944 ECOLOGY 



high significance in checking transpiration. The leaf and stem sur- 

 faces frequently are covered with hairs; spinescence also is common, 

 though its protective significance may not be important. The stomata 

 occur mainly on the more protected (chiefly the under) surfaces, and 

 often are at the bases of pits and specially protected by hairy coats or by 

 cutinized walls ; as a rule, they are not wide open. Many xerophytes are 

 succulent, containing large amounts of colorless sap or of latex ; oils and 

 resins often are abundantly developed. The osmotic pressure of the cell 

 sap often is very high, especially in shrubs and in plants of alkaline soil. 

 The conductive tracts are prominent, the vessels being larger and longer 

 and the walls thicker than in most plants ; lignification is prominent, 

 and annual rings are well developed. Bast fibers and other mechanical 

 elements reach their highest development in xerophytes. 



Some xerophytes, particularly the lichens, appear wanting in prominent 

 xerophytic structures, seeming able to withstand prolonged desiccation 

 without injury. Apart from the lichens and mosses, absorption through 

 aerial organs is relatively rare in xerophytes, though some of the epi- 

 phytic leaf-absorbing bromelias grow in dry climates. Tubers, corms, 

 and bulbs especially characterize arid climates, and it is obvious that 

 their ability to develop rapidly at the inception of a rainy season is a 

 character of great advantage. Xerophytic conditions usually are be- 

 lieved to favor the formation of flowers and fruits. 



Characteristic xerophytic associations. Perhaps the most repre- 

 sentative xerophytic region is the desert, and it is here that the features 

 above mentioned reach their most pronounced development. In general 

 the severity of the desert conditions increases as the rainfall decreases, 

 it being common to distinguish half deserts, such as the sagebrush plains, 

 from the more extreme deserts, such as those in which such plants as 

 the cacti, or the creosote bush, are dominant forms. Still more extreme 

 are the alkali deserts, in which excessive climatic aridity is supplemented 

 by a soil in which concentrated salts make absorption difficult. Suc- 

 culents with sap of high osmotic pressure seem best fitted for existence 

 under such conditions. There are habitats where the alkalinity is so 

 great that plant life is almost if not quite excluded. 



Many, but not all, alpine and arctic habitats have plants whose struc- 

 tures are chiefly xerophytic. Even though there is an abundant supply 

 of water, the soil often is so cold that absorption is difficult; conse- 

 quently plants without xerophytic structures are poorly fitted for such 

 habitats, except in alpine meadows or in similar situations, where the 



