196 STORAGE OF FOOD AND WATER 



divides tangentially and gives rise to several cell-layers constitut- 

 ing a typical water-storage tissue (Fig. 79). 



Water-storage tracheids sometimes occur as terminals of the 

 finer branches of leaf veins, as in Euphorbia splendens and 

 Townsendia cespitosa (Fig. 107); and occasionally the meso- 

 phyll cells have the characteristic wall thickenings of tracheids 

 and apparently serve in water storage. The tubers of the 

 potato and other fleshy underground parts serve for the stor- 

 age of water as well as of food. 



Fig. 107. — Water-storage tracheids in the leaf of Euphorbia splendens. b, b, water- 

 storage tracheids; d, mesophyll cells; c, branch from a milk tube. (After Haberlandt.) 



Frequently, and especially in xerophytes, cells or groups of 

 cells contain mucilage as a real cell content or as much thick- 

 ened cell-walls, the inner layers of which have become transformed 

 from cellulose to mucilage. Mucilage has a great affinity for 

 water, imbibing it with power and holding it with great tenacity. 

 When the amount of mucilage is considerable, as is frequently 

 the case in desert plants, such as the Aloes, cacti, certain species 

 of Astragalus, and many others, it plays an important part in 

 the water-storage 'function. 



Characteristics of Water-storage Tissues. — The water- 

 storage cells are characterized by having thin cellulose walls, 

 or walls, if thickened, having many pits or thin places. These 

 cells readily imbibe water when plenty is at hand, and when 

 the soil water is scarce they deliver their stores gradually to 

 those tissues, such as the photosynthetic and meristematic, in 



