248 PLANT PHYSIOLOGY 



a pound) when only 400 c. c. (or 400 pounds, i. e., about 50 gal- 

 lons) are supplied it, while the common wheats require about 

 50% more water to form the same amount of dry matter. Some 

 common plants may be arranged in ascending order of their water 

 requirement in northern Ohio as follows (Shantz) : millet, sorghum, 

 corn, sugar beet, barley, buckwheat, wheat, cabbage, cotton, 

 potato, oats, rye, legumes, and flax. Wheat and oats have a water 

 requirement about double that of millet, and legumes about three 

 times as much. Such figures, however, are somewhat relative, 

 since plants are generally somewhat less economical in their use 

 of water when they have plenty of it, i. e., in a moist soil, the 

 water requirement tends to be somewhat higher than in a dry 

 soil (Yuncker, 1916). 



Causes of Transpiration.— Why does this enormous water loss 

 occur? The simplest answer is that the plant cannot hinder it. 

 If one recalls the internal structure of the leaf (Fig. 9), it will be 

 remembered that the cells of the mesophyll are thin walled, that 

 they are soaked with water so as to permit the exchange of gases, 

 and that they border on air spaces from which diffusion into the 

 outer air is extremely easy. Under such circumstances, there is 

 a high gradient between the amount of water in the cells and that 

 in the air next to them. Consequently the cells must give off water. 

 They are built for gaseous exchange, but such a mechanism which 

 lets gases in and out must also permit the escape of water vapor. 

 All structures must be a compromise, bringing both advantages 

 and disadvantages, and nowhere is this better shown than in the 

 leaf. A plant which is built for manufacturing food easily, is built, 

 unfortunately, so that water can pass off easily. If plenty of water 

 is to be had, this is not a disadvantage since there is no object 

 in conserving it; but if water is scarce the plant is at a decided 

 disadvantage, and, if it is to survive at all, it must grow slowly 

 and conserve the water supply as carefully as possible. 



Transpiration as a Function. — Transpiration used to be con- 

 sidered a "function," but it is now seen to be an evil and a hin- 

 drance to the development of the plant even though it may en- 

 tail some minor advantages, among which have been proposed 

 (1) the aid to the "circulation" of water in the plant, (2) the 

 distribution of salts, and (3) the lowering of the temperature. 



The transpiration water stream which is drawn through the 

 plant as the result of the evaporation keeps the water coming 



