KEGIONAL DISTRIBUTION 



497 



452. What determines the occurrence of vegetation forms. 



The ecological type of the plants which inhabit any kind of 

 habitat depends usually on its soil (in the case of aquatics, 

 the water), its climate, or both. If we find a region with 



FIG. 371. Mangroves, halophytes with aerial roots 



The young seedlings are rooting in the beach sand and the thicket is 

 gradually pushing out toward the sea 



decidedly xerophytic vegetation, this fact may be due to any 

 one of several causes, as follows : 



(1) A climate with very little annual rainfall. 



(2) A climate with considerable rainfall during the year 

 but with long rainless periods. 



