HYDROHARMOSE 117 
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 (1. ¢., 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 com- 
parisons 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 
jeaf. The errors arising from more or less irregular distribution are elimi- 
nated by making counts near the tip, base, and middle of two or three ma- 
ture leaves. The most convenient unit of leaf.surface is the square deci- 
meter. 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: 7 =s5 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: taar5tt :50:75°. To avoid the large figures arising 
