RELATION TO ENVIRONMENT 



503 



hang quite vertically, and are alike upon both sides. In many 

 species of Acacia (Fig. 471) the characteristic pinnate leaves of 

 the young plant are 

 replaced by the 

 vertically flattened 

 phyllodes or leaf- 

 stalks, the blade of 

 the leaf having quite 

 disappeared. 



Differing from 

 these protective meas- 

 ures are those of a 

 host of herbaceous 

 plants, whose aerial 

 parts are produced 

 afresh each season, 

 the plant remaining 

 dormant during the 

 dry season, by means FIG. 471. Acacia sp. Leaves replaced by phyllodia, ph 

 of subterranean bulbs one ^ tliese > P^ 1 , has the leaf-lamiiia developed, as it 



always is in the seedling, 

 tubers, or similar 



structures. Bulbous and tuberous plants are especially common in 

 semiarid regions, like the great valleys of California and the Cape 

 region of Africa. 



In some tropical regions, like the northern part of South America, 

 where there is a marked dry season, the trees shed their leaves 

 during the dry period just as northern deciduous trees do on the 

 approach of winter. In California the Buckeye (PL X) does this, 

 being quite bare of leaves during the summer and autumn, and put- 

 ting forth its new leaves in midwinter. 



Epiphytes 



In temperate regions, where the competition among organisms is 

 not so keen as in the Tropics, epiphytes are not nearly so common, 

 and as a rule belong to the lower groups of plants, Lichens and 

 Mosses being the prevailing forms. In the Tropics, however, the 

 number of epiphytes is very large, and includes many characteristic 

 Ferns and Seed-plants. Where these grow in the deep shade of 

 moist forests, they seldom show xerophytic characters ; but where 

 they are exposed to the sun, the necessity for economizing water is 

 obvious, and they are then, as a rule, markedly xerophytic in their 

 structure. Of the xerophytic epiphytes, the Bromeliaceae, so charac- 

 teristic of the American Tropics, and represented in the southern 

 states by a few forms, e.g. the Spanish-moss (Tillandsia usneoides), 



