THE MATERIAL INCOME OF PLANTS 



321 



stomata, and on the under side of the olive leaf about 3700. The 

 following table (after Weiss) shows the numbers per square millimeter 

 in various common plants. 



So far as plants have been examined, it appears that a large majority 

 of mesophytes have less than 200 stomata to the square millimeter, 

 and a fair average is perhaps 150. (See Part III, p. 556, on variations 

 in the structure and distribution of stomata, and the causes thereof.) 



Transpiration. — Since the intercellular spaces arc bounded by moist 

 cell walls, freely permeable to water, they are always filled with air 

 which contains more or less water vapor. This vapor diffuses through 

 the stomata into the drier outer air, and being lost from the plant will 

 be replaced in whole or in part by water entering the root. At the 

 same time, since the walls of the epidermal cells contain a little water, 

 some evaporation takes place directly from them. The total evapora- 

 tion of water under these conditions is designated as transpiration 

 (see p. 323). 



Exit but no entry for water. — The aerial parts are constantly losing 

 water because they are permeable ; at the same time, there is practically 

 no opportunity for the admission of water, even when such parts are 

 deluged by it. Ordinarily rain comes into contact only with a nearly 

 waterproof surface, the cuticle. It cannot easily penetrate the minule 

 stomata, even when they occur on the upper surface of leaves, for there 



