upon the mountain vegetation of Colorado, the writer has brought the use of physical factor instruments to a logical conclusion, and has made the study of the habitat the basis of the whole work. Out of this investigation has come a new concept of vegetation (Clements 1904), namely, that it is to be regarded as a complex organism with structures and with functions susceptible of exact methods of study.

9. The evidence from historical development. This extremely brief resume of what has been accomplished in the several lines of ecological research makes evident the almost complete absence of correlation and of system. The whole field not merely lacks system, but it also demands a much keener perception of the relative value of the different tendencies already developed. It is inevitable from the great number of tyros, and of dilettante students of ecology in comparison with the few specialists, that the surface of the field should have received all of the attention. It is, however, both unfortunate and unscientific that great lines of development should be entirely unknown to all but a few. There is no other department of botany in which the superficial study of more than half a century ago still prevails to the exclusion of better methods, many of which have been known for a decade or more. It is clear, then, that the imperative need of ecology is the proper coordination of its various points of view, and the working out of a definite system which will make possible a ready recognition of that which is fundamental and of that which is merely collateral. The historical development, as is well understood, can furnish but a slight clue to this. It is a fact of common knowledge that the first development of any subject is general, and usually superficial also. True values come out clearly only after the whole field has been surveyed. For these reasons, as will be pointed out in detail later, the newer viewpoints are regarded as either the most important or the most fundamental. Experimental ecology will throw a flood of light upon plant structure and function, while exact methods of studying the habitat are practically certain of universal application in the future.

PRESENT STATUS OF ECOLOGY

10. The lack of special training. The bane of the recent development popularly known as ecology has been a widespread feeling that anyone can do ecological work, regardless of preparation. There is nothing in modern botany more erroneous than this feeling. The whole task of ecology is to find out what the living plant and the living formation are doing and have done in response to definite complexes of factors, i. e., habitats. In this sense, ecology is practically coextensive with botany, and the student of a local flora who knows a few hundred species is no more competent to do ecological work than he is to reconstruct the phylogeny of the vegetable kingdom, or to explain the transmission of ancestral qualities. The comprehensive and fundamental character of the subject makes a broad special training even more requisite than in more restricted lines of botanical inquiry. The ecologist must first of all be a botanist, not a mere cataloguer of plants, and he must also possess a particular training in the special methods of ecological research. He must be familiar with the various points of attack in this field, and he must know the history of his subject thoroughly. Ecology affords the most striking example of the prevalent evil of American botanical study, i. e., an indifference to, or an ignorance of the literature of the subject. The trouble is much aggravated here, however, by the breadth of the field, and the common assumption that a special training is unnecessary, if not, indeed, superfluous. Ignorance of the important ecological literature has been a most fertile source of crude and superficial studies, a condition that will become more apparent as these fields are worked again by carefully trained investigators.

11. Descriptive ecology. The stage of development of the subject at the present time may be designated as descriptive ecology, for purposes of discussion merely. This is concerned with the superficial description of vegetation in general terms, and results from the fact that the development has begun on the surface, and has scarcely penetrated beneath it. The organic connection between ecology and floristic has produced an erroneous impression as to the relative value of the two. Floristic has required little knowledge, and less preparation: it lends itself with insidious ease to chance journeys or to vacation trips, the fruits of which are found in vague descriptive articles, and in the multiplication of fictitious formations. While there is good reason that a record should be left of any serious reconnaissance, even though it be of a few weeks’ duration, the resulting lists and descriptive articles can have only the most rudimentary value, and it is absurd to regard them as ecological contributions at all. No statement admits of stronger emphasis, and there is none that should be taken more closely to heart by botanists who have supposed that they were doing ecological work. An almost equally fertile source of valueless work is the independent treatment of a restricted local area. The great readiness with which floristic lists and descriptions can be made has led many a botanist, working in a small area, or passing hurriedly through an extended region, to try his hand at formation making. From this practice have resulted scores of so-called formations, which are mere patches, consocies, or stages in development, or which have, indeed, no existence other than in the minds of their discoverers. The misleading definiteness which a photograph seems to give a bit of vegetation has been responsible for a surplus of photographic formations, which have no counterparts in nature. Indispensable as the photograph is to any systematic record of vegetation, its use up to the present time has but too often served to bring it into disrepute. There has been a marked tendency to apply the current methods of descriptive botany to vegetation, and to regard every slightly different piece of the floral covering as a formation. No method can yield results further from the truth. It is evident that the recognition and limitation of formations should be left absolutely to the broadly trained specialist, who has a thorough preparation by virtue of having acquainted himself carefully with the development and structure of typical formations over large areas.

12. The value of floristic. In what has been said above, there is no intent to decry the value of floristic. The skilled workman can spare the material which he is fashioning as readily as the ecologist can work without an accurate knowledge of the genera and species which make up a particular vegetation. Some botanists whose knowledge of ecology is that of the study or the laboratory have maintained that it is possible to investigate vegetation without knowing the plants which compose it. Ecology is to be wrought out in the field, however, and the field ecologist—none other, indeed, should bear the name—understands that floristic alone can furnish the crude material which takes form under his hands. It is the absolute need of a thorough acquaintance with the flora of a region which makes it impossible for a traveler to obtain anything of real ecological value in his first journey through a country. As the very first step, he must gain at least a fair knowledge of the floristic, which will alone take the major part of one or more growing seasons. This information the student of a local flora already has at the tip of his tongue; in itself it can not constitute a contribution to ecology, but merely the basis for one. In this connection, moreover, it can not be used independently, but becomes of value only after an acquaintance with a wide field. Floristic study and floristic lists, then, are indispensable, but to be of real value their proper function must be clearly recognized. They do not constitute ecology.

13. Reconnaissance and investigation. In striving to indicate the true value and worth of ecological study, it becomes necessary to draw a definite line between what we may term reconnaissance and investigation. By the former is understood the preliminary survey of a region, extending over one or two years. The objects of such a survey are to obtain a comprehensive general knowledge of the topography and vegetation of the region, and of its relation to the other regions about it. The chief purpose, however, is to gain a good working acquaintance with the flora: a reconnaissance to be of value must do this at all events. Certain general facts will inevitably appear during this process, but they will invariably need the confirmation of future study. It would be an advantage to real ecology if reconnaissance were to confine itself entirely to the matter of making a careful floristic survey. Investigation begins when the inquiry is directed to the habitat, or to the development and structure of the formation which it bears, i. e., when it takes up the manifold problems of the οἶκος. Such a study must be based upon floristic, but the latter becomes a part of investigation only in so far as it leads to it. Standing by itself, it is not ecological research: it is the preparation for it. This distinction deserves careful thought. The numerous recruits to ecology have turned their attention to what lay nearest to hand, with little question as to its value, or to where it might lead. The result has been to make reconnaissance far outweigh investigation in amount, and to give it a value which properly belongs to the latter. Furthermore, this mistaken conception has in many cases, without doubt, prevented its leading to valuable research work.

14. Resident investigation. Obviously, if reconnaissance is a superficial survey, and investigation thorough extensive study, an important distinction between them is in the time required. While one may well be the result of a journey of some duration, the other is essentially dependent upon residence. In the past the great disparity between the size of the field and the number of workers has made resident study too often an ideal, but in the future it will be increasingly the case that a particular region will be worked by a trained ecologist resident in it. This may never be altogether true of inaccessible and sterile portions of the globe. It may be pointed out, however, that, between the tropics and the poles, residence during the summer or growing period is in essence continuous residence. In the ultimate analysis, winter conditions have of course some influence upon the development of vegetation during the summer, but the important problems which a vegetation presents must be worked out during the period of development. For temperate, arctic, and alpine regions, then, repeated study during the growing period for a term of years has practically all the advantages of continuous residence. For all practical purposes, it is resident study.

15. The dangers of a restricted field. In the resident study of a particular region, the temptation to make an intensive investigation of a circumscribed area is very strong. The limits imposed by distance are alone a sufficient explanation of this, but it is greatly increased by the inclination toward detailed study for which a small field offers opportunity. This temptation can be overcome only by a general preliminary study of the larger region in which the particular field is located. The broader outlook gained in this way will throw needed light upon many obscure facts of the latter, and at the same time it will act as a necessary corrective of the tendency to consider the problems of the local field in a detached manner, and to magnify the value of the distinctions made and the results obtained. Such a general survey has the purpose and value of a reconnaissance, and is always the first step in the accurate and detailed investigation of the local area or formation. Each corrects the extreme tendency of the other, and thorough comprehensive work can be done only by combining the two methods. When the field of inquiry is a large area or covers a whole region, the procedure should be essentially the same. A third stage must be added, however, in which a more careful survey is made of the entire field in the light of the thorough study of the local area. The writer’s methods in the investigation of the Colorado vegetation illustrate this procedure. The summers of 1896, 1897, 1898 were devoted to reconnaissance; those of 1899–1904 were given to detailed and comprehensive study by instrument and quadrat of a highly diversified, representative area less than 20 miles square, while the work of the final summer will be the application of the results obtained in this localized area to the region traversed from 1896–98. This is practically the application of methods of precision to an area of more than 100,000 square miles. It also serves to call attention to another point not properly appreciated as yet by those who would do ecological work. This is the need of taking up field problems as a result of serious forethought, and not as a matter of accident or mere propinquity. A carefully matured plan of attack which contemplates an expenditure of time and energy for a number of years will yield results of value, no matter how much attention an area may have received. On the other hand, an aimless or hurried excursion into the least known or richest of regions will lead to nothing but a waste of time, especially upon the part of the ecologist, who must read the articles which result, if only for the purpose of making sure that there is nothing in them.

APPLICATIONS OF ECOLOGY

16. The subjects touched by ecology. The applications of ecological methods and results to other departments of botany, and to other fields of research are numerous. Many of these are both intimate and fundamental, and give promise of affording new and extremely fruitful points of view. It has already been indicated that ecology bears the closest of relations to morphology and histology on the one side, and to physiology on the other—that it is, indeed, nothing but a rational field physiology, which regards form and function as inseparable phenomena. The closeness with which it touches plant pathology follows directly from this, as pathology is nothing more than abnormal form and functioning. Experimental work in ecology is purely a study of evolution, and the facts of the latter are the materials with which taxonomy deals. Forestry has already been termed “applied ecology” and in its scientific aspects, which are its foundation, it is precisely the ecology of woody plants, and of the vegetation which they constitute. Apart from botany, the physical side of ecology is largely a question of soil physics, and of physiography. On the other hand, vegetation is coming more and more to be regarded as a fundamental factor in zoogeography and in sociology. Furthermore, with respect to the latter, it will be pointed out below that the principles of association which have been determined for plants, viz., invasion, succession, zonation, and alternation, apply with almost equal force to man.

17. Physiology and pathology. The effect of ecology in emphasizing the intrinsically close connection between physiology and morphology has already been mentioned. Its influence in normalizing the former by forcing it into the field as the place for experiment, and by directing the chief attention to the plant as an organism rather than a complex of organs, is also rapidly coming to be felt. Ecology will doubtless exert a corrective influence upon pathology in the near future. This is inevitable as the latter ceases to be the merely formal study of specific pathogenic organisms, and turns its attention to the cause of all abnormality, which is to be found in the habitat, whether this be physical, as when the water-content is low, or biotic, when a parasitic fungus is present. The relative ease with which specific diseases can be studied has helped to obscure the essential fact that the approach to pathology must be through physiology. Much indeed of the observational physiology of the laboratories has been pathology, and it will be impossible to draw a clear line between them until precise experiment in the habitat has come into vogue.

18. Experimental evolution. As a result of the extremely fragmentary character of the geological record, nothing is more absolute than that there can be no positive knowledge of the exact origin of a form or species, except in those rare cases of the present day, where the whole process has taken place under the eye of a trained observer. The origin of the plant forms known at present must forever lie without the domain of direct knowledge. If it were possible, by a marvel of ingenuity and patience to develop experimentally Myosurus from Selaginella, this would not be absolutely conclusive proof that Myosurus was first derived in this way. When all is said, however, this would be the very best of presumptive evidence. It must also be recognized that this is the nearest to complete proof that we shall ever attain, and with this in mind it becomes apparent at once that evidence from experiment is of paramount importance in the study of evolution (the origin of species).

The phase of experimental ecology which has to do with the plant has well been called experimental evolution. While this is a field almost wholly without development at present, there can be little question that it is to be one of the most fertile and important in the future. Attention will be directed first to those forms which are undergoing modification at the present time. The cause and direction of change will be ascertained, and its amount and rapidity measured by biometrical methods. The next step will be to actually change the habitat of representative types, and to determine for each the general trend of adaptation, as well as the exact details. By means of the methods used and the results obtained in these investigations, it will be possible to attack the much more difficult problem of retracing the development of species already definitely constituted. This will be accomplished by the study of the derived and the supposed ancestral form, but owing to the great preponderance of evolution over reversion, the study of the ancestral form will yield much more valuable results.

The general application of the methods of experimental ecology will mark a new era in the study of evolution. There has been a surplus of literary investigation, but altogether too little actual experiment. The great value of De Vries’ work lies not in the importance of the results obtained, but in calling attention to the unique importance of experimental methods in contributing to a knowledge of evolution. The development of the latter has been greatly hindered by the dearth of actual facts, and by a marked tendency to compensate for this by verbiage and dogmatism. This is well illustrated by the present position of the “mutation theory,” which, so far as the evidence available is concerned, is merely a working hypothesis. An incredible amount of bias and looseness of thought have characterized the discussion of evolution. It is earnestly to be hoped that the future will bring more work and less argument, and that the literary evolutionists will become less and less reluctant to leave the relative merits of variation and mutation to experiment.

19. Taxonomy. Taxonomy is classified evolution. It is distinct from descriptive botany, which is merely a cataloguing of all known forms, with little regard to development and relationship. The consideration of the latter is peculiarly the problem of taxonomy, but the solution must be sought through experimental evolution. The first task of the latter is to determine the course of modification in related forms, and the relationships existing between them. With this information, taxonomy can group forms according to their rank, i. e., their descent. The same method is applicable to the species of a genus, and, in a less degree, perhaps, to the genera which constitute a family. The use to which it may be put in indicating family relationships will depend largely upon the gap existing between the families concerned. While interpretation will always play a part in taxonomy, the general use of experiment will leave much less opportunity for the personal equation than is at present the case. Taxonomy, like descriptive botany, is based upon the species, but, while there may exist a passable kind of descriptive botany, there can be no real taxonomy as long as the sole criterion of a species is the difference which any observer thinks he sees between one plant and another. The so-called species of to-day range in value from mere variations to true species which are groups of great constancy and definiteness. The reasons for this are obvious when one recalls that “species” are still the product of the herbarium, not of the field, and that the more intensive the study, the greater the output in “species.” It would seem that careful field study of a form for several seasons would be the first requisite for the making of a species, but it is a precaution which is entirely ignored in the vast majority of cases. The thought of subjecting forms presumed to be species to conclusive test by experiment has apparently not even occurred to descriptive botanists as yet. Notwithstanding, there can be no serious doubt that the existing practice of re-splitting hairs must come to an end sooner or later. The remedy will come from without through the application of experimental methods in the hands of the ecologist, and the cataloguing of slight and unrelated differences will yield to an ordered taxonomy.

Experimental evolution will solve a taxonomic problem as yet untouched, namely, the effect of recent environment upon the production of species. It is well understood that some species grow in nature in various habitats without suffering material change, while others are modified to constitute a new form in each habitat. It is at once clear that these forms (or ecads) are of more recent descent than the species, i. e., of lower rank. It must also be recognized that a constant group and a highly plastic one are essentially different. If constancy is made a necessary quality of a species, one is a species, the other is not. If both are species, then two different kinds must be distinguished. Among the species of our manuals are found many ecads, alongside of constant and inconstant species. These can be distinguished only by field experiment, and their proper coordination is possible only after this has been done. Indeed, the whole question of the ability or the inability of environmental variation to produce constant species is one that must be referred to repeated and long-continued experiment in the field.

A minor service of considerable value can be rendered taxonomy by working over the diagnosis from the ecological standpoint. Many ecological facts are of real diagnostic value, while others are at least of much interest, and serve to direct attention to the plant as a living thing. The loose use of terms denoting abundance, which prevails in lists and manuals, should be replaced by the exact usage which the quadrat method has made possible for vegetation. The designation of habitats could be made much more exact, and the formation, as well as the habitat form or ecad, and the vegetation form or phyad, should be indicated in addition. The general terms drawn from pollination, seed-production, and dissemination might also be included to advantage.

20. Forestry, if the purely commercial aspects be disregarded, is the ecology of a particular kind of vegetation, the forest. Therefore, in pointing out the connection between them, it is only necessary to say that whatever contributes to the ecology of the forest is a contribution to forestry. There are, however, certain lines of inquiry which are of fundamental importance. First among these, and of primary interest from the practical point of view, are the questions pertaining to the distribution of forests and their structure. Of even greater significance are the problems of forest development, movement, and of reforestation, which are comprised in succession. The gradual invasion of the plains and prairies by the forest belt of the east and north is in full conformity with the laws of invasion, and the ecological methods to be employed here serve not merely to determine the actual conditions at present, but also to forecast them with a great deal of accuracy. The slow but certain development of forests on new soils, and their more rapid reestablishment where the woody vegetation has been destroyed by burning or lumbering, are ordinary phenomena of succession, for which the ecologist has already worked out the laws, and determined the methods of investigation. Having once ascertained the original and adjacent vegetation and the character of the habitat, the ecologist can indicate with accuracy not only the character of the new forest that will appear, but also the nature of the antecedent formations. A full knowledge of the character and laws of succession will prove of the greatest value to the forester in all studies of forestation and reforestation. Forests which now seem entirely unrelated will be seen to possess the most intimate developmental connection, and the fuller insight into the life history gained in this way will have a direct bearing upon methods of conservation, etc. It will further show that the forester must know other vegetations as well, since grassland and thicket formations have an intimate influence upon the course of the succession, as well as upon the advance of a forest frontier.

