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. 



