Research methods in ecology

by Frederic E. Clements

The amount of water absorbed may be obtained directly by subtracting the final weight of the jar and moist soil from their first weight, but a desirable check is obtained by taking the dry weight of jar and soil from the first, and the final weight of these, and subtracting the one from the other as indicated in the table. A second check is afforded by daily weighings, from which the amount of water transpired is determined. Since the two sunflower plants made practically no growth during the period of experiment, the exact correspondence between water absorbed and water lost is not startling, though it can not be expected that the results will always coincide.

This method has certain slight sources of error, all of which, it is thought, have been corrected in a new and more complete series of experiments now being carried on. The aeration of the soil is not entirely normal, as is also true of the capillary movements of the water, on account of the nonporous glass jar and the rubber cloth. Since the latter are necessary conditions of all accurate methods for measuring absorption and transpiration, the resulting error must be ignored. It can be reduced, however, by forcing air through the thistle tube from time to time. Sturdy plants, such as the sunflower, are the most satisfactory, since they recover more quickly from the shock of transplanting. Almost any plant can be used, however, if repotted in a loose sandy soil often enough. This permits the root system to develop normally, and also makes it possible to wash the soil away without injury to the root. The method is so recent that there has been no opportunity to test it in the field. It would seem that it can be applied without essential change to plants in their normal habitats. Very large herbs or plants with extensive root systems could not be used to advantage, and to be practicable the experiments would need to be carried on near the base station. The great value of the method, however, lies in its use as a check in determining the accuracy of other methods, and in practice it will often be found convenient and time-saving to use the latter, after they have once been carefully checked for different groups of species. This matter is further considered under measures of transpiration.

Fig. 31. Absorption and transpiration of Helianthus annuus. I and II, plants repotted in soil of known weight and water-content; III, plant undisturbed in the original soil; IV, potometer containing plant with cut stem; V, potometer with entire plant.

156. The quantitative relation of absorption and transpiration. Burgerstein[^[11]] has summarized the results of various investigators in the statement “that between the quantitative absorption of water on the one hand and emission on the other there exists no constant parallelism or proportion,” and he has cited the work of Kröber, and of Eberdt in proof. This statement holds, however, only for short periods of a few hours, or more rarely, a day, and even here its truth still remains to be conclusively demonstrated. The discrepancy between absorption and transpiration for a short period is often greater than for a longer time, but it is evident that a transient change in behavior or a small error in the method would inevitably produce this result. Eberdt found the discrepancy for a few hours to be 1–2 ccm. in an entire plant of Helianthus annuus, while for a whole day the water absorbed was 33.57 ccm. and the water lost 33.98 ccm. Kröber’s experiments with cut branches of Asclepias incarnata showed a maximum difference for 12 hours of 2.5 ccm., but the discrepancy for the first 24 hours was 1 ccm. and for the second 1.9 ccm. In both cases, the potometer was employed. Consequently, as will be shown later, Eberdt’s results are not entirely trustworthy, while those of Kröber, made with cut stems, are altogether unreliable. Hence, it is clear that the discrepancy is slight for a period of several days or weeks, and that it may be ignored without serious error, except in a few plants that retain considerable water as cell-sap, in consequence of extremely rapid growth. Accordingly, the amount of transpiration, which may be readily and accurately determined, can be employed as a measure of absorption that is sufficiently accurate for nearly all purposes. The truth of this statement may be easily confirmed. It is evident that the amount of water absorbed equals the amount transpired plus that retained by the plant as cell-sap, or used in the manufacture of organic compounds. In plants not actively growing, the amount lost equals that absorbed, as already shown in the experiment with Helianthus. According to Gain[^[12]], Dehérain has found that a plant rooted in ordinary soil transpired 680 kg. of water for each kilogram of dry substance elaborated. In Helianthus annuus, the dry matter is 10 per cent of the weight of the green plant. A well-grown plant weighing 1,000 grams, therefore, consists of 100 grams of dry matter and 900 of water. The length of the growing period for such a plant is approximately 100 days, during which it transpires 68 kilograms of water. Assuming the rate of transpiration and of growth to be constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap, and 1 gram to its dry weight. The amount of water in a gram of cellulose and its isomers is about ⅗. Consequently, the total water absorbed daily by the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent of the amount absorbed; in other words, only 1.4 per cent of the water absorbed is retained by the plant. From this it is evident that the simplest and most convenient measure of absorption under normal conditions can be obtained through transpiration, since the discrepancy between absorption and transpiration is scarcely larger than the error of any method applicable to the field. Conversely, the measure of absorption obtained by the process described in the preceding section serves also as a measure of transpiration. The determination of the latter in the field is so much simpler, however, that it is rarely desirable to apply the absorption method.

157. Measurement of transpiration. The water loss of a plant may be determined absolutely or relatively. Absolute or quantitative determinations are by (1) weighing, (2) collecting, or (3) measuring the water absorbed; relative values are indicated by hygroscopic substances. A number of methods have been employed more or less generally for measuring transpiration. The great majority of these can be used to advantage only in the laboratory, and practically all fail to meet the fundamental requirement for successful field work, namely, that the plant be studied under normal conditions in its own habitat. The following is a summary of the various methods, the details of which may be found in Burgerstein.

1. Weighing. This is the most satisfactory of all methods for determining water loss. It is more accurate than any other, and is unique in that it does not place the plant under abnormal conditions. On the score of convenience, moreover, it excels every other method capable of yielding quantitative results. Various modifications of weighing are employed, but none of these have all the advantages of a direct, simple weighing of the plant in its own soil.

2. Collecting the water transpired. This may be done by collecting and weighing the water vapor exhaled by a plant placed within a bell jar, or by weighing a deliquescent salt, such as calcium chloride, which is used to absorb the water of transpiration. The decisive disadvantage of these methods is that transpiration is carried on in an atmosphere far more humid than normal. If an excessive amount of salt is used, the air is abnormally dry. In both cases, the water loss decreases until it reaches a point much below the usual amount. Finally, all methods of this kind are open to considerable error, and are inconvenient, especially in field work. They are of relatively slight value in comparison with weighing.

3. Potometers. It has already been shown that the amount of water absorbed is a close measure of the amount transpired. In consequence, the potometer can be used to determine the amount of transpiration provided the absorption is not abnormal. It is rarely and only with much difficulty that this condition can be met. The use of cut stems and branches does not meet it, and even in the case of plants with roots, the results must be compared with those obtained from absorption experiments made with plants rooted in soil before they can be relied upon. This necessity practically puts the potometer out of commission for accurate work, unless future study may show a somewhat constant ratio between the absorption of a plant in its own soil and that of a plant placed in a potometer.

4. Measuring absolute humidity. The cog psychrometer makes it possible to determine the increased relative humidity produced within a glass cylinder or special tin chamber by a transpiring plant. From this result the absolute humidity is readily obtained, and by means of the latter the actual amount of water given off. The evident drawback to this method is that the increasing humidity within the chamber gives results entirely abnormal for the plant concerned.