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.
HYDROHARMOSE
ADJUSTMENT
152. Water as a stimulus. Plants are continually subjected to the action of the water of the soil and of the air; exception must naturally be made of submerged plants. The stimulus of soil water acts upon the absorbing organ, the root, while that of humidity affects the part most exposed to the air, viz., the assimilative organ, which is normally the leaf. But since both are simultaneous water stimuli, a clearer conception is gained of this operation if they are viewed as two phases of the same stimulus. This point of view receives further warrant from the essential and intimate relation of humidity and water-content as determined by the plant. They are in fact largely compensatory, as is shown at some length later. In determining the intensity of the two, a significant difference between them must be recognized. The total humidity of the air at any one time constitutes a stimulus to the leaf which it touches. This is not true of the total soil water. Part of the latter is not available under any circumstances, and can not affect the plant, at least directly. The chresard alone can act as a stimulus, but even this is potential in the great majority of cases, since the actual stimulus is not the water available but the water absorbed. The latter, moreover, contains many nutrient salts which are in themselves stimuli, but as they normally have little bearing upon the action of water as a stimulus they are to be considered only when present in excessive amounts.
153. The influence of other factors upon water. The amount of humidity is modified directly by temperature, wind, precipitation, and pressure, and, through these, it is affected by altitude, slope, exposure, and cover. Naturally, also, the evaporation of soil water has a marked influence. In determining water-content, atmospheric factors, with the exception of precipitation, are usually subordinate to edaphic ones. Soil texture, slope, and precipitation act directly in determining soil water, while temperature, wind, and pressure can operate only through humidity. This is likewise true of altitude, exposure, and cover, though the latter has in addition a profound effect upon run-off. Biotic factors can affect humidity or water-content only through the medium of another factor. Light in itself has no action upon either, but through its conversion into heat within the chloroplast, it has a profound effect upon transpiration. The following table indicates the general relation between water and the other physical factors of the habitat. The order of the signs, ±, denotes that the water increases and decreases with an increase and decrease of the factor, or the reverse, ∓.
| Humidity ± | Water-content ± |
|---|---|
| Temperature ∓ | Temperature ∓ |
| Wind ∓ | Wind ∓ |
| Precipitation ± | Precipitation ± |
| Pressure ± | Pressure ∓ |
| Soil texture 0 | Soil texture |
| Altitude ∓ | Porosity ∓ |
| Capillarity ± | |
| Slope ∓ | Slope ∓ |
| Exposure ∓ | Exposure ∓ |
| Cover ± | Cover ± |
154. Response. The normal functional responses to water stimuli are absorption, diffusion, transport, and transpiration. Of these, absorption and transpiration alone are the immediate response to soil water and humidity, respectively. Consequently they are the critical points of attack in studying the fundamental relation of the plant to the water of its habitat. In determining the pathway of the response, it is necessary to trace the steps in diffusion and transport, but, as these are essentially alike for all vascular plants, this task lies outside the scope of the work in hand. As previously suggested, the relation between absorption and transpiration is strictly compensatory, though, for obvious reasons, the amount of water transpired is usually somewhat less than the amount absorbed. Absorption falls below transpiration when extreme conditions cause temporary or permanent wilting; the two activities are essentially equal after a growing plant reaches maturity. In all cases, however, the rule is that an increase or decrease in water loss produces a corresponding change in the amount of water absorbed, and, conversely, variation in absorption produces a consequent change in transpiration. This is strictly true only when the stimuli are normal. For example, a decrease in humidity causes increased water loss, which, through diffusion and transport, is compensated by increased activity of the root surface. Frequently the water supply is insufficient to compensate for a greater stimulus, and the proper balance can be attained only by the closing of the stomata. In the case of excessive stimuli, neither compensation suffices, and the plant dies. Many mesophytes and all xerophytes have probably resulted from stimuli which regularly approached the limit of compensation for each, and often overstepped, but never permanently exceeded it. For hydrophytes, the danger arises from excessive water supply, not water loss. There is a limit to the compensation afforded by transpiration, which is naturally dependent upon the amount of plant surface exposed to the air. No compensation occurs in the case of submerged plants; floating hydrophytes possess a single transpiring leaf surface, while the leaves of amphibious plants behave as do those of mesophytes. The whole question of response to water stimuli thus turns upon the compensation for water loss afforded by water supply where the latter is moderate or precarious, and upon the compensation for water supply furnished by water loss where the supply is excessive, submerged plants excepted.
155. The measurement of absorption. As responses to measured stimuli of water-content and humidity, it is imperative that the amount of absorption and of transpiration be determined quantitatively. It is also extremely desirable that this be done in the normal habitat of the plant. A careful examination of the problems to be met quickly discloses the great difficulty of obtaining a direct and accurate measure of absorption under normal conditions, especially in the field. For this purpose, the ordinary potometric experiments by means of cut stems are valueless. The use of the entire plant in a potometer yields much more trustworthy results, though the fact that the root is under abnormal conditions can not be overlooked, especially in the case of mesophytes and xerophytes. While potometric conditions are less abnormal for amphibious plants, the error is not wholly eliminated, since the roots normally grow in the soil. The potometer can be made of value for quantitative work only by checking the results it gives by means of an instrument or a method in which the plant functions normally. In consequence, the potometer can not at present be used to measure absorption directly, though, as is further indicated in the discussion of transpiration, it is a valuable supplementary instrument, after the check mentioned has been applied to its use with a particular species.
An estimate of the amount of absorption may be obtained either in the field or in the control house by taking samples from the protected soil at different times. Since it is impossible to determine the weight of the area in which the roots lie, and since the soil water is often unequally distributed, this method can not yield exact results. An accurate method of measuring absorption under essentially normal conditions has been devised and tested in the control house. The essential feature of the process is the placing a plant in a soil containing a known quantity of water, and removing it after it has absorbed water from the soil for a certain period. In carrying out the experiment, a soil consisting of two parts of sod and one of sand was used, since the aeration is more perfect and the particles are more easily removed from the roots. The soil was completely dried out in a water bath and then placed in a five-inch battery jar. The latter, together with the rubber cloth used later to prevent evaporation, was weighed to the decigram. A weighed quantity of water was added, and the whole again weighed as a check. Two plants of Helianthus annuus were taken from the pots in which they had grown, and the soil was carefully washed from the roots. Each plant was weighed with its roots in a dish of water to prevent wilting, and then carefully potted, one in each battery jar. A thistle tube was placed in the soil of each jar to facilitate aeration, as well as the addition of weighed amounts of water, when necessary, and the rubber cloth attached in the usual manner to prevent evaporation. The entire outfit was weighed again, and the weighing repeated at 8:00 A.M. and 5:00 P.M. for five days, in order to determine the amount of transpiration and its relation to the water absorbed. The plants were kept in diffuse light to prevent excessive water loss while the roots were becoming established. At the close of the experiment, the jar and its contents were weighed finally. The plants were removed and weighed, the soil particles being shaken from the roots into the jar, which was also weighed. The results obtained were as follows:
| Wt. of pot and dry soil | Wt. of pot and wet soil | Total H2O | H2O left | H2O absorbed | H2O transpired | ||
|---|---|---|---|---|---|---|---|
| I | II | ||||||
| I | 1846.0 g. | 2218.0 g. | 2174.3 g. | 372.0 g. | 328.3 g. | 43.7 g. | 43.7 g. |
| II | 1886.7 g. | 2253.2 g. | 2221.6 g. | 366.5 g. | 334.9 g. | 31.6 g. | 31.6 g. |