5. Self-registering instruments. There are various methods for registering the amount of transpiration, based upon weighing, or upon the potometer. The Richard recording evaporimeter has all the advantages of weighing, inasmuch as the water loss is measured in this way, and in addition the amount is recorded upon a revolving drum, obviating the necessity of repeated attention in case it is desirable to know the exact course of transpiration. On the other hand, methods which depend upon the potometer, while graphic, are not sufficiently accurate to be of value.
6. The use of hygroscopic materials. Hygroscopic substances change their form or color in response to moisture. As they indicate comparative water loss alone, they are of value chiefly in the study of the stomatic surfaces of leaves. F. Darwin[[13]] has used strips of horn, awns of Stipa, and epidermis of Yucca to construct small hygroscopes for this purpose. In these instruments the error is large, but as no endeavor is made to obtain exact results, it is negligible. Filter paper impregnated with a 3–5 per cent aqueous solution of cobalt chloride is deep blue when dry. If a strip of cobalt paper is placed upon a leaf and covered with a glass slip it turns bright rose color, the rapidity of the change affording a clue to the amount of transpiration.
158. Field methods. The conditions which a satisfactory field method of measuring transpiration must fulfill have already been discussed; they are accuracy, simplicity, and normality. These conditions are met only by weighing the plant in its own soil and habitat. This has been accomplished by means of the sheet-iron soil box, already described under the determination of the chresard. The method is merely the familiar one of pot and balance, slightly modified for field use. The soil block, which contains the plant to be studied, is cut out, and the metal plates put in position as indicated in section 53. Indeed, it is a great saving of time and effort to determine transpiration and chresard in the same experiment; this is particularly desirable in view of the close connection between them. In this event, the soil block must be small enough not to exceed the load of a field balance. After the block is cut and encased, all the plants are removed, except the one to be studied. If several individuals of the same species are present, it is an advantage to leave all of them, since the error arising from individual variations of water loss may, in this way, be almost completely eliminated. A sheet of rubber or rubber cloth is carefully tied over the box to prevent evaporation from the soil. A broad band is passed under the box to aid in lifting it upon the scales. The latter must be of the platform type, and should have a capacity as great as consistent with the need for moving it about in the field. Weighings are made in the usual way, care being taken to free the surface of the box from soil. The aeration of the soil block is kept normal by removing the rubber for a few minutes from time to time, or by forcing air through a thistle tube. Water is also added through the latter, when it is desired to continue the experiment for a considerable period. After the study of transpiration is concluded, the rubber cloth is removed, soil samples taken, and the soil allowed to dry out until the plant becomes thoroughly wilted. If the box is weighed again, the difference represents the amount of available water. The per cent of chresard is also obtained in the usual way by taking samples for ascertaining the echard, and subtracting this from the holard. Field determinations of water loss yield the most valuable results when different habitat forms, or ecads, of the same species are used. There is little profit in comparing the transpiration of a typical sun plant, such as Touterea multiflora, with that of a shade plant, such as Washingtonia obtusa. But the simultaneous study of plants like Chamaenerium angustifolium, Gentiana acuta, Scutellaria brittonii etc., which grow in several different habitats, furnishes direct and fundamental evidence of the course of adjustment and adaptation.
Hesselmann[[14]], in his study of open woodlands in Sweden, has employed a method essentially similar to the preceding. Young plants of various species were transferred to pots in the field, where they were allowed to grow for several months before a series of weighings was made to determine the amount of transpiration. Since weighing is the measure used in each, both methods are equally accurate. The one has a certain advantage in that the pots are, perhaps, more easily handled, while the other has the advantage of maintaining the normal relation of soil and roots, a condition more or less impossible in a pot. In both instances the weighing should be done in the habitat, which was not the case in Hesselmann’s researches.
