RELATION OF WATER TO THE PROTOPLASM 65 
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 
adaptations 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 gene- 
rally furnished with a very prominent cuticle tending to 
check undue escape. 
