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 hairs 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 