One of the greatest aids which modern ecology can furnish forestry, however, is the method of determining the physical nature of the habitat. So far, foresters have been obliged to content themselves with a more or less superficial study of the structure of forest formations, without being able to do more than guess at the physical causes which control both structure and development. This handicap is especially noticeable in the case of forest plantings in non-forested regions, where it has been impossible to estimate the chances of success, or to determine the most favorable areas except by actual plantations. Equipped with the proper instruments for measuring water-content, humidity, light and temperature, the ecologist is able to determine the precise conditions under which reproduction is occurring, and to ascertain what non-forested areas offer the most nearly similar conditions. A knowledge of habitats and the means of measuring them enables the forester to discover the causes which control the vegetation with which he is already familiar, and to forecast results otherwise hidden. Furthermore, it makes it possible for him to enter a new region and to determine its nature and capabilities at a minimum of time and energy.

21. Physiography. Physiographic features play an important part in determining the quantity of certain direct factors of the habitat. Perhaps a more important connection between physiography and ecology is to be found in succession. The beginning of all primary, and of many secondary successions is to be sought in the physiographic processes which produce new habitats, or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of the land. The most pronounced influence of terrestrial successions is found in the stabilization which their ultimate stages exert upon land forms, even where these are highly immature. The chief effect of aquatic successions is to be found in the “silting up” and the formation of new land, which result from the action of vegetation upon silt-bearing waters. The closeness of the relation between succession and the forms of the land has led to the application of the term “physiographic ecology” to that part of the subject which deals with the development of vegetation, i. e., succession.

22. Soil physics. This subject is as much a part of ecology as is forestry. It is intrinsically that subdivision of ecology which deals with the edaphic factors of the habitat, and their relation to the plant. Since the basis is physics, there has been a general tendency to overvalue the determination of soil properties, and to ignore the fact that these are decisive only when considered with reference to the living plant. As the soil contains the water which is the factor of greatest importance to plants, soil physics is an especially important part of ecology. Its methods are discussed under the habitat.

23. Zoogeography. Since animals are free for the most part, and hence not confined so strictly to one spot as plants, their dependence upon the habitat is not so evident. The relation is further obscured by the fact that no physical factor has the direct effect upon them which water or light exerts upon the plant. Vegetation, indeed, as the source of food and protection, plays a more obvious, if not a more important part. This is especially true of anthophilous insects, but it also holds for all herbivorous animals, and, through them, for carnivorous ones. The animal ecology of a particular region can only be properly investigated after the habitats and plant formations have been carefully studied. Here, as in floristics, a great deal can be done in the way of listing the fauna, or studying the life habits of its species, without any knowledge of plant ecology; but an adequate study must be based upon a knowledge of the vegetation. Although animal formations are often poorly defined, there can be no doubt of their existence. Frequently they coincide with plant formations, and then have very definite limits. They exhibit both development and structure, and are subject to the laws of invasion, succession, zonation, and alternation, though these are not altogether similar to those known for plants, a fact readily explained by the motility of animals. Considered from the above point of view, zoogeography is a virgin field, and it promises great things to the student who approaches it with the proper training.

24. Sociology. In its fundamental aspects, sociology is the ecology of a particular species of animal, and has in consequence, a similar close connection with plant ecology. The widespread migration of man and his social nature have resulted in the production of groups or communities which have much more in common with plant formations than do formations of other animals. The laws of association apply with especial force to the family, tribe, community, etc., while the laws of succession are essentially the same for both plants and man. At first thought it might seem that man’s ability to change his dwelling-place and to modify his environment exempts him in large measure from the influence of the habitat. The exemption, however, is only apparent, as the control exerted by climate, soil, and physiography is all but absolute, particularly when man’s dependence upon vegetation, both natural and cultural, is called to mind.

The Essentials of a System

25. Cause and effect: habitat and plant. In seeking to lay the foundation for a broad and thorough system of ecological research, it is necessary to scan the whole field, and to discriminate carefully between what is fundamental and what is merely collateral. The chief task is to discover, if possible, such a guiding principle as will furnish a basis for a permanent and logical superstructure. In ecology, the one relation which is precedent to all others is the one that exists between the habitat and the plant. This relation has long been known, but its full value has yet to be appreciated. It is precisely the relation that exists between cause and effect, and its fundamental importance lies in the fact that all questions concerning the plant lead back to it ultimately. Other relations are important, but no other is paramount, or able to serve as the basis of ecology. Ecology sums up this relation of cause and effect in a single word, and it may be that this advantage will finally cause its general acceptance as the proper name for this great field.

In the further analysis of the connection between the habitat and the plant, it is evident that the causes or factors of the habitat act directly upon the plant as an individual, and at the same time upon plants as groups of individuals. The latter in no wise decreases the importance of the plant as the primary effect of the habitat, but it gives form to research by making it possible to consider two great natural groups of phenomena, each characterized by very different categories of effects. Ecology thus falls naturally into three great fundamental fields of inquiry: habitat, plant, and formation (or vegetation). To be sure, the last can be approached only through the plant, but as the latter is not an individual, but the unit of a complex from the formational standpoint, the formation itself may be regarded as a sort of multiple organism, which is in many ways at least a direct effect of the habitat. In emphasizing this fundamental relation of habitat and vegetation, it is imperative not to ignore the fact that neither plant nor formation is altogether the effect of its present habitat. A third element must always be considered, namely, the historical fact, by which is meant the ancestral structure. Upon analysis, however, this is in its turn found to be the product of antecedent habitats, and in consequence the essential connection between the habitat and the plant is seen to be absolute.

26. The place of function. In the foregoing it is understood that the immediate effect of the physical factors of the habitat is to be found in the functions of the plant, and that these determine the plant structure. Function has so long been the especial theme of plant physiology that methods of investigation are numerous and well known, and it is unnecessary here to consider it further than to indicate its general bearing. The essential sequence in ecological research, then, is the one already indicated, viz., habitat, plant, and formation, and this will constitute the order of treatment in the following pages. That portion of floristic which is not mere descriptive botany belongs to the consideration of the formation, and in consequence there will be no special treatment of floristic as a subdivision of ecology.

CHAPTER II. THE HABITAT

Concept and Analysis

27. Definition of the habitat. The habitat is the sum of all the forces or factors present in a given area. It is the exact equivalent of the term environment, though the latter is commonly used in a more general sense. As an ecological concept, the habitat refers to an area much more definite in character, and more sharply limited in extent than the habitat of species as indicated in the manuals. Since the careful study of habitats has scarcely begun, it is impossible to recognize and delimit them in an absolute sense. Visible topographic boundaries often exist, but in many cases, the limit, though actual, is not readily perceived. Contiguous habitats may be sharply limited, or they may pass into each other so gradually that no real line of demarcation can be drawn. Whatever variations they may show, however, all habitats agree in the possession of certain essential factors, which are universally present. On the other hand, a few factors are merely incidental and may be present or absent. The relative value and amount of these is probably similar for no two habitats, though the latter readily fall into groups with reference to the amount of some particular factor.

28. Factors. The factors of a habitat are water-content, humidity, light, temperature, soil, wind, precipitation, pressure, altitude, exposure, slope, surface (cover), and animals. To these should be added gravity and polarity, which are practically uniform for all habitats, and may, in consequence, be ignored in this treatise. Length of season, while it plays an important part in vegetation, is clearly a complex and is to be treated under its constituents. Of the factors given, all are regularly found in each habitat, though some are not constantly present. The first five, water-content, humidity, light, temperature, and soil are the most important, and any one may well serve as a basis for grouping habitats into particular classes with reference to quantity. As will be pointed out later, however, water-content and light furnish the most striking differences between habitats, and offer the best means of classification. As habitats are inseparable from the formations which they bear, the discussion of the kinds of habitats is reserved for chapter IV.

Classification of Factors

29. The nature of factors. The factors of a habitat are arranged in two groups according to their nature: (1) physical, (2) biotic. In the strict sense, the physical factors constitute the habitat proper, and are the real causative forces. No habitat escapes the influence of biotic factors, however, as the formation always reacts upon it, and the influence of animals is usually felt in some measure. Physical factors are further grouped into (1) climatic and (2) edaphic, with respect to source, or, better, the medium in which they are found. Climatic, or atmospheric factors are humidity, light, temperature, wind, pressure, and precipitation. Axiomatically, the stimuli which they produce are especially related to the leaf. Edaphic or soil factors are confined to the soil, as the term denotes, and are immediately concerned with the functions of the root. Water-content is by far the most important of these; the others are soil composition (nutrient-content), soil temperature, altitude, slope, exposure, and surface. The last four are of a more general character than the others, and are usually referred to as physiographic factors. Cover, when dead, might well be placed among these also, but as it is little different from the living cover in effect, it seems most logical to refer it to biotic factors.

30. The influence of factors. While the above classification is both obvious and convenient, a more logical and intimate grouping may be made upon the influence which the factor exerts. On this basis, factors are divided into (1) direct, (2) indirect, and (3) remote. Direct factors are those which act directly upon an important function of the plant and produce a formative effect: for example, an increase in humidity produces an immediate decrease in transpiration. They are water-content, humidity, and light. Other factors have a direct action: thus temperature has an immediate influence upon respiration and probably assimilation also, but it is not structurally formative. Wind has a direct mechanical effect upon woody plants, but it does not fall within our definition. Indirect factors are those that affect a formative function of the plant through another factor; thus a change in temperature causes a change in humidity and this in turn calls forth a change in transpiration; or, a change in soil texture increases the water-content, and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature, wind, pressure, precipitation, and soil composition. Remote factors are, for the most part, physiographic and biotic: they require at least two other factors to act as middlemen. Altitude affects plants through pressure, which modifies humidity, and hence transpiration. Slope determines in large degree the run-off during a rainstorm, thus affecting water-content and the amount of water absorbed. Earthworms and plant parts change the texture of the soil, and thereby the water-content. Indirect factors often exert a remote influence also, as may be seen in the effect which temperature and wind have in increasing evaporation from the soil, and thus reducing the water-content. This distinction between factors may seem insufficiently grounded. In this event, it should be noted that it centers the effects of all factors upon the three direct ones, water-content, humidity, and light. If it further be recalled that these are the only factors which produce qualitative structural changes, and that the classification of ecads and formations is based upon them, the validity of the distinction is clear.

The Determination of Factors

31. The need of exact measurement. Any serious endeavor to find in the habitat those causes which are producing modification in the plant and in vegetation can not stop with the factors merely. The next step is to determine the quantity of each. It is not sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all degrees, and it is impossible to institute comparisons between them without an exact measure of each factor. Similarly, one can not trace the adaptations of species to their proper causes unless the quantity of each factor is known. It is of little value to know the general effect of a factor, unless it is known to what degree this effect is exerted. For this purpose it becomes necessary to appeal to instruments, in order to determine the exact amount of each factor that is present in a particular habitat, and hence to determine the ratio between the stimulus and the amount of structural adjustment which results. The employment of instruments of precision is clearly indispensable for the task which we have set for ecology, and every student that intends to strike at the root of the subject, and to make lasting contributions to it, must familiarize himself with instrumental methods. One great benefit will accrue to ecology as soon as this fact is generally recognized. The use of instruments and the application of results obtained from them demand much patience and seriousness of purpose upon the part of the student. As a consequence, there will be a general exodus from ecology of those that have been attracted to it as the latest botanical fad, and have done so much to bring it into disrepute.

32. The value of meteorological methods. At the outset there must be a very clear understanding that weather records and readings have only a very general value. This is in spite of the fact that the instruments employed are of standard precision. An important reason for this lack of value is that readings are not made in a particular habitat; as a rule, indeed, they are made in towns and cities, and hence are far removed from masses of vegetation. They are usually taken at considerable heights, and give but a general indication of the conditions at the level of vegetation. The chief difficulty, however, is that the factors observed at weather stations—temperature, pressure, wind, and precipitation—are those which have the least value for the ecologist. It is true that a knowledge of the temperature and rainfall of a great region will afford some idea of the general character of its vegetation. A proper understanding of such a vegetation is, however, to be gained only through the exact study of its component formations. Ecology has already incurred sufficient censure as a subject composed of very general ideas, and the use of meteorological data, which can never be connected definitely with anything in the plant or the formation, should be discontinued. This must not be understood to mean that meteorological instruments can not be used in the proper place and manner, i. e., in the habitat.

33. Habitat determination. It is self-evident that determinations of factors by instruments can only be of value in the habitat where they are made. In other words, a habitat is a unit for purposes of measuring its factors, and measures of one habitat have no exact value in another. This fact can not be overstated. Thus, while it is perfectly legitimate, and indeed highly desirable, to locate thermographs in different mountain zones for ascertaining the rate at which temperature decreases with altitude, the data obtained in this way are not directly applicable in explanation of plant or formation changes, except when the same species occurs at different altitudes. Special methods are valuable and often absolutely necessary, but in view of the fact that the plant as well as the formation is the definite product of a definite habitat, the fundamental rule in instrumentation is that complete readings must be made within a habitat for that habitat alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to be investigated, as it is imperative that the station for making readings be located well within the formation, in order to avoid transition conditions. In vegetation, there are as many habitats as formations, and in addition a large number of new and denuded habitats upon which successions have not yet started; a knowledge of each formation or succession must rest ultimately upon the factors of its particular habitat.

34. Determinable and efficient differences. The instruments employed in studying habitats can not be too exact, as there is no adequate knowledge as yet concerning the real differences which exist between related or contiguous formations. This is particularly true of differences which are efficient in producing a recognizable structural change in plant or formation. Investigations made by the writer have shewn that standard instruments will measure differences of quantity quite too small to produce a visible reaction. Efficient differences are not the same for different factors, and perhaps also for the same factor when found in various combinations. They vary widely for different species, being in direct relation to the plasticity of the latter. The point necessary to bear in mind in formulating methods for habitat investigation and in making use of instruments is that standard instruments should be used for the very reason that we do not yet know the relation between determinable and efficient differences. On the other hand, it is unnecessary to insist upon absolute exactness as soon as it is found that the determinable difference lies well within the efficient one. This by no means indicates that instruments are not to be carefully standardized and frequently checked, or that accurate readings should not be made. It means that a slight margin of error may be permitted in a machine which registers well within the efficient difference for that factor, and that instruments that read to the last degree of nicety are not absolutely necessary. In the fundamental work of determining efficient differences, however, instruments can not have too great precision. Moreover, these differences must be based upon the most plastic species of a formation, and the readings must be made under normal conditions.

Instrumentation

35. Methods. In the field use of instruments two methods have been developed. The first in point of time was the method of simple instruments, devised especially for class work, and capable of being used only where a number of trained students are available. The method of automatic instruments was an immediate outgrowth of this, due to the necessity which confronts the solitary investigator of being in different habitats at the same time. In the gradual evolution of this subject, it has become possible to combine the two methods in such a way as to retain all the advantages of the automatic method, and most of those of the method of simple instruments.

36. Method of simple instruments. By simple instruments are denoted those that do not record, but must be read by the observer at the time. They are standard instruments of precision, but possess the disadvantage of requiring an observer for each one. They are well illustrated by the thermometers and psychrometers used by the Weather Bureau. In the hands of trained observers the results obtained are unimpeachable; in fact, standard simple instruments must be constantly employed to check automatic ones. As physical factors vary greatly through the day and through the year, it is all-important that the readings in habitats which are being compared should be made at the same instant. This requires a number of observers; as many as twelve stations have been read at one time, and there is of course no limit to the number. It is very important, also, that observers be carefully trained in the handling of instruments, and in reading them accurately and intelligently at the proper moment. In practice it has been found impossible to do such work in elementary classes, and, even in using small advanced classes, prolonged drill has been necessary before trustworthy results could be obtained. When a class has once been thoroughly trained in making accurate simultaneous readings, there is practically no limit, other than that set by time, to the valuable work that can be done, both in instruction and investigation.

37. Method of automatic instruments. The solitary investigator must replace trained helpers by automatic instruments or ecographs. These have the very great advantages of giving continuous simultaneous records for long periods, and of having no personal equation. They must be regulated and checked, to be sure, but as this is all done by the same person, the error is negligible. There is nothing more satisfactory in resident investigation than a series of accurate recording instruments in various habitats. Ecographs have two disadvantages. The chief perhaps is cost. The expense of a single “battery” which will record light, water-content, humidity, and temperature is about $250. Another difficulty is that they can be used only within a few miles of the base, since they require attention every week for regulation, change of record, etc. While this means that ecographs in their present form are not adapted to reconnaissance, this is not a real disadvantage, as the scattered observations possible on such a journey can best be made by simple instruments.

38. Combined methods. The best results by far are to be obtained by the combined use of simple and automatic instruments. This is particularly true in research, but it applies also to class instruction. The ecographs afford a continuous, accurate basal record, to which a single reading made at any time or place can be readily referred for comparison. On the other hand, it is an easy matter to carry a full complement of simple instruments on the daily field trips, and to make accurate readings in a score or more of formations in a single day. An isolated reading, especially of a climatic factor, has little or no value in itself, but when it can be compared with a reading made at the same time in the base station by an ecograph, it is the equivalent of an automatic reading. This method renders a set of simple instruments more desirable for a long trip or reconnaissance than a battery of automatic ones. It is practically impossible to carry the latter into the field, and in any event a continuous record is out of the question. As there are other tasks at such times also, it becomes evident that the taking of single readings which can be compared with a continuous record offers the most satisfactory solution.

Construction and Use of Instruments

39. The selection of instruments. In selecting and devising instruments for the investigation of physical factors, emphasis has first been laid upon accuracy. This is the result of a feeling that it is better to have instruments that read too minutely than those which do not make distinctions that are sufficiently close, particularly until more has been learned about efficient differences. On the other hand, no hesitation has been felt in employing instruments which are not absolutely accurate, when it was clear that the error was less than the efficient difference. Similarly, the margin of error practically eliminates itself in the case of simultaneous comparative readings, when the instruments have been checked to the same standard. Simplicity of construction and operation are of great importance, especially in saving time where a large number of instruments are in operation. Expense is likewise to be carefully considered. It is impossible to have too many instruments, but cost practically determines the number that can be obtained. It is further necessary to secure or invent both simple and automatic instruments for all factors, except such invariable ones as altitude, slope, etc. Simple instruments must be of a kind that can be easily carried, and so constructed that they can be used at a minimum of risk. The sling psychrometer, for example, is very readily broken in field use, and it has been replaced by a protected modification, the rotating form.

