=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.