EELATION OF WATER TO THE PEOTOPLASM 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. 