In describing the construction and operation of the many factor instruments, there has been no attempt to make the treatment exhaustive. Those instruments which the author has found of greatest value in his own work are given precedence, and the manner of using them is described in detail. Other instruments of value are also considered, though with greater brevity. Some of the most complex and expensive ones have been ignored, as it is altogether improbable that they can come into general use in their present form. While the conviction is felt that the methods described below will enable the most advanced investigators to carry on thorough work, it is hoped that they will be seen to be so fundamental, and so attractive, that they will appeal to all who are planning serious ecological study.

WATER-CONTENT

40. Value of different instruments. The paramount importance of water-content as a direct factor in the modification of plant form and distribution gives a fundamental value to the methods used for its determination. Automatic instruments for ascertaining the water in the soil are costly, in addition to being complicated, and often inaccurate. For these reasons, much attention has been given to developing the simpler but more reliable methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is always ready for instant use. In the determination of physiological water-content, it is practically indispensable. Indeed, the readiness with which geotome determinations of water-content can be made should hasten the universal recognition of the fact that it is the available, and not the total amount of water in the soil, which determines the effect upon the plant.

Geotome Methods

Fig. 1. Geotomes and soil can.

41. The geotome. In its simplest form, the geotome is merely a stout iron tube with a sharp cutting edge at one end and a firmly attached handle at the other. The length is variable and is primarily determined by the location of the active root surface of the plant. In xerophytic habitats, generally a longer tube is necessary than in mesophytic ones. The bore is largely determined by the character of the soil; for example, a larger one is necessary for gravel than for loam. Tubes of small bore also tend to pack the soil below them, and to give a correspondingly incomplete core. The best results have been obtained with geotomes of ½–1 inch tube. Each geotome has a removable rod, flattened into a disk at one end, and bent at the other, for forcing out the core after it has been cut from the soil. Sets of geotomes have been made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12– and 15–inch forms have been commonly used for herbaceous formations and layers. They are marked in inches so that a sample of any lesser depth may be readily taken. Such a device is very necessary for gravel soils and in mountain regions, where the subsoil of rock lies close to the surface.

Fig. 2. Fraenkel soil borer.

Fig. 3. American soil borer.

42. Soil borers. There is a large variety of soil borers to choose from, but none have been found as simple and satisfactory for relatively shallow readings as the geotome just described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the auger type are necessary. These are large and heavy, and of necessity slow in operation. They can not well be carried in an ordinary outfit of instruments, and the size of the soil sample itself precludes the use of such instruments far from the base station, except on trips made expressly for obtaining samples from deep-seated layers. For depths from two to eight feet, the Fraenkel borer is perhaps the most satisfactory, except for the coarser gravels: it costs $14 or $20 according to the length. For greater depths, or when a larger core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be made use of. This is a ponderous affair and can be employed only on special occasions. On account of the size of samples obtained by these borers, it is usually most satisfactory to take a small sample from the core at different depths. Frequently, indeed, a hand trowel may be readily used to obtain a good sample at a particular depth.

43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring movement, in order to avoid packing the soil. This difficulty is further obviated by deep notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-inch core, there is also less compression if it be cut five inches at a time. Repeated tests have shown, however, that the single compressed sample is practically as trustworthy as the one made in sections. The water-content of the former constantly fell within .5 per cent of that of the latter, and both varied less than 1 per cent from the dug sample used as a check. As soon as dug, the core is pressed out of the geotome by the plunger directly into an air-tight soil can. Bottles may be used as containers, but tin cans are lighter and more durable. Aluminum cans have been devised for this purpose, but on account of the expense, “Antikamnia” cans have been used instead. These are tested, and those that are not water-tight are rejected, although it has been found that, even in these, ordinary soils do not lose an appreciable amount of water in twenty-four hours. The lid should be screwed on as quickly as possible, and, as an added precaution, the cans are kept in a close case until they have been weighed. The cans are numbered consecutively on both lid and side in such a way that the number may be read at a glance. The numbers are painted, as a label wears off too rapidly, and scratched numbers are not quickly discerned.

Fig. 4. Field balance.

44. Weighing. Although soil samples have been kept in tight cans outside of cases for several days without losing a milligram of moisture, the safest plan is to make it a rule to weigh cans as quickly as possible after bringing them in from the field. Moreover, when delicate balances are available, it is a good practice to weigh to the milligram. At remote bases, however, and particularly in the field, and on reconnaissance, where delicate, expensive instruments are out of place, coarser balances, which weigh accurately to one centigram, give satisfactory results. The study of efficient water-content values has already gone far enough to indicate that differences less than 1 per cent are negligible. Indeed, the soil variation in a single square meter is often as great as this. The greatest difference possible in the third place, i. e., that of 9 milligrams, does not produce a difference of .1 of 1 per cent in the water-content value. In consequence, such strong portable balances as Bausch & Lomb 12308 ($2), which can be carried anywhere, give entirely reliable results. The best procedure is to weigh the soil with the can. Turning the soil out upon the pan or upon paper obviates one weighing, but there is always some slight loss, and the chances of serious mishap are many. After weighing, the sample is dried as rapidly as possible in a water bath or oven. At a temperature of 100° C. this is accomplished ordinarily in twenty-four hours; the most tenacious clays require a longer time, or a higher temperature. High temperatures should be avoided, however, for soils that contain much leaf mould or other organic matter, in order that this may not be destroyed. When it is necessary on trips, soil samples can be dried in the sun or even in the air. This usually takes several days, however, and a test weighing is generally required before one can be certain that the moisture is entirely gone. The weighing of the dried soil is made as before, and the can is carefully brushed out and weighed. The weight of aluminum cans may be determined once for all, but with painted cans it has been the practice to weigh them each time.

45. Computation. The most direct method of expressing the water-content is by per cents figured upon the moist soil as a basis. The ideal way would be to determine the actual amount of water per unit volume, but as this would necessitate weighing one unit volume at least in every habitat studied, as a preliminary step, it is not practicable. The actual process of computation is extremely simple. The weight of the dried sample, w¹, is subtracted from the weight of the original sample, w, and the weight of the can, w², is likewise subtracted from w. The first result is then divided by the second, giving the per cent of water, or the physical water-content. The formula is: (w − w¹)/(w − w²) = W. The result is expressed preferably in grams per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per cent of water-content may readily be figured on the basis of dry or moist soil.

46. Time and location of readings. Owing to the daily change in the amount of soil water due to evaporation, gravity, and rainfall, an isolated determination of water-content has very little value. It is a primary requisite that a basis for comparison be established by making (1) a series of readings in the same place, (2) a series at practically the same time in a number of different places or habitats, or (3) by combining the two methods, and following the daily changes of a series of stations throughout an entire season, or at least for a period sufficient to determine the approximate maximum and minimum. The last procedure can hardly be carried out except at a base station, but here it is practically indispensable. It has been followed both at Lincoln and at Minnehaha, resulting in a basal series for each place that is of the greatest importance. When such a base already exists, or, better, while it is being established, scattered readings may be used somewhat profitably. As a practical working rule, however, it is most convenient and satisfactory to make all determinations consecutively, i. e., in a series of stations or of successive days. Under ordinary conditions, the time of day at which a particular sample is taken is of little importance, as the variation during a day is usually slight. This does not hold for exposed wet soils, and especially for soils which have just been wetted by rains. In all comparative series, however, the samples should be taken at the same hour whenever possible. This is particularly necessary when it is desired to ascertain the daily decrease of water-content in the same spot. In the case of a series of stations, these should be read always in the same order, at the same time of day, and as rapidly as possible. When a daily station series is being run, i. e., a series by days and stations both, the daily reading for each place should fall at the same time. While there are certain advantages in making readings either early or late in the day, they may be made at any time if the above precautions are followed.

47. Location of readings. Samples should invariably be taken in spots which are both typical and normal, especially when they are to be used as representative of a particular area or habitat. A slight change in slope, soil composition, in the amount of dead or living cover, etc., will produce considerable change in the amount of water present. Where habitat and formation are uniform, fewer precautions are necessary. This is a rare circumstance, and as a rule determinations must be made wherever appreciable differences are in evidence. The problem is simpler when readings are taken with reference to the structure or modifications of a particular species, but even here, check readings in several places are of great value. The variation of water in a spot apparently uniform has been found to be slight in the prairies and the mountains. In taking three samples in spots a few inches to several feet apart, the difference in the amount of water has rarely exceeded 1 per cent, which is practically negligible. Gardner[^[2]] found that 16 samples taken to a depth of 3 inches, in as many different portions of a carefully prepared, denuded soil plot, showed a variation of 7½ per cent. This is partially explained by the shallowness of the samples, but even then the results of the two investigations are in serious conflict and indicate that the question needs especial study. It should be further pointed out that all readings should be made well within a particular area, and not near its edge, and that, in the case of large diversified habitats, it is the consocies and the society which indicate the obvious variations in the structure of the habitat.

48. Depth of samples. The general rule is that the depth of soil samples is determined by the layer to which the roots penetrate. The practice is to remove the air-dried surface in which no roots are found, and to take a sample to the proper depth. This method is open to some objection, as the actively absorbing root surfaces are often localized. There is no practical way of taking account of this as yet, except in the case of deep-rooted xerophytes and woody plants. It is practicable to determine the location of the active root area of a particular plant and hence the water-content of the soil layer, but in most formations, roots penetrate to such different depths that a sample which includes the greater part of the distance concerned is satisfactory. Some knowledge of the soil of a formation is also necessary, since shallow soils do not require as deep samples as others. The same is true of shaded soils without reference to their depth, and, in large measure, of soils supplied with telluric water. In all cases, it is highly desirable to have numerous control-samples at different depths. The normal cores are 12 or 15 inches; control-samples are taken every 5 inches to the depth desired, and in some cases 3–inch sections are made. It has been found a great saving of time to combine these methods. A 5–inch sample is taken and placed in one can, then a second one, and a third in like manner. In this way the water-content of each 5–inch layer is determined, and from the combined weight the total content is readily ascertained.

49. Check and control instruments. A number of instruments throw much light upon the general relations of soil water. The rain-gauge, or ombrometer, measures the periodical replenishment of the water supply, and has a direct bearing upon seasonal variation. The atmometer affords a clue to the daily decrease of water by evaporation, and thus supplements the rain-gauge. The run-off gauge enables one to establish a direct connection between water-content and the slope and character of the surface. The amount and rapidity of absorption are determined by means of a simple instrument termed a rhoptometer. The gravitation water of a soil is ascertained by a hizometer, and some clue to the hygroscopic and capillary water may be obtained by an artificial osmotic cell. All of these are of importance because they serve to explain the water-content of a particular soil with especial reference to the other factors of the habitat. It is evident that none of them can actually be used in exact determinations of the amount of water, and they will be considered under the factors with which, they are more immediately concerned.

Physical and Physiological Water

50. The availability of soil water. The amount of water present in a soil is no real index to the influence of water-content as a factor of the habitat. All soils contain more water than can be absorbed by the plants which grow in them. This residual water, which is not available for use, varies for different soils. It is greatest in the compact soils, such as clay and loam, and least in the loose ones, as sand and gravel. It differs, but to a much less degree, from one species to another. A plant of xerophytic tendency is naturally able to remove more water from the same soil than one of mesophytic or hydrophytic character. As the species of a particular formation owe their association chiefly to their common relation to the water-content of the habitat, this difference is of little importance in the field. In comparing the structure of formations, and especially that of the plants which are found in them, the need to distinguish the available water from the total amount is imperative. Thus, water-contents of 15 per cent in gravel and in clay are in no wise comparable. A coarse gravel containing 15 per cent of water is practically saturated. The plants which grow upon it are mesophytes of a strong hydrophytic tendency, and they are able to use 14½ or all but .5 out of the 15 per cent of water. In a compact clay, only 3½ of the 15 per cent are available, and the plants growing in it are marked xerophytes. It is evident that a knowledge merely of the physical water-content is actually misleading in such cases, and this holds true of comparisons of any soils which differ considerably in texture. After one has determined the physiological water for the great groups of soils, it is more or less possible to estimate the amounts in the various types of each. As an analysis is necessary to show how close soils are in texture, this is little better than a guess, and for accurate work it is indispensable that the available water be determined for each habitat. Within the same formation, however, after this has once been carefully ascertained, it is perfectly satisfactory to convert physical water-content into available by subtracting the non-available water, which under normal conditions in the field remains practically the same.

The importance of knowing the available water is even greater in those habitats in which salts, acids, cold, or other factors than the molecular attraction of soil particles increase the amount of water which the plant can not absorb. Few careful investigations of such soils have yet been made, and the relation of available to non-available water in them is almost entirely unknown. It is probable that the phenomena in some of these will be found to be produced by other factors.

51. Terms. The terms, physiological water-content, and physical water-content, are awkward and not altogether clear in their application. It is here proposed to replace them by short words which will refer directly to the availability of the soil water for absorption by the plant. Accordingly, the total amount of water in the soil is divided into the available and the non-available water-content. The terms suggested for these are respectively, holard (ὅλos, whole, ἅpδov, water), chresard (χοῆςις, use), and echard (ἕχω, to withhold).

52. Chresard determinations under control. The determination of the chresard in the field is attended with peculiar difficulties. In consequence, the method of obtaining it under control will first be described. The inquiry may be made with reference to soils in general or to the soil of a particular formation. In the last case, if the plants used are from the same formation, the results will have almost the value of a field determination. When no definite habitat is the subject of investigation, an actual soil, and not an artificial mixture, should be used, and the plants employed should be mesophytes. The individual plants are grown from seeds in the proper soils, and are repotted sufficiently often to keep the roots away from the surface. The last transfer is made to a pot large enough to permit the plant to become full-grown without crowding the roots. The pot should be glazed inside and out in order to prevent the escape of moisture. This interferes slightly with the aeration of the soil, but it will not cause any real difficulty. The plant is watered in such a way as to make the growth as normal as possible. After it has become well established, three soil samples are taken in such a manner that they will give the variation in different parts of the pot. One is taken near the plant, the second midway between the plant and the edge of the pot, and the third near the edge. The depth is determined by the size of the pot and the position of the roots. The holard is determined for these in the usual way, but the result is expressed with reference to 100 grams of dry soil; the average is taken as representative. The soil is then allowed to dry out slowly, as sudden drouth will sometimes impair the power of absorption and a plant will wilt although considerable available water remains. Plants often wilt in the field daily for several successive hot dry days, and become completely turgid again during the night. If the drying out takes place slowly, the plant will not recover after it has once begun to wilt. The proper time to make the second reading is indicated by the pronounced wilting of the leaves and shoots. Complete wilting occurs, as a rule, only after the younger parts have drawn for some time upon the watery tissues of the stem and root, by which time evaporation has considerably deceased the water in the soil. It is a well-known fact that young leaves do not wilt easily, especially in watery or succulent plants. Three samples are again taken and the average water-content determined as above. This is the non-available water or the echard. The latter is then computed on the basis of 100 grams of dry soil, and this result is subtracted from the holard to give the chresard in grams for each 100 grams of dry weight. The chresard may also be expressed with respect to 100 grams of moist soil. As a final precaution in basal work, it is advisable to determine the chresard for six individuals of the same species under as nearly the same conditions as possible. When it is desired, however, to find the average chresard for a particular soil, it is necessary to employ various species representing diverse phyads and ecads. Such an investigation is necessarily very complicated, and must be made the subject of special inquiry.

53. Chresard readings in the field. The especial difficulties which must be overcome in the field are the exclusion of rain and dew and the cutting off of the capillary water. It is evident, of course, that experiments of this sort must also be entirely free from outside disturbance. The choice of an area depends upon the scope of the study. If the chresard is sought for a particular consocies, the block of soil to be studied should show several species which are fairly representative. In case the chresard of a certain species is to be obtained, this species alone need be present, but it should be represented by several individuals. Check plots are desirable in either event, and at least two or three which are as nearly uniform as possible should be chosen. The size and depth of the soil block depends upon the plants concerned. It must be large enough that the roots of the particular individuals under investigation are not disturbed. There is a limit to the size of the mass that can be handled readily, and in consequence the test plants must not be too large or too deeply rooted. The task of cutting out the soil block requires a spade with a long sharp blade. After ascertaining the spread and depth of the roots, the block is cut so that a margin of several inches free from the roots concerned is left on the sides and bottom. If the block is to be lifted out of place, so that the sides are exposed to evaporation, this allowance should be greater. In some cases, it may be found more convenient to dig the plant up, place it in a large pot, and put the latter back in the hole. As a general practice, however, this is much less satisfactory.

After the block has been cut, it may be moved if the soil is sufficiently compact, and then allowed to dry out in its own formation or elsewhere. The results are most valuable in the first case, though it is often an advantage to remove blocks cut from shade or wet formations to dry, sunny stations where they will dry more rapidly. The most satisfactory and natural method, however, is to leave the block in place, and to prevent the reestablishment of capillary action by enclosing it within plates. This is accomplished by slipping thin sheet-iron plates into position along the cut surfaces. The plate for the bottom should be somewhat wider than the block, and is slipped into place by raising the block if the soil is not too loose; in the latter event, it is carefully driven in. The side plates are then pushed down to meet the former. The size of the plates depends upon the block; in general, plates of 1, 2, and 3 feet square, with the bottom plates a trifle larger, are the most serviceable. Access of rain and dew is prevented by an awning of heavy canvas which projects far enough beyond each side of the block to prevent wetting. The height will depend of course upon the size of the plants. The awning must be used only when rain or heavy dew is threatened, as the shade which it produces changes the power of the plant to draw water from the soil.

The time necessary to cause wilting varies with the habitat and the weather. When the block is large and in position, two or three weeks are required. This period of drying incidentally furnishes an excellent opportunity for determining the rate at which the particular soil loses water. The holard sample is taken daily for several days before the block is cut out, in order to obtain an average, care being taken of course to avoid a period of extreme weather. The echard samples are taken as soon as the wilting is sufficient to indicate that the limit of available water is reached. The air-dry soil above the roots is first removed. The treatment of the samples and the computation of the chresard are as previously indicated.

