90 



VEGETABLE PHYSIOLOGY 



a very great one, the degree of the development of the 

 cuticle having considerable influence upon its amount. It 

 is carried out much more freely through the thin walls of 

 the cells abutting upon the intercellular spaces, which, as 

 we have seen, communicate with the external air by means 

 of the stomata and the lenticels. Very little watery vapour 

 is given off by the latter, so that by far the greater amount 

 that is exhaled passes through the stomata. Transpiration 

 is consequently most copious from the leaves, the structure 

 of the lower side of which, in dorsiventral forms, is espe- 

 cially favourable to it (fig. 64). If a leaf is taken which has 

 stomata upon its under surface only, and the rates of watery 



FIG. 64. TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING 

 THE INTERCELLULAR SPACES OF THE INTERIOR. 



exhalation from the two sides are compared, it will be found 

 that the stomatal gives off considerably more vapour than 

 the other surface. 



A method first introduced by Stahl enables us to prove 

 with considerable facility that the escape of vapour through 

 the stomata is much greater than that through the cuticular 

 surface. It consists in applying to each side of a leaf which 

 has stomata only on the under surface, a piece of filter- 

 paper which has been impregnated with a solution of 

 cobalt chloride and dried. When dry this paper is blue in 

 colour, but it rapidly becomes pink when exposed to 

 moisture. A fresh dry leaf is taken and placed between 

 two pieces of the cobalt-paper and the whole put between 



