GENERAL RELATIONS
144. The nature of stimuli. Whatever produces a change in the functions of a plant is a stimulus. The latter may be a force or a material; it may be imponderable or ponderable; effect, not character, determines a stimulus. Consequently, reaction or response decides what constitutes a stimulus. The presence of the latter can be recognized only through an appreciable or visible response, since it is impossible to discriminate between an impact which produces no reaction and one which produces a merely latent one. From this it is evident that quantity is decisive in determining whether the impact becomes a stimulus. Plants grow constantly under the influence of many stimuli, all varying from time to time in amount. Small changes in these are so frequent that, in many cases at least, the plant no longer appreciably reacts to them. Such changes, though usually measurable, are not stimuli. Furthermore, it must be clearly recognized that plants which are in constant response to stimuli are stimulated anew by an efficient increase or decrease in the amount of any one of these. As is well known, however, such increase or decrease is a stimulus only within certain limits, and the degree of change necessary to produce a response depends upon the amount of the factor normally present. The entire absence of a force usually present, moreover, often constitutes a stimulus, as is evident in the case of light. The nature of the plant itself has a profound bearing upon the factors that act as stimuli. Many species are extremely labile, and react strongly to relatively slight stimuli; others are correspondingly stable, and respond only to stimuli of much greater force. Some light is thrown upon the nature of this difference by the behavior of ecads. A form which has grown under comparatively uniform conditions for a long time seems to respond less readily, and is therefore less labile than one which is subject to constant fluctuation. In many cases this is not true, however, and the degree of stability, i. e., of response, can only be connected in a general way with taxonomic position.
145. The kinds of stimuli. The factors of a habitat are external to the plant, and consequently are termed external stimuli. Properly speaking, all stimuli are external, but since the response is often delayed or can not be clearly traced, it may be permissible to speak of internal stimuli, i. e., those which appear to originate within the plant. These, however, are extremely obscure, and it is hardly possible to deal with them until much more is known of the action of external stimuli. Of the latter, certain forces, gravity and polarity, act in a way not at all understood, and as they are essentially alike for all plants and all habitats, they can here be ignored. Stimuli are imponderable when, like light and heat, they are measured with reference to intensity, and ponderable, when, as in the case of water-content, humidity, and salt-content, they can be expressed in mass or weight. It is undesirable to insist upon this distinction, however, since the real character of a stimulus is determined by its effect, and the latter is not necessarily dependent upon whether the stimulus is one of force or one of material. There is, however, a fundamental difference between factors with respect to their relation to the plant. Direct factors alone are stimuli, since indirect factors must always act through them. For example, the wind, its mechanical influence excepted, can affect the plant only in so far as it is converted into the stimulus of increased or decreased humidity. Consequently, the normal stimuli of the plants of a formation are: (1) water-content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. Soil, pressure, physiography, and biotic factors influence plants only through these, and are not stimuli, though exceptions must be made of biotic factors in the case of sensitive, insectivorous, and gall-producing plants.
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.
151. The method of working hypotheses. In the study of stimulus and response, where the unimpeachable facts are relatively few, and their present correlation slight, the working hypothesis is an indispensable aid. “The true course of inductive procedure ... consists in anticipating nature, in the sense of forming hypotheses as to the laws which are probably in operation, and then observing whether the combinations of phenomena are such as would follow from the laws supposed. The investigator begins with facts and ends with them. He uses such facts as are in the first place known to him in suggesting probable hypotheses; deducing other facts which would happen if a particular hypothesis is true, he proceeds to test the truth of his notion by fresh observations or experiments. If any result prove different from what he expects, it leads him either to abandon or to modify his hypothesis; but every new fact may give some new suggestion as to the laws in action. Even if the result in any case agrees with his anticipations, he does not regard it as finally confirmatory of his theory, but proceeds to test the truth of the theory by new deductions and new trials.”[[10]] In the treatment of adjustment and adaptation which follows, the method of multiple working hypotheses is uniformly employed. No apology is felt to be necessary for this, since the whole endeavor is to indicate the proper points of attack, and not to distinguish between that which is conjectural and that which is known. If an hypothesis occasionally seem to be stated too strongly, it is merely that it appears, after a survey of the problem from all sides, to explain the facts most satisfactorily. The final proof of any hypothesis, however, rests not only upon its ability to explain all the facts, but also upon the inability of other hypotheses to meet the same test. The discovery and examination of all possible hypotheses, and the elimination of those that prove inadequate are the essential steps in the method of working hypotheses.