70 VEGETABLE PHYSIOLOGY 
intercellular spaces, and hence often a serious interference 
with its processes of gaseous interchange, particularly 
respiration. Nor is the protoplasm unaffected by the 
flaccidity, for its health is in a certain degree dependent 
upon its being subjected to hydrostatic pressure by the 
water of the vacuole. 
The importance of the water supply, and indeed its 
necessity to the plant, explains the existence of certain 
subsidiary mechanisms for its absorption and storage 
which are occasionally met with. These will be considered 
in detail in a subsequent chapter, but a few of such adapta- 
tions may be noticed here. We frequently find particular 
aggregations of cells set apart for storage of water. The 
epidermis of certain parts frequently subserves this purpose, 
and many plants possess a considerable development of 
aqueous tissue, variously disposed, which forms a similar 
storehouse. The cells of this tissue contain little else than 
water, and thus serve to supplement the vacuoles of the 
ordinary cells. In plants that inhabit dry, arid soils such 
as sandy deserts there are often other adaptations relating 
to water storage. Such plants are often covered with large 
bladder-like hairs which hold a considerable quantity of 
liquid. Plants which are exposed to conditions threatening 
too copious evaporation are generally furnished with a very 
prominent cuticle tending to check undue escape. 
