RESPONSE OF THE PLANT TO ITS SURROUNDINGS 275 



to a considerable extent by covering the warm earth like 

 a blanket during winter, and thus protecting tender seeds 

 and shoots that otherwise would not be able to survive. 



313. Light may be of all 

 degrees of intensity, from the 

 blazing sun of the treeless 

 plain to the darkness of caves 

 and cellars where no green 

 thing can exist. Between 

 these extremes are number- 

 less intermediate stages : the 

 dark ravines on the northern 

 side of mountains, the dense 

 shade of beech and hemlock 

 forests, and the light, lacy 

 shadows of the pines, each FlG 41 i._ Dogwood> a tree tolerant 



characterized by its peculiar * shade, growing and blooming in a deeply 

 f . . wooded glen. 



form of vegetation. Absence 



of light, too, is usually accompanied by a lowering of tempera- 

 ture and a reduction of transpiration, factors which tend to 

 accentuate the difference 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 deli- 

 cate, and having no need to guard 

 against excessive transpiration, they 

 wither rapidly when cut or exposed 

 to too great intensity of heat and 

 _ ___ light. 

 FIG. 412. A red cedar grown 314. Winds affect vegetation, not 



in a barren, wind-beaten situa- ] ag O the manner of Seed dis- 



tion. J 



tribution and the conveyance of pol- 

 len, but directly by increasing transpiration, and necessitat- 

 ing the development of strong holdfasts in plants growing 

 upon mountain sides and in other exposed situations. The 

 nature of the region from which they blow whether moist, 