54. Chresard values of different soils. The following table gives the water-content values of six representative soils. The per cents of holard (at saturation) and of echard are those determined by Hedgcock[^[3]] with six mesophytes as test plants for each soil. The chresard has been computed directly from these.

The first column indicates the per cent based upon the dry weight, the second upon the weight of the moist soil.

While these can not be considered absolute for a particular soil other than the ones investigated, they are found to correspond somewhat closely to the results obtained for other soils of the respective groups. For accurate research, the chresard must of course be ascertained for each formation with respect to its peculiar plants and soil. The influence of the ecad in more or less determining the echard is also shown by Hedgcock, who found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat higher for shade plants than for heliophytes.

Records and Results

55. The field record. It is superfluous to point out that a definite form for field records saves much time and prevents many mistakes. The exact form may be left to personal taste, but there are certain features which are essential. Many of these are evident, while others may seem unnecessary; all, however, have been proved by experience to have some value in saving time or in preventing confusion. The two fundamental maxims of field work are that nothing is too trivial to be of importance, and that no detail should be entrusted to the memory. The field record should contain in unmistakable terms all that the field has yielded. These statements apply with especial force to water-content, in many senses the most important of physical factors. The precise character of the record depends upon the way in which the readings are made, whether scattered or in series. As the day-station series is of the greatest importance, the record is adapted for it especially, but it will also serve for all readings. The record is chronological, since this is the only convenient method for the field. A proper form for a field record of water-content is the following:

A general designation of the soil composition is a material aid, especially where there is a difference in the core. For example, in a mountain forest or meadow, the upper layer will usually be mold, the lower sand or gravel. A careful estimate of the relation between the two throws much light upon the chresard. Under “sample” the number taken to reach the desired depth, if more than one, is indicated by placing the number before the depth, thus 2:10. When two or more full cores are included in the same sample for a check, the order is 10:2. It has already been shown, however, that these precautions are not necessary for ordinary purposes. In computing the holard and echard, there is no need to show the figuring, if the process is checked and then proved. Notes upon sky conditions aid in explaining the daily decrease in water-content. The amount of rain and the period during which it falls are of great importance in understanding the fluctuations of the holard. Under community it is highly desirable to have a list of all the species, but it is impossible to include this in the table, and a glance at the formation list will show them. The form indicated above serves for a day-station series, a daily series in one station for any number of check series in one spot, and for scattered readings. In many cases the echard will not be determined, but on account of its primary importance, there should be a space for it, especially since it may be desirable to determine it at some later time.

56. The permanent record. This should be kept by formation, or if the latter exhibits well-defined associations, the formational record may be divided accordingly. This may seem an unnecessary expenditure of time, but a slight experience in finding the water-content values of a particular habitat, when scattered through a chronological field record, will be convincing. The form of permanent record is the same as for the field, except that the column for the formation and that for the society are often unnecessary.

57. Sums and means. From the great difficulty of determining the absolute water-content, and of obtaining a standard of comparison between soils on account of the varying ratio between bulk and weight, water-content sums are impracticable. For the same reasons, means of actual water-content are practically impossible, and the mean water-content must be expressed in per cents. Daily readings are not essential to a satisfactory mean. In fact, a single reading at each extreme enables one to approximate the real mean very closely; thus, the average of 26 readings in the prairie formation is 18 per cent. The extremes are 5 per cent and 28 per cent, and their average 16.5 per cent. A few readings properly scattered through moist and dry periods will give a reliable mean, as will also a series of daily readings from one heavy rain through a long dry period. The one difficulty with the last method is that such periods can not well be determined beforehand. Means permit ready comparison between habitats, but in connecting the modifications of a species with water-content as a cause, the extremes are significant as indicating the range of conditions. Furthermore, the extremes, i. e., 5 per cent and 28 per cent, make it possible to approximate the mean, 18 per cent, while the latter gives little or no clue to the extremes. It is hardly necessary to state that means and extremes should be determined for a certain habitat, or particular area of it, and that the results may be expressed with reference to holard and chresard.

58. Curves. The value of graphic methods and the details of plotting curves are reserved for a particular section. It will suffice in this place to indicate the water-content curves that are of especial value. Simple curves are made with regard to time, place, or depth. The day curve shows the fluctuations of the water-content of one station from day to day or from time to time. The station curve indicates the variation in water from station to station, while the depth curve represents the different values at various depths in the same station. These may be combined on the same sheet in such a way that the station curves of each day may be compared directly. Similar combinations may be used for comparing the day curves, or the depth curves of different stations, but these are of less importance. A combination of curves which is of the greatest value is one which admits of direct comparison between the station curves of saturation, holard, chresard, and echard.

HUMIDITY

59. Instruments. As a direct factor, humidity is intimately connected with water-content in determining the structure and distribution of plants. The one is in control of water loss; the other regulates water supply. Humidity as a climatic factor undergoes greater fluctuation in the same habitat, and the efficient difference is correspondingly greater. Accordingly, simple instruments are less valuable than automatic ones, since a continuous record is essential to a proper understanding of the real influence of humidity. As is the rule, however, the use of simple instruments, when they can be referred to an ecographic basis, greatly extends the field which can be studied. In investigation, both psychrometer and psychrograph have their proper place. In the consideration of simple instruments for obtaining humidity values, an arbitrary distinction is made between psychrometers and hygrometers. The former consist of a wet and a dry bulb thermometer, while the latter make use of a hygroscopic awn, hair, or other object.

Psychrometers

60. Kinds. There are three kinds of psychrometer, the sling, the cog, and the stationary. All consist of a wet bulb and a dry bulb thermometer set in a case; the first two are designed to be moved or whirled in the air. The same principle is applied in each, viz., that evaporation produces a decrease in temperature proportional to the amount of moisture in the air. The dry bulb thermometer is an ordinary thermometer, while the wet bulb is covered with a cloth that can be moistened. The former indicates the normal temperature of the air, the latter gives the reduced temperature due to evaporation. The relative humidity of the air is ascertained by means of the proper tables, from two terms, i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling and the cog psychrometers alone are in general use. The stationary form has been found to be unreliable, because the moisture, as it evaporates from the wet bulb, is not removed, and, in consequence, hinders evaporation to the proper degree.

Fig. 5. Sling psychrometer.

61. The sling psychrometer. The standard form of this is shown in the illustration, and is the one used by the Weather Bureau. This instrument can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of a metal frame to which are firmly attached two accurately standardized thermometers, reading usually from –30° to 130°. The frame is attached at the uppermost end to a handle in such fashion that it swings freely. The wet bulb thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth for the wet bulb should be always of the same texture and quality; the standard used by the Weather Bureau can be obtained from the instrument makers. A slight difference in texture makes no appreciable error, but the results obtained with different instruments and by different observers will be more trustworthy and comparable if the same cloth be used in all cases. The jacket for the wet bulb may be sewed in the form of a close-fitting bag, which soon shrinks and clings tightly. It may be made in the field by wrapping the cloth so that the edges just overlap, and tying it tightly above and below the bulb. In either case, a single layer of cloth alone must be used. The cloth becomes soiled or thin after a few months’ constant use and should be replaced. It is a wise precaution to carry a small piece of psychrometer cloth in the field outfit.

Fig. 6. Cog psychrometer.

62. Readings. All observations should be made facing the wind, and the observer should move one or two steps during the reading to prevent the possibility of error. The cloth of the wet bulb is moistened with water by means of a brush, or, much better, it is dipped directly into a bottle of water. Distilled water is preferable, as it contains no dissolved material to accumulate in the cloth. Tap-water and the water of streams may be used without appreciable error, if the cloth is changed somewhat more frequently. The temperature of the water is practically negligible under ordinary conditions. Readings can be made more quickly, however, when the temperature is not too far from that of the air. The psychrometer is held firmly and swung rapidly through the air when the space is not too confined. Where there is danger of breakage, it is swung back and forth through a short arc, pendulum-fashion. As the reading must be made when the mercury of the wet bulb reaches the lowest point, the instrument is stopped from time to time and the position of the column noted. The lowest point is often indicated by the tendency of the mercury to remain stationary; as a rule it can be noted with certainty when the next glance shows a rise in the column. In following the movement, and especially in noting the final reading, great care must be taken to make the latter before the mercury begins to rise. For this reason it is desirable to shade the psychrometer with the body when looking at it, and to take pains not to breathe upon the bulbs nor to bring them too near the body. At the moment when the wet bulb registers the lowest point, the dry bulb should be read and the results recorded.

63. Cog psychrometer. This instrument, commonly called the “egg-beater” psychrometer, has been devised to obviate certain disadvantages of the sling psychrometer in field work, and has entirely supplanted the latter in the writer’s own studies. It is smaller, more compact, and the danger of breaking in carriage or in use is almost nil. It has the great advantage of making it possible to take readings in a layer of air less than two inches in thickness, and in any position. Fairly accurate results can even be obtained from transpiring leaves. The instrument can readily be made by a good mechanic, at a cost for materials of $1.75, which is less than half the price for the sling form. A single drawback exists in the use of short, Centigrade thermometers, inasmuch as tables of relative humidity are usually expressed in Fahrenheit. It is a simple matter, however, to convert Centigrade degrees into Fahrenheit, mentally, or the difficulty may be avoided by the conversion table shown on page [47], or by constructing a Centigrade series of humidity tables. The fact that the wet and dry bulbs revolve in the same path has raised a doubt concerning the accuracy of the results obtained with this instrument. Repeated comparisons with the sling psychrometer have not only removed this doubt completely, but have also proved that the standardization of the thermometers has been efficient.

64. Construction and use. A convenient form of egg-beater is the Lyon (Albany, New York), in which the revolving plates can be readily removed, leaving the axis and the frame. The thermometers used are of the short Centigrade type. They are 4½ inches long and read from –5° to 50°. Eimer and Amend, 205 Third Ave., New York city, furnish them at 75 cents each. The thermometers are carefully standardized and compared, and then grouped in pairs that read together. Each pair is used to construct a particular psychrometer. Each thermometer is strongly wired to one side of the frame, pieces of felt being used to protect the tube and increase the contact. The frame is also bent at the base angles to permit free circulation of air about the thermometer bulbs. The bulb of one thermometer is covered with the proper cloth, and the psychrometer is finished. Since the frame revolves with the thermometers, it is necessary to pour the water on the wet bulb, or to employ a pipette or brush. The thermometer bulbs are placed in the layer to be studied, and the frame rotated at an even rate and with moderate rapidity. The observation is further made as in the case of the sling psychrometer. As the circle of rotation is less than three inches in diameter, and the layer less than an inch, in place of nearly three feet for the sling form, the instrument should not be moved at all for extremely localized readings, but it must be moved considerably, a foot or more, if it is desirable to obtain a more general reading.

65. Hygrometers. While there are instruments designed to indicate the humidity by means of a hygroscopic substance, not one of them seems to be of sufficient accuracy for use in ecological study. The difficulty is that the hygroscopic reaction is inconstant, rather than that the instruments are not sufficiently sensitive. A number of hygrometers have been tested, and in all the error has been found to be great, varying usually from 10–20 per cent. In the middle of the scale they sometimes read more accurately, but toward either extreme they are very inexact. It seems probable that an accurate hygrometer can be constructed only after the model of the Draper psychrograph. Its weight and bulk would make it an impossible instrument for field trips, and the expense of one would provide a dozen psychrometers. In consequence, it does not seem too sweeping to say that no hygrometer can furnish trustworthy results. Of simple instruments for humidity, the psychrometer alone can be trusted to give reliable readings. Crova’s hygrometer, used by Hesselmann, is not a hygrometer in the sense indicated. As it is much less convenient to handle and to operate than the cog psychrometer, it is not necessary to describe it.

Psychrographs

Fig. 7. Draper psychrograph.

66. The Draper psychrograph. A year’s trial of the Draper psychrograph in field and planthouse has left little question of its accuracy and its great usefulness. Essentially, it consists of a band of fine catgut strings, which are sensitive to changes in the moisture-content of the air. The variations in the length of the band are communicated to a long pointer carrying an inking pen. The latter traces the record in per cent of relative humidity on a graduated paper disk, which is practically the face of an eight-day clock. The whole is enclosed in a metal case with a glass front. A glance at the illustration will show the general structure of the instrument. Continued psychrometric tests demonstrate that the margin of error is well within the efficient difference for humidity, which is taken to be 5 per cent. In the field tests of the past summer, two psychrographs placed side by side in the same habitat did not vary 1 per cent from each other. The same instruments when in different habitats did not deviate more than 1 per cent from the psychrometric values, except when the air approached saturation. For humidities above 90 per cent, the deviation is considerable, but as these are temporary and incident upon rainfall, the error is not serious. For humidities varying from 10–85 per cent, the psychrograph is practically as accurate as the psychrometer. Per cents below 10 are rare, and no tests have been made for them.

Fig. 8. Instrument shelter, showing thermograph and psychrograph in position.

67. Placing the instrument. The psychrograph should be located in a place where the circulation of the air is typical of the station observed. A satisfactory shelter will screen the instrument from sun and rain, and at the same time permit the air to pass freely through the perforations of the metal case. The form shown in figure 8 meets both of these conditions. A desirable modification is effected by fastening a strip about the cover of such depth as to prevent the sun’s rays from striking the case except when the sun is near the horizon. A cross block is fastened on the post of the shelter after being exactly leveled. The psychrograph rests upon this block, which is three feet above the ground in order to avoid the influence of radiation. The instrument is held in position by slipping the eye over a small-headed nail driven obliquely. It does not hang from the latter, but must rest firmly upon the cross block. The post is set to a depth that prevents oscillation in the wind, which is liable to obscure the record. In shallow mountain soils stability is attained by fastening a broad board at the base of the post before setting it. When two or more psychrographs are established in different habitats, great pains are taken to set them up in exactly the same way. The shelters are alike, the height above the soil the same, and the instruments all face the south.

68. Regulating and operating the instrument. When two or more psychrographs are to be used in series, they must be compared with each other in the same spot for several days until they run exactly together with respect to per cent of humidity and to time. During this comparison they are checked by the psychrometer and so regulated that they register the proper humidity. When a single instrument is used alone as the basis to which simple readings may be referred, all regulating may well be done after the instrument is in position. This is a simple process; it is accomplished by obtaining the relative humidity beneath the shelter and at the proper height by a psychrometer. The pen hand is then moved to the proper line on the disk by means of the screws at its base. These are reached by removing the lettered glass face. The thumbscrew on the side opposite the direction in which the pen is to move is released, and the opposite screw simultaneously tightened, until the pen remains upon the proper line. Experience has proved that the record sheet should be correctly labeled and dated before being placed on the disk. In the press of field duties, records labeled after removal are liable to be confused. It is likewise a great saving of time to write the date of the month in the margin of each segment. Care is taken to place the sheet on the disk in the same position each time; this can easily be done by seeing that the sharp point on the disk penetrates the same spot on the paper. A single drop of ink in the pen will usually give the most satisfactory line. A thin line is read most accurately. If the pen point is too fine, however, the ink does not flow readily, and the point should be slightly blunted by means of a file. More often the line is too broad and the pen must be carefully pointed. Occasionally the pen does not touch the sheet, and it becomes necessary to bend the hand slightly. This is a frequent difficulty if the records are folded or wrinkled, and consequently the sheets should always be kept flat.

69. The weekly visit. Psychrographs must be visited, checked, rewound, and inked every week. Whenever possible this should be done regularly at a specified day and hour. This is especially desirable if the same record sheet is used for more than one week. Time and energy are saved by a fixed order for the various tasks to be done at each visit. After opening the instrument the disk is removed, and the clock wound, and, if need be, regulated. The record sheet is replaced, the disk again put on the clock arbor, and the pen replenished with a drop of ink. A psychrometer reading is made, and the results in terms of relative humidity noted at the proper place on the disk sheet. If the psychrograph vary more than 1 per cent, it is adjusted to read accurately. In practice it has been found a great convenience to keep each record sheet in position for three weeks, and the time may easily be extended to four. In this event, the pen is carefully cleaned with blotting paper at each visit, and is then refilled with an ink of different color. To prevent confusion, the three different colored inks are always used in the same order, red for the first week, blue for the second, and green for the third. The advantages of this plan are obvious: fewer records are used and less time is spent in changing them. The records of several weeks are side by side instead of on separate sheets, and in working over the season’s results, it is necessary to handle but a third as many sheets.

The Draper psychrograph is made by the Draper Manufacturing Company, 152 Front St., New York city. The price is $30. A few record sheets and a bottle of red ink are furnished with it. Additional records can be obtained at 3 cents each. The inks are 25–50 cents per bottle, depending upon the color.

Humidity Readings and Records

70. The time of readings. If simple instruments alone are used for determining humidity, readings are practically without value unless made simultaneously through several stations, or successively at one. When it is possible to combine these, and to make psychrometer readings at different habitats for each hour of the day, or at the same hour for several days, the series is of very great value. Single readings are unreliable on account of the hourly and daily variations of humidity, but when these changes are recorded by a psychrograph, such readings at once become of use, whether made in the same habitat with the recording instrument or elsewhere. In the latter case, one reading will tell little about the normal humidity of the habitat, but several make a close estimate possible. When a series of psychrographs is in use, accurate observations can be made to advantage anywhere at any time. As a rule, however, it has been found most convenient to make simple readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford much evidence in regard to the daily range. A good time also is that at which the temperature maximum occurs each day, but this is movable and in the press of field work can rarely be taken advantage of. A very fair idea of the daily mean humidity is obtainable by averaging the readings made at the hours already indicated. The comparison of single readings with the psychrograph record should not be made at a time when a rapid change is occurring, as the automatic instrument does not respond immediately. Such a condition is usually represented by a sudden rain, and is naturally not a satisfactory time for single readings in any event.

Fig. 9. Atmometer.

71. Place and height. As stated above, the psychrograph is placed three feet above the surface of the ground in making readings for the comparison of stations. In low, herbaceous formations, the instrument is usually placed within a few inches of the soil in order to record the humidity of the air in which the plants are growing. In forest formations, the moisture often varies considerably in the different layers. This variation is easily determined by simultaneous psychrometer readings in the several layers, or, if occasion warrants, a series of psychrographs may be used. In field work the rule has been to make observations with the psychrometer at 6 feet, 3 feet, and the surface of the soil, but the reading at the height of 3 feet is ordinarily sufficient. Humidity varies so easily that several readings in different parts of one formation are often desirable. In comparing different formations, the readings should be made in corresponding situations, for example, in the densest portion of each.

