ECOLOGICAL FACTORS 



239 



in the form of sudden floods, or at fixed intervals, causing 

 an alternation of wet and dry seasons. Moreover, the 

 moisture of the soil or the atmosphere may be impregnated 

 with minerals or gases which may affect the vegetation 

 independently of the actual amount of water absorbed. 



344. Light may be of all degrees of intensity, from the 

 blazing sun of the treeless plain to the darkness of caves 

 and cellars where nothing but mold and slime can exist. 

 Between these extremes are numberless intermediate stages ; 

 the dark ravines on the northern side of mountains ; the 

 dense shade of beech and hemlock forests ; the light, lacy 

 shadows of the pines; each characterized by its peculiar 

 form of vegetation. Absence of light, too, is usually ac- 

 companied by a lowering of temperature and reduction of 

 transpiration, factors which tend to accentuate the differ- 

 ence between sun plants and shade plants, giving to the 

 latter some of the characteristics of aquatic vegetation. 

 Generally, the tissues of these are thin and delicate, and 

 having no need to guard against excessive transpiration 

 they wither rapidly when broken. 



345. Winds affect vegetation not only 

 as to the manner of seed distribution, 

 as in the case of tumbleweeds and 



winged fruits, 

 but directly by 

 increasing tran- 

 spiration, and 

 necessitating 

 the develop- 

 ment of strong 

 holdfasts in 



^^Sg^jg" Plants growing 

 upon mountain 



sides and in other exposed situations. 



The nature of the region from which they blow whether 



moist, dry, hot, cold, etc., is also an important factor. 



In a district open to sea breezes, live oaks, which require 



462. A red cedar 

 grown under normal 

 conditions. 