The slight value of the potometer, which has had a vogue far beyond its merits, is indicated by the following table. These results were obtained from three plants of Helianthus annuus; III was left undisturbed in the pot where it had been growing, IV was placed in a potometer, after the root had been cut off, and V was an entire plant placed in a potometer. The amount of transpiration is indicated in grams per square decimeter of leaf surface. The plants were kept in diffuse light, except for a period of two hours (8:00 to 10:00 A.M.) on the last day, when they were in full sunshine at a temperature of 75° F. Plant IV wilted so promptly in the sunshine that it was found necessary to conclude the experiment in diffuse light.
| 8 A.M. | 5 P.M. | 8 A.M. | 5 P.M. | 8 A.M. | 5 P.M. | 8 A.M. | 10 A.M. | 5 P.M. | 8 A.M. | Total | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| III | 2.9 | 7.3 | 2.4 | 6.0 | 1.7 | 1.6 | 2.0 | 3.4 | 2.0 | 1.8 | 31.1 |
| IV | 4.7 | 7.2 | 2.9 | 2.3 | 1.0 | 0.6 | 0.9 | 0.5 | 0.5 | 0.4 | 21.0 |
| V | 3.7 | 5.3 | 3.2 | 4.8 | 2.5 | 1.6 | 3.0 | 2.6 | 1.6 | 2.6 | 30.9 |
The cut plant, IV, lost more water the first day than either of the others, but the water loss soon decreased, and at the end of the period was almost nil. The total transpiration for III and V is much the same, but the range of variation for periods of 12 hours is from +2 to –1 gram. This experiment is taken as a fair warrant that the use of cut stems in potometers can not give accurate results. It is inconclusive, however, as to the merits of potometric values obtained by means of the entire plant, and further studies are now being made with reference to this point.
159. Expression of results. From the previous discussion of the relation between them, it follows that an expression of the amount of transpiration likewise constitutes an expression of absorption. It is very desirable also that the latter be based upon root surface and chresard, but the difficulty of determining the former accurately and readily is at present too great to make such a basis practicable. In expressing transpiration in exact terms, the fact that plants of the same species or form are somewhat individual in their behavior must be constantly reckoned with. In consequence, experiments should be made upon two or three individuals whenever possible, in order to avoid the error arising from this source.
Water loss may be expressed either in terms of transpiring surface or of dry weight. Since there is no constant relation between surface and weight, the terms are not interchangeable or comparable, and in practice it is necessary to use one to the exclusion of the other. Obviously, surface furnishes by far the best basis, on account of its intimate connection with stomata and air-spaces, a conclusion which Burgerstein (l. c., p. [6]) has shown by experiment to be true. For the best results, the whole transpiring surface should be determined. This is especially necessary in making comparisons of different species. In those studies which are of the greatest value, viz., ecads of the same species, it is scarcely desirable to measure stem and petiole surfaces, unless these organs show unusual modification. The actual transpiring surface is constituted by the walls of the cells bordering the intercellular spaces, but, since it is impossible to determine the aggregate area of these, or the humidity of the air-spaces themselves, the leaf surface must be taken as a basis. Since the transpiration through the stomata is much greater than that through the epidermal walls, the number of stomata must be taken into account. Since they are usually less abundant on the upper surface, their number should be determined for both sides of the leaf. The errors arising from more or less irregular distribution are eliminated by making counts near the tip, base, and middle of two or three mature leaves. The most convenient unit of leaf surface is the square decimeter. The simplest way to determine the total leaf area of a plant is to outline the leaves upon a homogeneous paper, or to print them upon a photographic paper. The outlines are then cut out and weighed, and the leaf area obtained in square decimeters by dividing the total weight by the weight of a square decimeter of the paper used. The area may also be readily determined by means of a planimeter.
160. Coefficient of transpiration. At present it does not seem feasible to express the transpiration of a plant in the form of a definite coefficient, but it is probable that the application of exact methods to each part of the problem will finally bring about this result. Meanwhile the following formula is suggested as a step toward this goal: t = g(u/l)LHT, in which t, the transpiration relation of a plant, is expressed by the number of grams of water lost per hour, on a day of sunshine, by one square decimeter of leaf, considered with reference to the stomata of the two surfaces, and the amount of the controlling physical factors, light, humidity, and temperature, at the time of determination. For Helianthus annuus, this formula would appear as follows: t = 2(²⁰⁰⁄₂₅₀) : 1 : 50 : 75°. To avoid the large figures arising from the extent of surface considered, the number of stomata per square decimeter is divided by 10,000. This amounts to the number per square millimeter, and time may consequently be saved by using this figure directly. While this formula obviously leaves much to be desired, it has the great advantage of making it possible to compare ecads of one species, or species of the same habitat or of different habitats, upon an exact basis of factor, function, and structure.