72. Check instruments. Humidity is so readily affected by temperature, wind, and pressure, that a knowledge of these factors is essential to an understanding of its fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for ascertaining relative humidity, and is determined once for all when the altitude of a station has been carefully established. The temperature is obtained directly from the dry bulb reading. Its value is fundamental, as the amount of moisture in a given space is directly affected by it; like pressure, it also is taken account of in the formula. The movement of the air has an immediate influence upon moisture by mixing the air of different habitats and layers. So far as the plant is concerned, it has practically the effect of increasing or decreasing the humidity by the removal of the air above it. Thus, while the anemometer can furnish no direct evidence as to the amount of variation, it is of aid in explaining the reason for it. Likewise, the rate of evaporation as indicated by a series of atmometers, affords a ready method of estimating the comparative effect of humidity in different habitats. Potometers and other instruments for measuring transpiration throw much light upon humidity values. Since they are concerned with the response of the plant to humidity, they are considered in the following chapter.

73. Humidity tables. To ascertain the relative humidity, the difference between the wet and dry bulb readings is obtained. This, with the dry bulb temperature, is referred to the tables, where the corresponding humidity is found. A variation in temperature has less effect than a variation in the difference; in consequence, the dry bulb reading is expressed in the nearest unit, and the difference reckoned to the nearest .5. The humidity varies with the air pressure. Hence, the altitude must be determined for the base station, and for all others that show much change in elevation. Within the ordinary range of growing-period temperatures, the effect of pressure is not great. For all ordinary cases, it suffices to compute tables for pressures of 30, 29, 27, 25, and 23 inches. The following table indicates the decrease in pressure which is due to altitude.

The fluctuations of pressure due to weather are usually so slight that their influence may be disregarded. An excellent series of tables of relative humidity is found in Marvin’s Psychrometric Tables, published by the U. S. Weather Bureau, and to be obtained from the Division of Publications, Washington, D. C., for 10 cents. A convenient field form is made by removing the portion containing the tables of relative humidity, and binding it in stiff oilcloth.

Fig. 10. Conversion scale for temperatures.

74. Sums, means, and curves. An approximate humidity sum can be obtained by adding the absolute humidities for each of the twenty-four hours, and expressing the results in grains per cubic foot. It is possible to establish a general ratio between this sum and the transpiration sum of the plant, but its value is not great at present. Means of absolute and of relative humidity are readily determinable from the psychrograph records; the latter are the most useful. The mean of relative humidity for the twenty-four hours of a day is the average of the twenty-four hour humidities. From these means the seasonal mean is computed in the same manner. A close approximation, usually within 1 degree, may be obtained in either case by averaging the maximum and minimum for the period concerned. Various kinds of curves are of value in representing variation in humidity. Obviously, these must be derived from the psychrograph, or from the psychrometer when the series is sufficiently complete. The level curve indicates the variation in different stations at the same time. These may be combined in a series for the comparison of readings made at various heights in the stations. The day or point curve shows the fluctuations during the day of one point, and the station curve the variation at different heights in the same station. The curves of successive days or of different stations may of course be combined on the same sheet for comparison. Level and station curves based upon mean relative humidities are especially valuable.

75. Records. A field form is obviously unnecessary for the psychrograph. The record sheets constitute both a field and permanent record. The altitude and other constant features of the station and the list of species, etc., are entered on the back of the first record sheet, or, better, they are noted in the permanent formation record. For psychrometer readings, whether single or in series, the following record form is employed:

On page [47] is given a table for the conversion of Centigrade into Fahrenheit temperatures. This may be done mentally by means of the formula F = C/5 × 9 + 32°.

LIGHT

76. Methods. All methods for measuring light intensity, which have been at all satisfactory, are based upon the fact that silver salts blacken in the light. The first photographic method was proposed by Bunsen and Roscoe in 1862; this has been taken up by Wiesner and variously modified. After considerable experiment by the writer, however, it seemed desirable to abandon all methods which require the use of “normal paper” and “normal black” and to develop a simpler one. As space is lacking for a satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited below.[^[4]] Simple photometers for making light readings simultaneously or in series were constructed in 1900, and have been in constant use since that time. An automatic instrument capable of making accurate continuous records proved to be a more difficult problem. A sunshine recorder was ultimately found which yields valuable results, and very recently a recording photometer which promises to be perfectly satisfactory has been devised. Since the hourly and daily variations of sunlight in the same habitat are relatively small, automatic photometers are perhaps a convenience rather than a necessity.

The Photometer

Fig. 11. Photometer, showing front and side view.

77. Construction. The simple form of photometer shown in the illustration is a light-tight metal box with a central wheel upon which a strip of photographic paper is fastened. This wheel is revolved by the thumbscrew past an opening 6 mm. square which is closed by means of a slide working closely between two flanges. At the edge of the opening, and beneath the slide is a hollow for the reception of a permanent light standard. The disk of the thumbscrew is graduated into twenty-five parts, and these are numbered. A line just beneath the opening coincides with the successive lines on the disk, and indicates the number of the exposure. The wheel contains twenty-five hollows in which the click works, thus moving each exposure just beyond the opening. The metal case is made in two parts, so that the bottom may be readily removed, and the photographic strip placed in position. The water-photometer is similar except that the opening is always covered with a transparent strip and the whole instrument is water-tight. These instruments have been made especially for measuring light by the C. H. Stoelting Co., 31 W. Randolph street, Chicago, Ill. The price is $5.

78. Filling the photometer. The photographic paper called “solio” which is made by the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for photometric readings. The most convenient size is that of the 8 × 10 inch sheet, which can be obtained at any supply house in packages of a dozen sheets for 60 cents. New “emulsions,” i. e., new lots of paper, are received by the dealers every week, but each emulsion can be preserved for three to six months without harm if kept in a cool, light-tight place. Furthermore, all emulsions are made in exactly the same way, and it has been impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm. wide is cut lengthwise from the 8 × 10 sheet. This must be done in the dark room, or at night in very weak light. The strip is placed on the wheel, extreme care being taken not to touch the coated surface, and fixed in position by forcing the free ends into the slit of the wheel by a piece of cork 8–9 mm. long. The wheel is replaced in the case, turned until the zero is opposite the index line, and the instrument is ready for use.

79. Making readings. An exposure is made by moving the slide quickly in such a way as to uncover the entire opening, and the standard if the exposure is to be very short. Care must be taken not to pull the slide entirely out of the groove, as it will be impossible to replace it with sufficient quickness. The time of exposure can be determined by any watch after a little practice. It is somewhat awkward for one person to manage the slide properly when his attention is fixed upon a second hand. This is obviated by having one observer handle the watch and another the photometer, but here the reaction time is a source of considerable error. The most satisfactory method is to use a stop-watch. This can be held in the left hand and started and stopped by the index finger. The photometer is held against it in the right hand in such a way that the two movements of stopping the watch and closing the slide may be made at the same instant. The length of exposure is that necessary to bring the tint of the paper to that of the standard beside it. A second method which is equally advantageous and sometimes preferable does away with the permanent standard in the field and the need for a stop-watch. In this event, the strip is exposed until a medium color is obtained, since very light or very deep prints are harder to match. This is later compared with the multiple standard. In both cases, the date, time of day, station, number of instrument and of exposure, and the length of the latter in seconds are carefully noted. The instrument is held with the edge toward the south at the level to be read, and the opening uppermost in the usual position of the leaf. When special readings are desired, as for isophotic leaves, reflected light, etc., the position is naturally changed to correspond. In practice, it is made an invariable rule to move the strip for the next exposure as soon as the slide is closed. Otherwise double exposures are liable to occur. When a strip is completely exposed it is removed in the dark, and a new one put in place. The former is carefully labeled and dated on the back, and put away in a light-tight box in a cool place.

Fig. 12. Dawson-Lander sun recorder.

80. The Dawson-Lander sun recorder. “The instrument consists of a small outer cylinder of copper which revolves with the sun, and through the side of which is cut a narrow slit to allow the sunshine to impinge on a strip of sensitive paper, wound round a drum which fits closely inside the outer cylinder, but is held by a pin so that it can not rotate. By means of a screw fixed to the lid of the outer cylinder, the drum holding the sensitive paper is made to travel endwise down the outer tube, one-eighth of an inch daily, so that a fresh portion of the sensitive surface is brought into position to receive the record.” The instrument is driven by an eight-day clock placed in the base below the drum. The slit is covered by means of a flattened funnel-shaped hood, and the photographic strip is protected from rain by a perfectly transparent sheet of celluloid. The detailed structure of the instrument is shown in figure 12. This instrument may be obtained from Lander and Smith, Canterbury, England, for $35.

In setting up the sunshine recorder, the axis should be placed in such a position that the angle which it makes with the base is the same as the altitude of the place where the observations are made. This is readily done by loosening the bolts at either side. The drum is removed, the celluloid sheet unwound by means of the key which holds it in place, the sensitive strip put in position, and the sheet again wound up. Strips of a special sensitive paper upon which the hours are indicated are furnished by the makers of the instrument, but it has been found preferable to use solio strips in order to facilitate comparison with the standards. The drum is placed on the axis, and is screwed up until it just escapes the collar at the top of the spiral. The clock is wound and started, and the outer cylinder put on so that the proper hour mark coincides with the index on the front of the base.

As a sunshine recorder, the instrument gives a perfect record, in which the varying intensities are readily recognizable. Since the cylinder moves one-half inch in an hour, and the slit is .01 of an inch, the time of each exposure is 72 seconds. This gives a very deep color on the solio paper, which results in a serious error in making comparisons with the standard. On account of the hood, diffuse light is not recorded when it is too weak to cast a distinct shadow. It seems probable that this difficulty will be overcome by the use of a flat disk containing the proper slit, and in this event the instrument will become of especial value for measuring the diffuse light of layered formations. The celluloid sheet constitutes a source of error in sunlight on account of the reflection which it causes. This can be prevented by using the instrument only on sunny days, when the protection of the sheet can be dispensed with.

81. The selagraph. This instrument is at present under construction, and can only be described in a general way. In principle it is a simple photometer operating automatically. It consists of a light-tight box preferably of metal, which contains an eight-day lever clock. Attached to the arbor of the latter is a disk 7 inches in diameter bearing on its circumference a solio strip 1 cm. wide and 59 cm. long. The opening in the box for exposure is 6 mm. square and is controlled by a photographic shutter. The latter is constructed so that it may be set for 5, 10, or 20 seconds, since a single period of exposure can not serve for both sun and shade. The shutter is tripped once every two hours, by means of a special wheel revolving once a day. Each exposure is 6 mm. square, and is separated by a small space from the next one. Twelve exposures are made every 24 hours, and 84 during the week, though, naturally, the daytime exposures alone are recorded. Comparisons with the multiple standard are made exactly as in the case of the simple photometer. The selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.

Standards

82. Use. The light value of each exposure is determined by reference to a standard. When the photometer carries a permanent standard, each exposure is brought to the tint of the latter, and its value is indicated by the time ratio between them. Thus, if the standard is the result of a 5–second exposure to full sunlight at meridian, and a reading which corresponds in color requires 100 seconds in the habitat concerned, the light of the latter is twenty times weaker or more diffuse. Usually, the standard is regarded as unity, and light values figured with reference to it, as .05. With the selagraph such a use of the standard is impossible, and often, also, with the photometer it is unnecessary or not desirable. The value of each exposure in such case is obtained by matching it with a multiple standard, after the entire strip has been exposed. The further steps are those already indicated. After the exact tint in the standard has been found, the length of the reading in seconds is divided by the time of the proper standard, and the result expressed as above.

83. Making a standard. Standards are obtained by exposing the photometer at meridian on a typically clear day, and in the field where there is the least dust and smoke. Exception to the latter may be made, of course, in obtaining standards for plant houses located in cities, though it is far better to have the same one for both field and control experiment. Usable standards can be obtained on any bright day at the base station. Indeed, valuable results are often secured by immediate successive sun and shade readings in adjacent habitats, where the sun reading series is the sole standard. Preferably, standards should be made at the solstices or equinoxes, and at a representative station. The June solstice is much to be preferred, as it represents the maximum light values of the year. Lincoln has been taken as the base station for the plains and mountains. It is desirable, however, that a national or international station be ultimately selected for this purpose, in order that light values taken in different parts of the world may be readily compared.

84. Kinds of standards. The base standard is the one taken at Lincoln (latitude 41° N.) at meridian June 20–22. This is properly the unit to which all exposures are referred, but it has been found convenient to employ the Minnehaha standard as the base for the Colorado mountains, in order to avoid reducing each time. Relative standards are frequently used for temporary purposes. Thus, in comparing the light intensities of a series of formations, one to five standards are exposed on the solio strip before beginning the series of readings. Proof standards are the exposed solio strips, which fade in the light, and can, in consequence, be kept only a few weeks without possibility of error. The fading can be prevented by “toning” the strip, but in this event the exposures must be fixed in like manner before they can be compared. This process is inconvenient and time-consuming. It is also open to considerable error, as the time of treatment, strength of solution, etc., must be exactly equivalent in all instances. Permanent standards are accurate water-color copies of the originals obtained by the photometer. These have the apparent disadvantage of requiring a double comparison or matching, but after a little practice it is possible to reproduce the solio tints so that the copy is practically indistinguishable from the original. The most satisfactory method is to make a long stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and then to reject such parts of the stroke as do not match exactly. Permanent standards fade after a few month’s use, and must be replaced by parts of the original stroke. Single standards are made by one exposure, while multiple ones have a series of exposures filling a whole light strip. These are regularly obtained by making the exposures from 1–10 seconds respectively, and then increasing the length of each successive exposure by 2 seconds. Single exposures of 1–5 seconds as desired usually serve as the basis for permanent standards, but a multiple standard may also be copied in permanent form. Exposures for securing standards must be made only under the most favorable conditions, and the length in seconds must be exact. The use of the stop-watch is imperative, except where access may be had to an astronomical clock with a large second hand, which is even more satisfactory. The length of time necessary for the series desired is reckoned beforehand, and the exposures begun so that the meridian falls in the middle of the process.

Single standards are exceedingly convenient in photometer readings, but they are open to one objection. In the sunshine it is necessary to make instant decision upon the accuracy of the match, or the exposure becomes too deep. In the shade where the action is slower, this difficulty is not felt. For this reason it is usually desirable to check the results by a multiple standard, and in the case of selagraph records, where the various exposures show a wide range of tint, light values are obtainable only by direct comparison with the multiple standard. The exact matching of exposure and standard requires great accuracy, but with a little practice this may be done with slight chance of error by merely moving the exposure along the various tints of the standard until the proper shade is found. The requisite skill is soon acquired by running over a strip of exposures several times until the comparisons always yield the same results for each. The margin of error is practically negligible when the same person makes all the comparisons, and in the case of two or three working on the same reading the results diverge little or not at all. The efficient difference for light is much more of a variable than is the case with water-content. It has been determined so far only for a few species, all of which seem to indicate that appreciable modification in the form or structure of a leaf does not occur until the reduction in intensity reaches .1 of the meridian sunlight at the June solstice. The error of comparison is far less than this, and consequently may be ignored, even in the most painstaking inquiry.

Readings

85. Time. The intensity of the light incident upon a habitat varies periodically with the hour and the day, and changes in accord with the changing conditions of the sky. The light variations on cloudy days can only be determined by the photometer. While these can not be ignored, proper comparisons can be instituted only between the readings taken on normal days of sunshine. The sunlight varies with the altitude of the sun, i. e., the angle which its rays make with the surface at a given latitude. This angle reaches a daily maximum at meridian. The yearly maximum falls on June 22, and the angle decreases in both directions through the year to a minimum on December 22. At equal distances from either solstice, the angle is the same, e. g., on March 21 and September 23. At Lincoln (41° N. latitude) the extremes at meridian are 73° and 26°; at Minnehaha (39°) they are 75° and 28°. The extremes for any latitude may be found by subtracting its distance in degrees north of the two tropics from 90. Thus, the 50th parallel is 26.5° north of the tropic of Cancer, and the maximum altitude of the sun at a place upon it is 63.5°. It is 73.5° north of the tropic of Capricorn, and the minimum meridional altitude is 16.5°.

The changes in the amount of light due to the altitude of the sun are produced by the earth’s atmosphere. The absorption of light rays is greatest near the horizon, where their pathway through the atmosphere is longest, and it is least at the zenith. The absorption, and, consequently, the relative intensity of sunlight, can be determined at a given place for each hour of any sunshiny day by the use of chart 13. This chart has been constructed for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The curves which show the altitude of the sun at the various times of the day and the year have been constructed by measurements upon the celestial globe. Each interval between the horizontal lines represents 2 degrees of the sun’s altitude. The vertical lines indicate time before or after the apparent noon, the intervals corresponding to 10 minutes. If the relative intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent noon for this day must first be determined. A glance at the table shows that the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The apparent noon at Lincoln is found by adding 26 minutes 49 seconds, the difference in time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23 minutes later. The sun’s altitude is accordingly 36°. If the intensity of the light which reaches the earth’s surface when the sun is at zenith is taken as 1, the table of the sun’s altitudes gives the intensity at 3:00 P.M. on March 12 as .85.

For places with a latitude differing by several degrees from that of Lincoln, it is necessary to construct a new table of altitude curves from the celestial globe. It is quite possible to make a close approximation of this from the table given, since the maximum and minimum meridional altitude, and hence the corresponding light intensity, can be obtained as indicated above. For Minnehaha, which is on the 105th meridian, and for other places on standard meridians, i. e., 60°, 75°, 90°, and 120° W., the table of apparent noon indicates the number of minutes to be added to 12 noon, standard time, when the sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of a standard meridian is respectively faster or slower than the latter. The exact difference in minutes is obtained from the difference in longitude by the equation, 15° = 1 hour. Thus, Lincoln, 96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is consequently 26 minutes 49 seconds slower, and this time must always be added to the apparent noon as determined from the chart. At a place east of a standard meridian, the time difference is, of course, subtracted.

Fig. 13. Chart for the determination of the sun’s altitude, and the corresponding light intensity.

