106 Field, Forest, and Wayside Flowers 
In tropical plants it is comparatively thick; for 
were it not so, the ardent sun would soon parch 
all the juices out of the foliage. The oleander, in 
its native soil, has to endure long droughts, and its 
leaves are provided with skins four times as thick 
as those of some leaves which grow in moist cli- 
mates. But, thick or thin, the leaf-skin must not 
keep the air away from the green cells, or the 
little chlorophyll-grains would get no carbonic acid 
to digest, and the luckless vegetable would die 
of starvation. Neither must it totally check the 
evaporation of the water which has ascended 
from the roots. So the leaf-skin is full of pores, 
through which air and vapor can pass freely. 
To these openings botanists give the name of 
‘*stomata” or ‘‘mouths’’ (Fig. 20). They open 
into passages which are channelled out, as it were, 
in the fleshy part of the leaf, and their office is 
best described by the term transpiration. They 
enable the leaves to breathe out any moisture 
which may be contained in them over and above 
the plant’s immediate needs. Thus the ‘‘transpira- 
tion” of a plant-body is comparable to the per- 
spiration of an animal body. 
During rainy or misty weather, when leaves nat- 
urally contain more fluid than they need, these 
