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 Fia. 411.— Dogwood, a tree tolerant 
characterized by its peculiar of shade, growing and blooming in a deeply 
form of vegetation. Absence osuhis bay 
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. 
Fic. 412.—Aredcedargrown 314. Winds affect vegetation, not 
in a barren, wind-beaten situa- only as to the manner of seed dis- 
tion. ‘ E 
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, 
