Research methods in ecology

by Frederic E. Clements

146. The nature of response. Since plants have no special organs for the perception of stimuli, nor sensory tracts for their transmission, an external stimulus acting upon a plant organ is ordinarily converted into a response at once. The latter as a rule becomes evident immediately; in some cases it is latent or imperceptible, or some time elapses before the chain of responses finds visible expression. A marked decrease in humidity calls forth an immediate increase of transpiration, but the ultimate response is seen in the closing of the stomata. A response to decreased light intensity, on the other hand, is much less rapid and obvious. This difference is largely due to the fact that the functional response is more marked, or at least more perceptible in one case than in the other.

Response is the reaction of the plant to a stimulus; it begins with the impact of an efficient factor, and ends only with the consequent final readjustment. The immediate reaction is always functional. The nature and intensity of the stimulus determine whether this functional response is followed by a corresponding change in structure. The consideration of this theme consequently gains in clearness if a functional and a structural response be distinguished. The chief value of this distinction lies in the fact that many reactions are functional alone; it serves also to emphasize the absolute interdependence of structure and function, and the imperative need of considering both in connection with the common stimulus. For these reasons, the logical treatment is to connect each stimulus with its proper functional change, and, through this, with the corresponding modification of structure. For the sake of convenience, the term adjustment is used to denote response in function, and adaptation, to indicate the response in structure.

147. Adjustment and adaptation. The adjustment of a plant to the stimuli of its habitat is a constant process. It is the daily task, seen in nutrition and growth. So long as these take place under stimulation by factors which fall within the normal variation of the habitat, the problems belong to what has long been called physiology. When the stimuli become unusual in degree or in kind, by a change of habitat or a modification in it, adjustment is of much greater moment and is recorded in the plant’s structure. These structural records are the foundation of proper ecological study. Since they are the direct result of adjustment, however, this affords further evidence that a division of the field into ecology and physiology is illogical and superficial. Slight or periodical adjustment may concern function alone; it may be expressed in the movement of parts or organs, such as the closing of stomata or changes in the position of leaves, in growth, or in modifications of structure. This expression is fundamentally affected by the nature of the factor and is in direct relation to the intensity of the latter. Adaptation comprises all structural changes resulting from adjustment. It includes both growth and modification. The latter is merely growth in response to unusual stimuli, but this fact is the real clue to all evolution. Growth is periodic, and in a sense quantitative; it results from the normal continuous adjustment of the plant to the stimuli of its proper habitat. In contrast, modification is relatively permanent and qualitative; it is the response to stimuli unusual in kind or intensity. A definite knowledge of the processes of growth is indispensable to an understanding of modification. In the fundamental task of connecting plant and habitat, it is the modification of the plant, and not its growth, which records the significant responses to stimuli. For this reason the discussion of adaptation in the pages that follow is practically confined to modification of structure. This is particularly desirable, since growth has long been the theme of physiological study, while modification has too often been considered from the structural standpoint alone. The comparatively few studies that have taken function into account have been largely empirical; in them neither stimulus nor adaptation has received anything approaching adequate treatment.

148. The measurement of response. The amount of response to a stimulus is proportional to the intensity of the factor concerned. This does not mean that the same stimulus produces the same response in two distinct species, or necessarily in two plants of one species. In these cases the rule holds only when the plants or species are equally plastic. For each individual, however, this quantitative correspondence of stimulus and response is fundamental. It is uncertain whether an exact or constant ratio can be established between factor and function; the answer to this must await the general use of quantitative methods. There can be no doubt, however, that within certain limits the adjustment is proportional to the amount of stimulus, whereas reaction is well known to be abnormal or inhibited beyond certain extremes. It is quite erroneous to think that reaction is independent of quantity of stimulus, or to liken the stimulating factor to “the smallest spark (which) by igniting a mass of powder, produces an enormous mechanical effect.”[^[8]] Such a statement is only apparently true of the action of mechanical stimuli upon the few plants that may properly be said to possess irritability, such as sensitive plants and certain insectivorous ones. Of the normal relation of response to direct factors, water, light, etc., it is entirely untrue. Axiomatically, there is ordinarily an essential correspondence, also, between the amount of adjustment and of adaptation. This correspondence is profoundly affected, however, by the structural stability of the plant.

From the preceding it follows that the measurement of response and the relating it to definite amounts of direct factors as stimuli are two of the most fundamental tasks of ecology. The exact determination of physical factors has no value apart from its use for this purpose. It is perfectly clear that precise methods of measuring stimuli call for similar methods in determining the amount of adjustment and of adaptation. The problem is a difficult one, and it is possible at present only to indicate the direction which its development should take, and to describe a few methods which will at least serve as a beginning. To cover the ground adequately it is necessary to measure response by adjustment and by adaptation separately, and in the latter to find a measure for the individual and one for the species. The one is furnished by the methods of morphology and the other by biometry.