The actual differences in the light intensity from hour to hour and day to day, which are caused by variations in the sun’s altitude, are not as great as might be expected. For example, the maximum intensity at Lincoln, June 22, is .98; the minimum meridional intensity December 22 is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00 A.M. and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.M. the range in intensity is from .90 to .98 merely. On December 22, the greatest intensity is .52, the least .20 (the latter at 8:00 A.M. and 4:00 P.M. approximately). If the growing season be taken as beginning with the 1st of March and closing the 1st of October, the greatest variation in light intensity at Lincoln within a period of 10 hours with the meridian at its center (cloudy days excepted) is from .33 to .98. In a period of 8 hours, the extremes are .65 to .98, i. e., the greatest variation, .3, is far within the efficient difference, which has been put at .9. For the growing period, then, readings made between 8:00 A.M. and 4:00 P.M. on normal sunshiny days may be compared directly, without taking into account the compensation for the sun’s altitude. Until the efficient difference has been determined for a large number of species, however, it seems wise to err on the safe side and to compensate for great differences in time of day or year. In all doubtful cases, the intensity obtained by the astronomical method should also be checked by photometric readings. A slight error probably enters in, due to reflection from the surface of the paper, and to temperature, but this is negligible.

86. Table for determining apparent noon

87. Place. The effect of latitude upon the sun’s altitude, and the consequent light intensity have been discussed in the pages which precede. Latitude has also a profound influence upon the duration of daylight, but the importance of the latter apart from intensity is not altogether clear. The variation of intensity due to altitude has been greatly overestimated; it is practically certain, for example, that the dwarf habit of alpine plants is not to be ascribed to intense illumination, since the latter increases but slightly with the altitude. It has been demonstrated astronomically that about 20 per cent of a vertical ray of sunlight is absorbed by the atmosphere by the time it reaches sea level. At the summit of Pike’s Peak, which is 14,000 feet (4,267 meters) high, the barometric pressure is 17 inches, and the absorption is approximately 11 per cent. In other words, the light at sea level is 80 per cent of that which enters the earth’s atmosphere; on the summit of Pike’s Peak it is 89 per cent. As the effect of the sun’s altitude is the same in both places, the table of curves on page [57] will apply to both. Taking into account the difference in absorption, the maximum intensity at sea level and at 14,000 feet on the fortieth parallel is .98 and 1.09 respectively. The minimum intensities between 8:00 A.M. and 4:00 P.M. of the growing period are .64 and .71 respectively. The correctness of these figures has been demonstrated by photometer readings, which have given almost exactly the same results. Such slight variations are quite insufficient to produce an appreciable adjustment, particularly in structure. They are far within the efficient difference, and Reinke[^[5]] has found, moreover, that photosynthetic activity in Elodea is not increased beyond the normal in sunlight sixty times concentrated. In consequence, it is entirely unnecessary to take account of different altitudes in obtaining light values.

The slope of a habitat exerts a considerable effect upon the intensity of the incident light. If the angle between the slope and the sun’s ray be 90°, a square meter of surface will receive the maximum intensity, 1. At an angle of 10°, the same area receives but .17 of the light. This relation between angle and intensity is shown in the table which follows. The influence of the light, however, is felt by the leaf, not by the slope. Since there is no connection between the position of the leaf and the slope of the habitat, the latter may be ignored. In consequence, it is unnecessary to make allowances for the direction of a slope, viz., whether north, east, south, or west, in so far as light values are concerned. The angle which a leaf makes with its stem determines the angle of incidence, and hence the amount of light received by the leaf surface. This is relatively unimportant for two reasons. This angle changes hourly and daily with the altitude of the sun, and the intensity constantly swings from one extreme to the other. Moreover, the extremes 1.00 and 0.17, even if constant, are hardly sufficient to produce a measurable result. When the angle of the leaf approaches 90°, there is the well-known differentiation of leaf surfaces and of chlorenchym, but this has no relation to the angle of incidence.

Table of Intensity at Various Angles

In the sunlight, it makes no difference at what height a light reading is taken. In forest and thicket as well as in some herbaceous formations, the intensity of the light, if there is any difference, is greatest just beneath the foliage of the facies. In forests especially, the light is increasingly diffuse toward the ground, particularly where layers intervene. In woodland formations, moreover, the exact spot in which a reading is made must be carefully chosen, unless the foliage is so dense that the shade is uniform. A very satisfactory plan is to take readings in two or more spots where the shade appears to be typical, and to make a check reading in a “sunfleck,” a spot where sunlight shows through. In forests and thickets, the sunflecks are fleeting, and the light value is practically that of the shade. In passing into open woodland and thicket, the sunflecks increase in size and permanence, until finally they exceed the shade areas in amount and become typical of the formation.

Reflected and Absorbed Light

88. The fate of incident light. The light present in a habitat and incident upon a leaf is not all available for photosynthesis. Part is reflected or screened out by the epidermis, and a certain amount passes through the chlorenchym, except in very thick leaves. The light absorbed is by far the greatest in the majority of species. Many plants with dense coatings of hairs reflect or withhold more light than they absorb, and the amount of light reflected by a thick cuticule is likewise great. As light is imponderable, the actual amount absorbed or reflected by the leaf can not be determined. It is possible, however, to express this in terms of the total amount received, by means of readings with solio paper, and the knowledge thus obtained is of great importance in interpreting the modifications of certain types of leaves. For example, a leaf with a densely hairy epidermis may receive light of the full intensity, 1; the amount reflected or screened out by the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the majority of cases, however, the absorbed light is considerably more than the amount reflected or transmitted.

Fig. 14. Leaf print: exposed 10 m., 11 A.M. August 20. The leaves are from sun and shade forms of Bursa bursa-pastoris, Rosa sayii, Thalictrum sparsiflorum, and Machaeranthera aspera. In each the shade leaf prints more deeply.

89. Methods of determination. If results are to be of value, reflected and transmitted light must be determined in the habitat of the plant simultaneously with the total light which a leaf receives. An approximation of the light reflected from a leaf surface is secured by placing the photometer so that the light reflected is thrown upon the solio strip. A much more satisfactory method, however, is to determine it in connection with the amount of light transmitted through the epidermis. This is done by stripping a piece of epidermis from the upper surface of the leaf and placing it over the slit in the photometer for an exposure. An exposure in the full light of the habitat is made simultaneously with another photometer, or immediately afterward upon the same strip. When the epidermis is not too dense, both exposures are permitted to reach the same tint, and the relation between them is precisely that of their lengths of exposure. Ordinarily the two exposures are made absolutely simultaneous by placing the epidermis over half of the opening, leaving the other half to record the full light value, and the results, or epidermis prints, are referred to a multiple standard. The difference between the two values thus obtained represents the amount of reflected light together with that screened by the epidermis. The amount of light transmitted through the leaf may be measured in the same way by using the leaf itself in place of the epidermis alone. The time of exposure is necessarily long, however, and it has been found practicable to obtain leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time that the epidermis print is made. In a few species both the upper and lower epidermis can be removed and the amount of light absorbed determined directly by exposing the strip covered with the chlorenchym. Generally, however, this must be computed by subtracting the sum of the per cents of reflected and transmitted light from 100 per cent, which represents the total light.

Fig. 15. Leaf print: exposure as before. Sun and shade leaves of Achillea lanulosa, Capnoides aureum, Antennaria umbrinella, Galium boreale, and Potentilla propinqua.

90. Leaf and epidermis prints. In diphotic leaves the screening effect of the lower epidermis may be ignored. Isophotic sun leaves, i. e., those nearly upright in position or found above light-colored, reflecting soils, are usually strongly illuminated on both sides, and the absorbed light can be obtained only by measuring the screening effect of both epiderms. Shade leaves and submerged leaves often contain chloroplasts in the epidermis, and the above method can not be applied to them. In fact, in habitats where the light is quite diffuse, practically all incident light is absorbed. The rare exceptions are those shade leaves with a distinct bloom. In addition to their use in obtaining the amount of light absorbed, both leaf and epidermis prints are extremely interesting for the direct comparison of light relations in the leaves of species belonging to different habitats. The relative screening value of the upper and lower epidermis, or of the corresponding epiderms of two ecads or two species, is readily ascertained by exposing the two side by side in sunshine, over the slit in the photometer. For leaf prints fresh leaves are desirable, though nearly the same results can be obtained from leaves dried under pressure. The leaves are grouped as desired on the glass of a printing frame, and covered with a sheet of solio. They are then exposed to full sunlight, preferably at meridian, and the prints evaluated by means of the multiple standard. This method is especially useful in the comparison of ecads of one species. These differences due to transmitted light are very graphic, and can easily be preserved by “toning” the print in the usual way.

Expression of Results

91. Light records. The actual photographic records obtained by photometer and selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning” modifies the color of the exposure materially, and changes its intensity so that it can not be compared with readings not fixed. It would involve a great deal of inconvenience to make all comparisons by means of toned strips and standard, even if it were not for the fact that it is practically impossible to obtain exactly the same shade in lots toned at different times. The field record, if carefully and neatly made, may well take the place of a permanent one. The form is the following:

92. Light sums, means, and curves. Owing to the fact that the selagraph has not yet been used in the field, no endeavor has been made to determine the light value for every hour of the day in different habitats. Consequently there has been no attempt to compute light sums and means. Photometer readings have sufficed to interpret the effect of light in the structure of the formation, and of the individual, but they have not been sufficiently frequent for use in ascertaining sums and means. The latter are much less valuable than the extremes, especially when the relative duration of these is indicated. Means, however, are readily obtained from the continuous records. Light sums are probably impracticable, as the factor is not one that can be expressed in absolute terms. The various kinds and combinations of light curves are essentially the same as for humidity. The level curve through a series of habitats is the most illuminating, but the day curve of hour variations is of considerable value. The curve of daily duration, based upon full sunlight, is also of especial importance for plants, and stations which receive both sun and shade during the day.

TEMPERATURE

93. In consequence of its indirect action, temperature does not have a striking effect upon the form and structure of the plant, as is the case with water and light. Notwithstanding, it is a factor of fundamental importance. This is especially evident in the character and distribution of vegetation. It is also seen in the germination and growth of plants, in the length of season, and in the important influence of temperature upon humidity, and hence upon water-content. Because of its intimate relation with the comfort of mankind, the determination of temperature values has received more attention than that of any other factor, and excellent simple and recording instruments are numerous. For plants, it is also necessary to employ instruments for measuring soil temperatures. The latter unquestionably have much less meaning for the plant than the temperatures of the air, but they have a direct influence upon the imbibition of water, and upon germination.

Thermometers

94. Air thermometers. The accurate measurement of temperature requires standard thermometers. Reasonably accurate instruments may be standardized by determining their error, but they are extremely unsatisfactory in practice, since they result in a serious waste of time. Accurate thermometers which read to the degree are entirely serviceable as a rule, but instruments which read to a fraction of a degree are often very much to be desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are delicate and require careful handling, but even in class work this has proved to be an advantage rather than otherwise. In making readings of air temperatures with such thermometers, constant precautions must be taken to expose the bulb directly to the wind and to keep it away from the hand and person.

95. Soil thermometers. The thermometer described above has been used extensively for soil temperatures. The determination of the latter is conveniently combined with the taking of soil samples, by using the hole for a temperature reading. When carefully covered, these holes can be used from day to day throughout the season without appreciable error, even in gravel soils. Repeated tests of this have been made by simultaneous readings in permanent and newly made holes, and the results have always been the same. It has even been found that the error is usually less than 1 degree when the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in the fact that the thermometer must be raised to make the reading. With a little practice, however, the top of the column of mercury may be raised to the surface and read before the change of temperature can react upon it. This is especially important in very moist or wet soils where the bulb becomes coated with a film of moisture. This evaporates when the bulb is brought into the air, and after a moment or two the mercury slowly falls.

Fig. 16. Soil thermometer

Regular soil thermometers are indispensable when readings are desired at depths greater than 12–18 inches. They possess several disadvantages which restrict their use almost wholly to permanent stations. It is scarcely possible to carry them on field trips, and the time required to place them in the soil renders them practically useless for single readings. Moreover, the instruments are expensive, ranging in price from $7 for the two-foot thermometer, to $19 for the eight-foot one. When it is recognized that deep-seated temperatures are extremely constant and that the slight fluctuations affect, as a rule, only the relatively stable shrubs and trees, it is evident that such temperatures are of restricted importance. Still, in any habitat, they must be ascertained before they can well be ignored, though it is unwise to spend much time and energy in their determination. Soil thermometers of the form illustrated may be obtained from H. J. Green, Brooklyn.

96. Maximum-minimum thermometers. These are used for determining the range of temperature within a given period, usually a day. Since they are much cheaper than thermographs, they can replace these in part, although they merely indicate the maximum and minimum temperatures for the day, and do not register the time when each occurs. The maximum is a mercurial thermometer with a constriction in the tube just above the bulb; this allows the mercury to pass out as it expands, but prevents it from running back, thus registering the maximum temperature. The minimum thermometer contains alcohol. The column carries a tiny dumbbell-shaped marker which moves down with it, but will not rise as the liquid expands. This is due to the fact that the fluid expands too slowly to carry the marker upward, while the surface tension causes it to be drawn downward as the fluid contracts. The minimum temperature is indicated by the upper end of the marker. In setting up the thermometers, they are attached by special thumbscrews to a support which holds them in an oblique position. The minimum is placed in a special holder above the maximum which rests on a pin that is used also for screwing the pivot-screw into position. The support is screwed tightly to the cross-piece of a post, or in forest formations it is fastened directly to a board nailed upon a tree trunk. A shelter has not been used in ecological work, although it is the rule in meteorological observations. The minimum thermometer is set for registering by raising the free end, so that the marker runs to the end of the column. The mercury of the maximum is driven back into the bulb by whirling it rapidly on the pivot-screw after the pin has been taken out. This must be done with care in order that the bulb may not be broken. As soon as the instrument comes to rest, it is raised and the pin replaced, great care being taken to lift it no higher than is necessary. When the night maximum is sought, the thermometer should be whirled several times in order to drive the column sufficiently low. Usually, in such cases, a record is made of this point to make sure that the maximum read is the actual one. If the pivot-screw is kept well oiled, less force will be required to drive the mercury back. In practice, the thermometers have been observed at 6:00 A.M. and 6:00 P.M. each day, thus permitting the reading of the maximum-minimum for both day and night. Pairs of maximum-minimum thermometers are to be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, Baltimore, Maryland, at a cost of $8.25.

Fig. 17. Maximum-minimum thermometer.

Fig. 18. Terrestrial radiation thermometer.

Fig. 19. Draper thermograph.

97. Radiation thermometers. These are used to determine the radiation in the air, and from the soil, i. e., for solar and terrestrial radiation. The latter alone has been employed in the study of habitats, chiefly for the purpose of ascertaining the difference in the cooling of different soils at night. The terrestrial radiation thermometer is merely a special form of minimum thermometer, so arranged in a support that the bulb can be placed directly above the soil or plant to be studied. It is otherwise operated exactly like the minimum thermometer, and the reading gives the minimum temperature which the air above the plant or soil reaches, not the amount of radiation. As a consequence, these instruments are valuable only where read in connection with a pair of maximum-minimum thermometers in the air, or when read in a series of instruments placed above different soils or plants.

98. Thermographs. Two types of standard instruments are in general use for obtaining continuous records of air temperatures. These are the Draper thermograph, made by the Draper Manufacturing Company, 152 Front St., New York city ($25 and $30), and the Richard thermograph sold by Julien P. Friez, Baltimore ($50). After careful trial had demonstrated that they were equally accurate, the matter of cost was considered decisive, and the Draper thermograph has been used exclusively in the writer’s own work. This instrument closely resembles the psychrograph manufactured by the same company. It is made in two sizes, of which the larger one is the more satisfactory on account of the greater distance between the lines of the recording disk. The thermometric part consists of two bimetallic strips, the contraction and expansion of which are communicated to a hand carrying a pen. The latter traces a line on the record sheet which is attached to a metal disk made to revolve by an eight-day clock. In practice the thermograph is set up in the shelter which contains the psychrograph, and in exactly the same manner. The clock is wound, the record put in place, and the pen inked in the same way also. The proper position of the pen is determined by making a careful thermometer reading under the shelter, and then regulating the pen hand by means of the screws at the base of it. A similar test reading is also made each week, when the clock is rewound. A record sheet may be left in position for three weeks, the pen being filled each week with a different ink. The fixed order of using the inks is red, blue, and green as already indicated.

Fig. 20. Shelter for thermograph.

Owing to the fact that they are practically stationary, soil thermographs are of slight value, except at base stations. Here, the facts that they are expensive, that the soil temperatures are of relatively little importance, and that they can be determined as easily, or nearly so, by simple thermometers, make the use of such instruments altogether unnecessary, if not, indeed, undesirable. In a perfectly equipped research station, they undoubtedly have their use, but at ordinary stations, and in the case of private investigators, their value is in no wise commensurate with their cost.

Readings

99. Time. The hourly and daily fluctuations of the temperature of the air render frequent readings desirable. It is this variation, indeed, which makes single readings, or even series of them, inconclusive, and renders the use of a recording instrument almost imperative in the base station at least. Undoubtedly, a set of simultaneous readings at different heights in one station, or at the same height in different stations, especially if made at the maximum, have much value for comparison, but their full significance is seen only when they are referred to a continuous base record. Such series, moreover, furnish good results for purposes of instruction. In research work, however, it has been found imperative to have thermographs in habitats of widely different character. With these as bases, it is possible to eke them out with considerable satisfaction by means of maximum-minimum thermometers in less different habitats, or in different parts of the same habitat. Naturally these are less satisfactory, and are used only when expense sets a limit to the number of thermographs. In a careful analysis of a single habitat, more can be gained by one base thermograph supplemented by three pairs of maximum-minimum thermometers in dissimilar areas of the habitat than by two thermographs, and the cost is the same.

Fig. 21. Richard thermograph.

