38 THE VEGETATIVE FUNCTIONS OF PLANTS 
vapor from within the leaves may not be able readily to 
pass off, and will accumulate in drops on the surface 
of the leaves, forming dew. The passing off of water is 
not confined to the stomata (stomatal transpiration), 
but may take place through portions of the epidermis 
where there are no stomata, the water passing through the 
cuticle (cuticular transpiration). 
39. Control of Transpiration. The rate of transpiration 
is controlled by both external and internal factors. If 
FIG. 30. Gasteria nigricans. Succulent leaves, with thick cuticle 
serving for the storage of water. 
the outer air is very humid, water cannot evaporate into 
it as rapidly as when it is less humid. On humid days, 
therefore, transpiration will be diminished. It is in 
recognition of this fact that gardeners, in " wetting down " 
a plant house, do not confine the water to the plants, 
