RESPONSE OF THE PLANT TO ITS SURROUNDINGS 283 



into the stem, after the manner of the cactus and prickly 

 euphorbia ; or their leaves may become thick and fleshy so 

 as to resist evaporation and retain large amounts of mois- 

 ture, as in the case of the yucca and century plant. They 

 also frequently develop a thick, hard epidermis, or cover 

 themselves with protective h^irs and scales. 



The principal types of xerophyte plants are : (1) the li- 

 chens, mosses, and saxifrages found on bald rocks and moun- 

 tain cliffs ; (2) sand plants, such as cockspur grass, sand spurry, 

 wiregrass, and the like, inhabiting sea beaches and pine 

 barrens ; (3) the sage brush, greasewood, and switch plants 

 of our Western alkali plains ; (4) the cactus and yuccas of 

 southern California, Arizona, and Mexico ; (5) the acacias, 

 agaves, and hardy " chapparal " thickets of southern Texas 

 and Mexico. The first class are of importance as the pio- 

 neers and pathfinders of the xerophyte community. In 

 tropical and polar deserts alike they are the first settlers, 

 and by aiding in the disintegration of rocks and their gradual 

 conversion into soil, they pave the way for the coming of 

 the higher plants, and it may be of man himself. 



321. Partial xerophytes. -- Plants exposed to periodic 

 and occasional droughts frequently provide against hard 

 times by laying up stores of nourishment in bulbs and root- 

 stocks and retiring underground until the stress is over. 

 This is known as the geophilous, or earth-loving, habit. 

 Others, as some of the lichens, and the little resurrection 

 fern (Polypodium incanum, Figs. 419, 420), so common on the 

 trunks of oaks and elms in the Southern States, make no 

 resistance, but wither away completely during dry weather, 

 only to waken again to vigorous life with the first shower. 



322. Physiological xerophytes. Plants growing in thin 

 or poor soil, such as that on denuded hillsides, fresh railroad 

 cuts, and newly graded streets, are apt to take on a more or 

 less xerophytic character, even though there may be no lack 

 of moisture. Such soils are called " new " because the 

 mineral elements in them have not been exposed long enough 



