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). Ifa leaf is taken which has 
stomata upon its under surface only, and the rates of watery 
Fic. 64.—TRansversE SEcTION oF THE Buave or A Lear, 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 
