RESPONSE OF THE PLANT TO ITS SURROUNDINGS 275 



Fig. 411. — Dogwood, a tree tolerant 

 of shade, growing and blooming in a deeply 

 wooded glen. 



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 

 characterized by its peculiar 

 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. 



314. Winds affect vegetation, not 

 only as to the manner of seed dis- 

 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, 



Fig. 412. — A red cedar grown 

 in a barren, wind-beaten situa- 

 tion. 