100. Place and height. For general air temperatures, thermograph and thermometer readings are made at a height of 3 feet (1 meter). Soil temperatures are regularly taken at the surface and at a depth of 1 foot. When a complete series of simultaneous readings is made in one station, the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 10, and 15 inches in the soil. When sun and shade occur side by side in the same formation, as is true of many thickets and forests, surface readings are regularly made in both. Similarly, valuable results are obtained by making simultaneous readings on the bare soil, on dead cover, and upon a leaf, while the influence of cover is readily ascertained by readings upon it and beneath it. A full series of station readings made at the same time upon north, east, south, and west slopes is of great importance in studying the effects of exposure.

Expression of Results

101. Temperature records. Neither field nor permanent form is required for thermographic records, other than the record sheet itself, which contains all the necessary information in a fairly convenient form. Although the temperature of a particular hour and day can not be read at a mere glance, it can be obtained so easily that it is a waste of time to make a tabular copy of each record sheet. For thermometer readings, either single or in series, the following form is used:

102. Temperature sums and means. The amount of heat, i. e., the number of calories received within a given time by a definite area of plant surface, can be determined by means of a calorimeter. From this the temperature sum of a particular period may be obtained by simple addition. In the present condition of our knowledge, it is impossible to establish any exact connection between such results and the functional or growth effect that can be traced directly to heat. As a consequence, temperature sums do not at present contribute anything of value to an understanding of the relation between cause and effect. The mean daily temperature is readily obtained by averaging twenty-four hour-temperatures recorded by the thermograph. The method employed by Meyen[^[6]], of deriving the mean directly from the maximum and minimum for the day, is not accurate; from a large number of computations, the error is always more than two degrees. On the other hand, the mean obtained by averaging the maximum and minimum for the day and night has been found to deviate less than 1 degree from the mean proper. This fact greatly increases the value of maximum-minimum instruments if they are read daily at 6:00 A.M. and 6:00 P.M.

103. Temperature curves. The kinds and combinations of temperature curves are almost without number. The simple curves of most interest are those for a series of stations or habitats, based upon the level of three feet, or the surface, or the daily mean. The curves for each station representing the different heights and depths and the season curve of the daily means for a habitat are also of much importance. One of the most illuminating combinations is that which groups together the various level curves for a series of habitats. Other valuable combinations are obtained by grouping the curves of daily means of different habitats for the season, or the various station curves.

104. Plant temperatures. The direct effects of temperature as seen in nutrition and growth can be ascertained only by determining the temperature of plant tissues. The temperatures of the air and of the soil surface have an important effect upon humidity, and water-content, and through them upon the plant, but heat can influence assimilation, for example, only in so far as it is absorbed by the assimilating tissue. The temperatures of the leaf, as the most active nutritive organ of the plant, are especially important. While it is a well-known fact that internal temperatures follow those of the air and soil closely, though with varying rapidity of response, this holds less for leaves than for stems and roots. Owing to the very obvious difficulties, practically nothing has yet been done in this important field. A few preliminary results have been obtained at Minnehaha, which serve to show the need for such readings. Gravel slide rosettes in an air temperature of 24° C. and a surface temperature of 40° C. gave the following surface readings: Parmelia, 40°, Eriogonum, 38.6°, Arctostaphylus, 35°, Thlaspi, 31.8°, and Senecio, 31°. The leaf of Eriogonum flavum, which is smooth above and densely hairy below, indicated a temperature of 31.8° when rolled closely about the thermometer bulb with the smooth surface out, and 28° when the hairy surface was outside. The surface readings of the same leaf were .5°–1° higher when made upon the upper smooth surface. This immediately suggests that the lower surface may be modified to protect the leaf from the great heat of the gravel, which often reaches 50° C. (122° F.).

PRECIPITATION

105. General relations. As the factor which exerts the most important control upon water-content and humidity, rainfall must be carefully considered by the ecologist. It is such an obvious factor, and is usually spoken of in such general terms that the need of following it accurately is not evident at once. When it is recognized that the fluctuations of water-content are directly traceable to it, it becomes clear that its determination is as important as that of any indirect factor. This does not mean, however, that the amount of yearly rainfall is to be taken from the records of the nearest weather station, and the factor dismissed. Like other instruments, the rain gauge must be kept at the base station of the area under study, and when this is extensive or diverse, additional instruments should be put into commission. While the different parts of the same general climatic region may receive practically the same amount of precipitation during the year, it is not necessarily true that the rainfall of any particular storm is equally distributed, especially in the mountains. Nothing less than an exact knowledge of the amount of rain that falls in the different areas will make it possible to tell how much of the water-content found at any particular time in these represents merely the chance differences of precipitation.

The forms of precipitation are rain, dew, hail, snow, and frost. Of these, hail is too infrequent to be taken into account, while frost usually occurs only at the extremes of the growing season, and in its effect is rather to be reckoned with temperature. Snow rarely falls except during the period of rest, and, while it plays an important part as cover, it is merely one of several factors that determine the water-content of the soil at the beginning of spring. The influence of dew is not clearly understood. It is almost always too slight in amount and too fleeting to affect the water-content of the soil. It seems probable that it may serve by its own evaporation to decrease in some degree the water loss from the soil, and from bedewed plants. If, however, the dew is largely formed by the water of the soil and of the plant, as is thought by some, then it is negligible as a reinforcement of water-content. From the above, it is evident that rainfall alone exerts a profound effect upon the habitat, and it is with its measurement that the ecologist is chiefly concerned.

Fig. 22. Rain gauge showing construction.

106. The rain gauge, as the illustration shows, is a cylindrical vessel with a funnel-shaped receiver at the top, which is 8 inches in diameter. The receiver fits closely upon a narrower brass vessel or measuring tube in which the rain collects. The ratio of surface between receiver and tube is 10 to 1. For readings covering a general area, the rain-gauge is placed in the open, away from buildings or other obstructions, and is sunken in the ground sufficiently to keep it upright. In localities where winds are strong, it is usually braced at the sides also or supported by a wooden frame. In measuring the amount of rain in the measuring tube, the depth is divided by ten in order to ascertain the actual rainfall. The depth is measured by inserting the measuring-rod through the hole in the funnel until it touches the bottom. It is left for a second or so, quickly withdrawn, and the limit of the wetted portion noted. In the case of standard rods, the actual rainfall is read directly in hundredths, so that the division by ten is unnecessary. After each reading, the measuring tube is carefully drained, replaced, and the receiver put in position. No regular time for making readings is necessary. During a rainy period, it is customary to make a measurement each day, but it has been found more satisfactory for ecological purposes to measure each shower, and to record its duration. These two facts furnish a ready clue to the relative amount of run-off in each fall of rain. The measurement of snowfall is often made merely by determining its depth. For comparison with rainfall, the rain gauge with receiver and tube withdrawn is used. The snow which falls is melted, poured into the measuring tube, and measured in the ordinary way. The U. S. Weather Bureau standard rain gauge, with measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25.

107. Precipitation records. From the periodic character of precipitation, rainfall sums, means, and curves have little importance in the careful study of the habitat. The rainfall curve for the growing season is an aid in explaining the curve of water-content, and the mean rainfall of a region gives some idea of its vegetation, though even here the matter of its distribution is of primary importance. The rain and snow charts published by the U. S. Weather Bureau furnish data of some importance for the general study of vegetation, but it is evident that they can play little part in a system which is founded upon the habitat. Precipitation records, for reasons of brevity and convenience, are united with wind records, and the form will be found under the discussion of this factor.

WIND

Fig. 23. Simple anemometer.

108. Value of readings. On account of its direct effect upon humidity, and its consequent influence upon water-content, the part which wind plays in a habitat can not be ignored in a thorough investigation. It is an important element in exposure, and accordingly has a marked mechanical effect upon the vegetation of exposed habitats, alpine slopes, seacoasts, plains, etc. Owing to its inconstancy and its extreme variation in velocity, single wind readings are absolutely without value. When read in series, anemometers give some information upon the comparative air movement in different habitats, but the chance of error is great, except when the breeze is steady. Anemographs alone give real satisfaction. Accurate results, however, are not obtainable without a series of two or more in different habitats, and it is still an open question whether the results obtained justify the expense. For a completely equipped base station, anemometer, anemograph, and wind vane are desirable instruments, but the study of the habitat has by no means reached the stage of precision in which their general use is necessary.

Fig. 24. Standard anemometer.

109. The anemometer in its simplest form is adapted only to readings made under direct observation, as a sudden change in the direction of the wind reverses the movement of the indicator needle. This simple wind gauge, shown in figure 23, has been used for instructional purposes, and to a slight extent, also, in ascertaining the effect of cover. In constant winds, successive single readings are found to have value, but, ordinarily, the observations must be simultaneous. Careful tests of this simple instrument show that it is essentially accurate. It may be obtained from the C.H. Stoelting Company, 31 W. Randolph St., Chicago, for $25. The standard anemometer (Fig. 24) is practically a recording instrument up to 1,000 miles, but as the dials run on without any indication of the total number of revolutions, it must be visited and read each day. This renders its use difficult for habitats which are some distance apart. When exact determinations of wind values become necessary, the most successful method is to establish a series of three standard anemometers. One of these should be placed upon the most exposed part of a typically open habitat, the second in the most protected part of the same habitat, while the third is located in the midst of a representative forest formation. If the two habitats are close together, the daily visits can be made without serious inconvenience. The reading of the registering dials requires detailed explanation, and for this the reader is referred to the printed directions which accompany the instrument. In setting up the anemometer it must be borne in mind that the ecologist desires the wind velocity for a particular habitat. In consequence, the precautions which the meteorologist takes to place the instrument at a certain height and well away from surrounding obstructions do not hold here. Standard anemometers are furnished by H. J. Green, and J. P. Friez for $25 each.

The anemograph is an anemometer electrically connected with an automatic register. It is the only instrument adapted to continuous weekly records in different habitats, but the price, $75 ($25 for the anemometer and $50 for the register) is practically prohibitive, at least until a complete series of ecographs for other factors has been obtained.

110. Records. The following form is used as a combined record for precipitation and wind:

SOIL

111. Soil as a factor. In determining the value of the soil as a factor in a particular habitat, it must be clearly recognized that its importance lies solely in the control which it exerts upon water-content and nutrient-content. The former is directly connected with the texture or fineness of the soil, the latter with its chemical nature. Accordingly, the structure of the soil and its chemical composition are the fundamental points of attack. These are not at all of equal value, however. Water is both a food, and a solvent for the nutrient salts of the soil. Furthermore, the per cent of soluble salts, as determined in mechanical analyses, is practically the same for all ordinary soils. Indeed, the variations for the same soil types are as great as for entirely different types. For these reasons, soluble salt-content may be ignored except where it is readily seen to be excessive, as in alkaline soils; and determinations of chemical composition are necessary only in those soils which contain salts or acids to an injurious degree, e. g., alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, on the other hand, in the usual absence of excessive amounts of solutes, absolutely controls the fate of the water that enters the ground, in addition to its influence upon the run-off. It determines the amount of gravitation water lost by percolation, as well as the water that can be raised by capillarity. The resultant of these, the total soil water or holard, is hence an effect of structure, while the size and compactness of the particles are conclusive factors in controlling the chresard. It must be recognized, however, that these are all factors which enable us to interpret the amount of holard or chresard found in a particular soil. They have no direct important effect upon the plant, but influence it only in so far as they affect the water present.

112. The value of soil surveys. The full appreciation of the preeminent value of water-content, particularly of the chresard, greatly simplifies the ecological study of soils. The ecologist is primarily concerned with soil water only in its relation to the plant, and while a fair knowledge of soil structure is essential to a proper understanding of this, he has little concern with the detailed study of the problems of soil physics. For the sake of a proper balance of values, he must avoid the tendency noted elsewhere of ignoring the claims of the plant, and of studying the soil simply as the seat of certain physical phenomena. Accordingly, it is felt that mechanical and chemical analyses, determinations of soluble salt-content, etc., have much less value than has been commonly supposed. The usual methods of soil survey, which pay little or no attention to water-content, and none at all to available water, are practically valueless for ecological research. This statement does not indicate a failure to appreciate the importance of the usual soil methods for many agricultural problems, such as the use of fertilizers, conservation of moisture, etc., though even here to focus the work upon water-content would give much more fundamental and serviceable results. For these reasons, slight attention will be paid to methods of mechanical and chemical analysis. In their stead is given a brief statement of the origin, structure, and character of soils with especial reference to water-content.

113. The origin of soils. Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its fragments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. These processes are usually concomitant, although, as a rule, one is more evident than the other. The relation between them is dependent upon the character of the rock and the forces which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a rule disintegrate more rapidly than they decompose; sedimentary rocks, on the other hand, tend to decompose more rapidly than they disintegrate. In many cases the two processes go hand in hand. This difference is the basis for the distinction, first proposed by Thurmann, between those rocks which weather with difficulty and those which weather readily. The former were called dysgeogenous, the latter eugeogenous. Thurmann restricted the application of the first term to those rocks which produce little soil, but it seems more logical to apply dysgeogenous to those in which disintegration is markedly in excess of decomposition, and eugeogenous to those rocks that break down rather readily into fine soils. With respect to the general character of the soil formed, rocks are pelogenous, clay-producing, psammogenous, sand-forming, or pelopsammogenous, producing mixed clay and sand. The first two are divided into perpelic, hemipelic, oligopelic, perpsammic, etc., with reference to the readiness with which they are weathered, but this distinction is not a very practicable one. The grouping of soils into silicious, calcareous, argillaceous, etc., with reference to the chemical nature of the original rock, is of no value to the ecologist, apart from the general clue to the physical properties which it furnishes.

114. The structure of soils. The water capacity of a soil is a direct result of the fineness of the particles. Since the water is held as a thin surface film by each particle or group of them, it follows that the amount of water increases with the water-holding surface. The latter increases as the particles become finer and more numerous, and thus produce a greater aggregate surface. The upward and downward movements of water in the soil are likewise in immediate connection with the size of particles. The upward or capillary movement increases as the particles become finer, thus making the irregular capillary spaces between them smaller, and magnifying the pull exerted. On the contrary, the downward movement of gravitation water, i. e., percolation, is retarded by a decrease in the size of the soil grains and hastened by an increase. Hence, the two properties, capillarity and porosity, are direct expressions of the structure of the soil, i. e., of its texture or fineness. Capillarity, however, increases the water-content of the upper layers permeated by the roots of the plant, while porosity decreases it. On the basis of these properties alone, soils would fall into two groups, capillary soils and porous soils, the former fine-grained and of high water-content, the latter coarse-grained and with relatively little water. A third factor, however, of great importance must be taken into account. This is the pull exerted upon each water film by the soil particle itself. This pull apparently increases in strength as the film grows thinner, and explains why it finally becomes impossible for the root-hairs to draw moisture from the soil. This property, like capillarity, is most pronounced in fine-grained soils, such as clays, and is least evident in the coarser sands and gravels. It seems to furnish the direct explanation of non-available water, and, in consequence, to indicate that the chresard is an immediate result of soil texture.

Fig. 25. Sieves for soil analysis.

115. Mechanical analysis. From the above it is evident that, with the same rainfall, coarse soils will be relatively dry, and fine soils correspondingly moist. However, this difference in holard is somewhat counterbalanced by the fact that the chresard is much greater in the former than in the latter. The basis of these relations can be obtained only from a study of the texture of the soil. The usual method of doing this is by mechanical analysis. This is far from satisfactory, since the use of the sieves often brings about the disaggregation of groups of particles which act as units in the soil. Furthermore, the analysis affords no exact evidence of the compactness of the soil in nature, and tests of capillarity and porosity made with soil samples out of position are open to serious error. Nevertheless, mechanical analyses furnish results of some value by making it possible to compare soils upon the basis of texture. For ecological purposes, minute analyses are undesirable; their value in any work is doubtful. A separation of soil into gravel, sand, and silt-clay is sufficient, since the relative proportion of these will explain the holard and chresard of the soil concerned. The latter are also affected in rich soils, especially of forests, by the organic matter present. If this is in a finely divided condition, the amount is determined by calcining. When a definite layer of leafmold is present, as in forests and thickets, its water-value is found separately, since its power of retaining water is altogether out of proportion to its weight.

116. Kinds of soils. It is very doubtful whether it is worth while to attempt to distinguish soils upon the basis of mechanical analysis. Unquestionably, the most satisfactory method is to distinguish them with respect to holard and chresard, and to regard texture as of secondary importance. A series of soil classes which comprise various soil types has been proposed by the U. S. Bureau[^[7]] of Soils as follows: (1) stony loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based entirely upon mechanical analyses, and in some cases are too closely related to be useful. The line between them can nowhere be sharply drawn. Indeed, the variation within one class is so great that soils have frequently been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely related to Oxnard sandy loam (26–37–18–12) and to Afton fine sandy loam (28–43–18–8) than to Coral sand (61–29–3–4), Galveston sand (6–91–1–1), or Salt Lake sand (84–15–1–0). Elsinore sandy loam (8–38–35–10) is much nearer to Hanford fine sandy loam (9–36–33–14) than to Billings sandy loam (1–60–22–11) or to Utuado sandy loam (48–23–19–8). The soil types are much more confused, and for ecological purposes at least are entirely valueless. Lake Charles fine sandy loam has the composition, 1–34–52–9; Vernon fine sandy loam, 1–37–54–7, while many other so-called types show nearly the same degree of identity.

117. The chemical nature of soils. The effect of alkaline and acid substances in the soil upon water-content and the activities of the plant is far from being well understood. It is generally recognized that salts and acids tend to inhibit the absorptive power of the root-hairs. In the case of saline soils, this inhibitive effect seems to be established, but the action of acids in bogs and swamps is still an open question. It is probable that the influence of organic acid has been overestimated, and that the curious anomaly of a structural xerophyte in a swamp is to be explained by the stability of the ancestral type and by the law of extremes. Apart from the effect which excessive amounts of acids and salts may have in reducing the chresard, the chemical character of the soil is powerless to produce structural modification in the plant. Since Thurmann’s researches there has been no real support of the contention that the chemical properties of the soil, not its physical nature, are the decisive factors in the distribution and adaptation of plants. It is not sufficient that the vegetation of a silicious soil differs from that of a calcareous one. A soil can modify the plants upon it only though its water-content, or the solutes it contains. Hence, the chemical composition of the original rock is immaterial, except in so far as it modifies these two factors. Humus, moreover, while an important factor in growth, has no formative influence beyond that which it exerts through water-content.