A primary requisite for any method for measuring adjustment is that it be applicable to field conditions. Many instruments for measuring transpiration, for example, are valueless, not because they are inaccurate, but because the plant studied is under abnormal conditions. To avoid the latter is absolutely necessary, a fact which makes it peculiarly difficult to devise a satisfactory field method. After the latter has been found and applied, it becomes possible to check other methods by it, and to give them real value. The final test of a field method is three-fold: (1) the plant must be studied while functioning normally in its own habitat; (2) the method must give accurate results; and (3) it must permit of extensive and fairly convenient application in the field. Until methods of this character, some of which are described later, have been employed for some time, it is impossible to connect definite intensities of factor stimuli with measured amounts of adjustment. Ultimately, it seems certain that researches will regularly take this form.

Adaptation is primarily indicated by changes in the arrangement and character of the cells of the plant. Since these determine the form of each organ, morphology also furnishes important evidence in regard to the course of adaptation, but form can be connected certainly with adjustment only through the study of cellular adaptation. In tracing the modifications of cell and of tissue, the usual methods of histology, viz., sectioning and drawing, suffice for the individual. It is merely necessary to select plants and organs which are as nearly typical as can be determined. The question of quantity becomes paramount, however, since it often gives the clue to qualitative changes, and hence it is imperative that complete and accurate measurements of cells, tissues, and organs be made. These measurements, when extended to a sufficiently large number of plants, serve to indicate the direction of adaptation in the species. They constitute the materials for determining biometrically the mean of adaptation for the species and the probable evolution of the latter. In its present development, biometry contains too much mathematics, and too little biology. This has perhaps been unavoidable, but it is to be hoped that the future will bring about a wise sifting of methods, which will make biometry the ready and invaluable servant of all serious students of experimental evolution. This condition does not obtain at present, and in consequence it seems unwise to consider the subject of biometry in this treatise.

149. Plasticity and fixity. As the product of accumulated responses, each species is characterized by a certain ability or inability to react to stimuli. Many facts seem to indicate that the degree of stability is connected with the length of time during which the species is acted upon by the same stimuli. It seems probable that plants which have reacted to sunlight for hundreds of years will respond less readily to shade than those which have grown in the sun for a much shorter period. This hypothesis is not susceptible of proof in nature because it is ordinarily impossible to distinguish species upon the basis of the time during which they have occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained only by field and control experiments, in which the time of occupation of any habitat is definitely known. Even in this case, however, it is clear that antecedent habitats will have left effects which can neither be traced nor ignored. Additional support is given this view by the fact that extreme types, both ecological and taxonomic, are the most stable. Intense xerophytes and hydrophytes are much more fixed than mesophytes, though the intensity of the stimulus has doubtless as great an influence as its duration. Composites, labiates, grasses, orchids, etc., are less plastic than ranals, rosals, etc., but there are many exceptions to the apparent rule that fixity increases with taxonomic complexity. At present it seems quite impossible to suggest an explanation of the rule. Recent experiments indicate that there may be ancestral fixity of function, as well as of structure. It has been found, for example, that the flowers of certain species always react normally to the stimuli which produce opening and closing, while others make extremely erratic response. If further work confirms this result and extends it to other functions, the necessity of arriving at a better understanding of fixity will be greatly emphasized.

It is impossible to make progress in the study of adaptation without recognizing the fundamental importance of ancestral fixity as a factor. E. S. Clements[^[9]] has shown that a number of species undergo pronounced changes in habitat without showing appreciable modification. Consequently, it is incorrect to assume that each habitat puts a structural impress upon every plant that enters it. For this reason, the writer feels that the current explanation of xerophytic bog plants, etc., is probably wrong, and that the discrepancy between the nature of the habitat and the structure of the plant is to be explained by the persistence of a fixed ancestral type. The anomaly is scarcely greater than in cases that have proved capable of being explained.

150. The law of extremes. When a stimulus approaches either the maximum or minimum of the factor for the species concerned, response becomes abnormal. The resulting modifications approach each other and in some respects at least become similar. Such effects are found chiefly in growth, but they occur to some degree in structure also. It is imperative that they be recognized in nature as well as in field and control experiment, since they directly affect the ratio between response and stimulus. The data which bear upon the similarity of response to extremes of different factors are too meager to permit the formulation of a rule. It is permissible, however, to suggest the general principle that extreme stimuli produce similar growth responses, and to emphasize the need of testing its application to adaptation proper.