468 



ECOLOGICAL GROUPS 



earth, or before the culmination of the severest drought of sum- 

 mer, may be regarded as a habit gradually acquired by decidu- 

 ous trees and shrubs for their own protection. The duration of 

 the period of leaflessness depends on the length of the danger- 

 ous season. Grapevines, for instance, in central Europe are leafy 

 during about six months and leafless during the following six. 

 But near Cairo, Egypt, the leafless period is only two months 

 long, and in very warm and moist climates the vines are ever- 

 green. So, too, cherry trees are evergreen in Ceylon, and 

 beeches in Madeira. 



A large shrubby Euphorbia* common in southern Italy, is 

 found absolutely leafless during July and August, when grow- 

 ing on the faces of limestone cliffs. But in moist soil, within 



a stone's throw of the 

 leafless plants, there 

 may be found others 

 profusely leafy. 



443. Halophytes. A 

 halophyte is a plant 

 which can thrive in a 

 soil containing much 

 common salt or other 

 saline substances. The 

 seaside is the principal 

 region of halophytic 

 vegetation, but many 

 halophytic plants are 

 also to be found in the neighborhoods about salt springs and the 

 " alkali " lands of the Southwest and the Pacific slope. 



The mangrove tree (Fig. 363) is one of the most remarkable 

 of halophytes. It grows in shallow water along the seashore, 

 and sends out many aerial roots which at length find their way 

 down into the salt mud. In this way it collects drift material 

 and gradually extends the shore line farther out to sea. 



1 E. dendroides. 



FIG. 363. The mangrove, a halophyte 

 After W. M. Davis 