PHYSIOGRAPHY

118. Factors. The physiographic factors of a definite habitat are altitude, exposure, slope, and surface. In addition, topography is a general though less tangible factor of regions, while the dynamic forces of weathering, erosion and sedimentation play a fundamental role in the change of habitats. It is evident, however, that these, except where they affect the destruction of vegetation directly, can operate upon the plant only through more direct factors, such as water, light, and temperature. While they are themselves not susceptible of measurement, they can often be expressed in terms of determinable factors, i. e., slope, exposure, and surface. Fundamentally, they constitute the forces which change one habitat into another, and, in consequence, are really to be considered as the factors which produce succession. The static features of physiography, altitude, etc., lend themselves readily to determination by means of precise instruments. These factors, though by no means negligible, are remote, and consequently their mere measurement is insufficient to indicate the nature or extent of their influence upon the plant. It is necessary to determine also the manner and degree in which they affect other factors, a task yet to be done. Readings of altitude, slope, and exposure are so easily made that the student must carefully avoid the tendency to let them stand at their own value, which is slight. Instead, they should be made the starting point for ascertaining the differences which they produce in water-content, humidity wind, and temperature.

Altitude

119. Analysis into factors. Of all physiographic features, altitude is the most difficult to resolve into simple factors. Because of general geographic relations, it has a certain connection with rainfall, but this is vague and inconstant. Obviously, in its influence upon the plant, altitude is really pressure, and in consequence its effect is exerted upon the climatic and not the edaphic factors of the habitat. Theoretically, the decrease of air pressure in the increased altitude directly affects humidity, light, and temperature. Actually, while there is unquestionably a decrease in the absorption of the light and heat rays owing to the fact that they traverse less atmosphere, which is at the same time less dense, this seems to be negligible. Photometric readings at elevations of 6,000 and 14,000 feet have so far failed to show more than slight differences, which are altogether too small to be efficient. The effect upon humidity is greater, but the degree is uncertain. Continuous psychrographic records at different elevations for a full season, at least, will be necessary to determine this, since the psychrometric readings so far made, while referred to a base psychrograph, are too scattered to be conclusive. Finally, the length of the season, itself a composite, is directly dependent upon the altitude. This relation, though obscure, rests chiefly upon the rarefaction of the air which prevents the accumulation of heat in both the soil and the air.

Fig. 26. Aneroid barometer.

120. The barometer. To secure convenience and accuracy in the determination of altitude, it is necessary to use both a mercurial and an aneroid barometer. The latter is by far the most serviceable for field work, but it requires frequent standardizing by means of the former. The mercurial form is much more accurate and should be read daily in the base station. It is practically impossible to carry it in the field, except in the so-called mountain form, which is of great service in establishing the altitudes of a series of stations. In use the aneroid barometer may be checked daily by the mercurial standard, or it may be set at the altitude of the base station, thus giving a direct reading. After the normal pressure at the base has once been ascertained, however, the most satisfactory method is to set the aneroid each day by the standard, at the same time noting the pressure deviation in feet of elevation (see p. [46]). The absolute elevation of the various stations of a series may be determined either by adding or subtracting this deviation from the actual reading at the station, or by noting the change from the base station, and then adding or subtracting this from the normal of the latter. When it is impossible to check the aneroid by means of a mercurial barometer, the average of a series of readings made at different days at one station, especially if taken during settled weather, will practically eliminate the daily fluctuations, and yield a result essentially accurate. Even in this event, the accuracy of the aneroid should be checked as often as possible, since the mechanism may go wrong at any time. The barograph, while a valuable instrument for base stations, is not at all necessary. These instruments can be obtained from all makers of meteorological apparatus, such as H. J. Green, and J. P. Friez. Aneroid barometers reading to 16,000 feet cost about $20; the price of the Richards aneroid barograph is $45. Ordinary observatory barometers cost $30–$40; the standard instrument sells at $75–$100. The mountain barometer, which is altogether the most serviceable for the ecologist, ranges from $30–$55, depending upon accessories, etc.

Slope

Fig. 27. Mountain barometer: (a) in carrying case; (b) set up for use.

121. Concept. This term is used in the ordinary sense to indicate the relation of the surface of a habitat to the horizon. Although it is a complex of factors, or rather influences several factors, these are readily determinable. The primary effect of slope is seen in the control of run-off and drainage, and consequently of water-content, although these are likewise affected by soil texture and by surface. Slope, moreover, as a concomitant of exposure, has an important bearing upon light and heat by virtue of determining the angle of incidence, and also upon wind, and, through it, upon the distribution of snow. At present, while it can be expressed definitely in degrees, it has not yet been connected quantitatively with more direct factors. This is, however, not a difficult task, and it is probable that we shall soon come to express slope principally in amount of run-off, and of incident heat.

122. The clinometer. In the simplest form, this instrument is merely a semicircle of paper, with each half graduated from 1–90°. It is mounted on a board and placed base upward, upon a wooden strip, 2 feet long and 2 inches wide, which has a true edge. At the center of the circle is attached a line and plummet for reading the perpendicular. A more convenient form is shown in figure 28, which is both clinometer and compass. This also necessitates the use of a basing strip to eliminate the inequalities of the surface. The dial face is graduated to show inches of rise per yard, as well as the number of degrees, but the latter, as the simpler term, is preferable for ecological work. In making a reading, the basing strip is placed upon a representative area of the slope, and pressed down firmly to equalize slight irregularities. The clinometer is moved slightly along the upper edge, causing the marker to swing freely. After the latter comes to rest, the instrument is carefully turned upon its back, when the angle of the slope in degrees may be read directly. Two or three such readings in different areas will suffice for the entire habitat, unless it be extremely irregular. The clinometer with compass may be obtained from the Keuffel and Esser Company, 111 Madison St., Chicago, Illinois, for $5.

Fig. 28. Combined clinometer and compass.

123. The trechometer. For measuring the effect of slope upon run-off, a simple instrument called the trechometer (τρέχω, to run off) has been devised. This consists merely of a metal tank, 3 × 4 × 12 inches, holding 144 cubic inches of water, with an opening ¼ × 12 inches at the base in front, closed by a tight-fitting slide. Three metal strips, 2 × 12 inches, are fastened to the front of the tank in such a way as to enclose a square foot of soil into which the strips penetrate an inch. In the front strip is an opening, 1 inch square, provided with a drip from which the run-off is collected in a measuring vessel. In use, the instrument is put in position with the metal rim forced down 1 inch into the soil; the tank is filled, the graduate put in place, and the slide raised. The run-off for a square foot is the amount of water caught by the graduate, and is represented in cubic inches per square foot. For obtaining results which express slope alone, comparisons must be made upon the same soil, from which all cover, dead and living, has been removed. They must be as closely together in time as possible, at least during the same day, as rain or evaporation will cause considerable error. It is obvious that with the same slope or on a level the trechometer may also be used to advantage to determine the absorptive power of soils of different texture. It serves well a similar purpose when used in different habitats to measure the composite action of slope, soil, and cover in dividing the rainfall into run-off and absorbed water.

Exposure

124. Exposure refers primarily to the direction toward which a slope faces, i. e., its exposition or insolation with respect to sun and wind. It is not altogether separable from slope, however, inasmuch as the angle of the slope has some effect upon the degree of exposure. The chief influence of exposure is exerted through temperature, since slopes longest exposed to the sun’s rays receive the most heat. This is supplemented in an important degree by the fact that a group of rays 1 foot square will occupy this area only on slopes upon which they fall at right angles. In all other cases the rays are spread over a longer area, with a consequent reduction in the amount of heat received. This effect is felt principally in evaporation from the soil, and in soil temperatures. For the leaf, it is largely if not entirely negligible, since the angle of incidence is determined by the position of the leaf, which is the same for each species whether on the level or upon a slope. On this account, exposure has little or no bearing upon light, except that the total amount of light received by the aggregate vegetation of a slope will be greater than for a level area of the same size. The effect of wind varies with the exposure. It is naturally most pronounced in those directions from which the prevailing dry or cold winds blow, and it is greatly emphasized by the fact that the opposite exposure is correspondingly protected. The influence of wind, especially in producing evaporation from the plant and the soil, increases with the slope, since the mutual protection of the plants, or that afforded the soil by the cover, is much reduced. Finally, the distribution of the snow by the wind, a matter of considerable importance for early spring vegetation, is largely determined by exposure.

Exposure is expressed directly in terms of direction, to which is added the angle of the slope. A good field compass, reading to twelve points, is sufficient. It should be checked, of course, by the declination of the needle at the place under observation. A convenient instrument is the one already mentioned, in which compass and clinometer are combined, since these are regularly used at the same time.

125. Surface. The most important consideration with respect to surface is the presence or absence of cover, and the character of the latter. With the exception of snow, cover is, however, a question of vegetation, living and dead, and consequently is to be referred to the discussion of biotic factors. The surface of the soil itself often shows irregularities which must be taken into account. Such are the rocks of boulder and rock fields, the hummocks of meadows and bogs, the mounds of prairie dog towns, the innumerable minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The influence of these is not profound, but they do have an appreciable effect upon the run-off, temperature, and wind. In many cases, this is distinctly measurable, but as a rule little more can be done than to indicate that the surface is even or uneven, and to describe the degree and kind of unevenness.

126. Record of physiographic factors. Altitude, slope, exposure, and surface are essentially constant factors, and are determined once for all, after a few check readings have been made, except in those relatively rare habitats in which dynamic forces are very active. The form of record used is the following:

127. Topography. As heretofore indicated, questions pertaining to the form and development of the land concern groups of habitats within which each habitat is the unit of investigation after the manner already laid down. A knowledge of topography is essential to the accurate mapping of a region, for which the simple methods of plane table and contour work are employed, while the geology of the surface is of primary importance in the study of successions.

BIOTIC FACTORS

128. Influence and importance. Biotic factors are animals and plants. With respect to influence they are usually remote, rarely direct. Nevertheless, they often play a decisive part in the vegetation. Their effect is, as a rule, felt directly by the formation rather than the habitat, but in either case the one reacts upon the other. Such factors are not themselves susceptible of exact measurement, but their influence upon the habitat is usually measurable in terms of the physical factors affected. In the case of biotic factors, it must be distinctly understood that these are not properly factors of the habitat as a physical complex, but that they are rather to be considered as reactions exerted by the effect, or formation, upon the cause or habitat. This is most especially true of plants.

129. Animals. The activities of man fall into two classes: (1) those that destroy vegetation, and (2) those that modify it. There are rare instances also where the work of man has changed a new or already denuded habitat. In the cases where the vegetation is destroyed, the habitat itself is sufficiently changed to permit the effect to be measured by physical factor instruments. Otherwise, the influence is felt only by the formation, as when man makes possible the migration of weeds, and it can be measured in terms of invasion by the quadrat alone. It becomes especially evident, then, in the case of man’s activities, that where they produce a denuded habitat they are to be regarded as factors in the habitat; when they merely affect the formation, this is not strictly true. The changes wrought by other animals are essentially the same as those produced by man. They are not so marked nor so important, but their relation to habitat and formation is the same. As a rule, however, they affect the habitat much less than they do the formation.

130. Plants. As a dead cover, vegetation is a factor of the habitat proper, but it has relatively little importance, since it occurs regularly during the resting period. Its chief effects are in modifying soil temperature, and in holding snow and rain, and thereby increasing the water-content. By its gradual decay, moreover, it not only adds humus to the soil, but it thereby increases the water-retaining capacity of the latter also. The cover of living vegetation reacts upon the habitat in a much more vital fashion, exerting a powerful effect upon every physical factor of the habitat. The factors thus affected are distinctly measurable though it is often impossible to determine just how much of the factor is directly traceable to the vegetation. This is a simple problem in the case of most aerial factors, especially light, but it is extremely difficult for soil factors, such as water-content and soil texture. In the case of all habitats covered with formations, by far the great majority, it is impossible as well as unnecessary to separate the physical factors of the habitat proper from the reaction upon them which the plant covering exerts. Indeed, the great differentiation of habitats is largely due to the universal principle that in vegetation the effect or formation always reacts upon the cause or habitat in such a way as to modify it. As fundamental causes of succession, the discussion of the various reactions of vegetation is reserved for another place.

Methods of Habitat Investigation

131. The use of the various instruments previously described depends largely upon the preponderance of simple instruments or recording ones. The former necessitate a number of well-trained assistants; the latter require only a part of the time of one investigator. For the most satisfactory results, however, an assistant is all but indispensable. Since simple instruments are most easily obtained because of their cheapness, and are especially adapted to purposes of instruction, the method of using them will be described first, and then that of ecograph batteries.

THE METHOD OF SIMPLE INSTRUMENTS

Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation.

132. Choice of stations. This method is based upon simultaneous readings by means of simple instruments in a series of habitats, or of stations in a single habitat. Such readings are necessary for the variable atmospheric factors, humidity, light, temperature, and wind. Frequent readings suffice for water-content and precipitation, while only two or three determinations, enough to check out the error, are necessary for the constant factors, altitude, slope, exposure, and surface. An account of the exact procedure employed in class study at Lincoln and Minnehaha will best serve to illustrate the use of this method. The series of stations chosen at Lincoln were primarily within a single formation, for the purpose of determining the physical factor variation in different areas. One series was located in the prairie-grass formation (Koelera-Andropogon-psilium), and consisted of the following stations: (1) low prairie, (2) crest of ridge I, (3) northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of ridge II, (6) bare crest of ridge II, (7) thicket ravine. The other series was established in the bur-oak-hickory forest (Quercus-Hicoria-hylium) at the following stations: (1) thicket, (2) woodland, (3) knoll in forest, (4) depression in forest, (5) level forest floor, (6) nettle thicket, (7) brook bank. At Minnehaha the series was primarily one of different formations: (1) the pine formation (Pinus-xerohylium), (2) the gravel slide formation (Pseudocymopterus-Mentzelia-chalicium), (3) east slope of spruce forest (Picea-Pseudotsuga-hylium), (4) ridge in the spruce forest, (5) north slope of spruce forest, (6) brook bank in forest, (7) the thicket formation (Quercus-Cercocarpus-lochmodium), (8) the aspen formation (Populus-hylium). When permanent or temporary quadrats are established, they are ordinarily used as regular stations, since this enables one to refer the physical factor readings to a few definite individual plants, as well as to the entire formation. The transects in figure 29 illustrate two of the above series of stations.

133. Time of readings. The frequency of simple readings and the times at which they are made must be regulated largely by opportunity and convenience. In addition to making readings once or twice a week throughout the season, the series should be read at least once every day for a representative week or two. It is also very desirable to have a series for each hour of a typical day, or of two days, one of which is clear, the other cloudy. When a single daily reading is made, it should be taken at or as near meridian as possible. The usual series is the one obtained by simultaneous observations at the same level in different stations. An important series is also secured by simultaneous readings at the various levels of the same station, though it is not necessary to take this series frequently.

Fig. 30. A denuded station in the aspen formation.

134. Details of the method. After the stations have been selected by a careful preliminary survey of the habitat or series of habitats, their location is indicated by a small flag bearing a number, in case there is no danger of these being disturbed. Otherwise, less conspicuous stakes are used. The ordinary practice is to visit each station of the series, and to take readings of water-content, altitude, slope, and exposure. On the first trip these are all made by the instructor, but after a short time the determination of each factor may be assigned in rotation to each of the students. After these constant factors have been read and recorded, one student is equipped with photometer, thermometer, and psychrometer, and, if desirable, anemometer, and left at the first station. At each succeeding station the same plan is followed, so that at the end of the series the constant factors have all been read, and there is an observer at each station prepared to make readings of the variable ones. The task of acquainting the students with the operation of photometer, psychrometer, etc., can best be done in class or at a previous field period, as it is evident that they must be familiar with the instruments before they can use them accurately in the field series. The details of operation have already been given and need not be repeated here. The task of obtaining readings at the same moment may be met by supplying each observer with a watch, which runs exactly with all the others, or by making observations upon signal. The second means has been found most successful in practice, since the signal fixes the attention at the exact moment. The best plan is for the instructor to occupy a commanding position somewhere near the middle of the series, and to give the signals by shout or whistle at the proper interval. Considerable care and experience are necessary to do the last satisfactorily. Sufficient time must be given for the operation of the instrument and the making of the record. In addition, a period must be permitted to elapse which is long enough for every instrument to reach the proper reading. For example, in a series which contains a gravel slide and a forest, the thermometer which has just been used for an air reading will require four or five times as long an interval to respond to the temperature of the gravel as to that of the cool forest floor. In such series, the instructor should regularly take his place in the station where the response is slowest or greatest. He must record the exact time of each signal, and note any general changes of sky or wind that produce temporary fluctuations at the time of reading. When the readings extend over a whole day, the usual plan is to begin at the last station and take a second series of water-content samples, noting the exact time in order that the rate of water loss may be determined. A check series of physiographic factors may be made at this time also, or this may be left for future visits. While it is unnecessary to take soil samples oftener than once a day, it is important to make at least one series at each visit. Sometimes it becomes desirable to know the rate of water loss in different stations during the day, and in this event, samples are taken at one or two hour intervals for the entire day.

In making simultaneous readings at the different levels of one station, the observers are grouped in one spot in such a way that they do not interfere with the correct reading of each instrument. Readings of this sort are most valuable in the case of temperature, which shows greater differences at the various levels. Important differences of humidity and wind also are readily obtained, and, in layered formations, marked variations in the amount of light. In the open, the ordinary levels for temperature are 6 feet, 3 feet, surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6 feet, 3 feet, and surface. In forests the same levels are used for comparison with formations in the open, but a more desirable series for light especially is secured by making readings at the height of, or better, just below the various layers. Series of this sort are likewise made on signal. The best time of day is that of a period in which the middle station is read near meridian, since the variation due to time is sufficiently small to permit fairly accurate comparisons between the readings for the different stations.

135. Records. The form used for recording the observations made by means of simple instruments is shown below. It is hardly necessary to state that it may be readily modified to suit the needs of different investigators. Ordinarily, each sheet is used for the records of one habitat or series alone, but for convenience sake, the records of two different series are here combined. The figures given are taken from records for the prairie and forest formations at Lincoln.