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 (/. 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 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 

 leaf. 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 



zc 

 formula is suggested as a step toward this goal: t^=^g 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 Helianthiis animus, this formula would 



200 . . 



appear as follows : t^2 : 1 150 :75 . To avoid the large figures arismg 



