No. 17] FLOWERING PLANTS AND FERNS 23 
variety of broad-leafed trees. The taller trees receive the 
direct light from above; beneath this canopy a lower tier of 
trees receive upon their foliage such lght as sifts through 
the overlapping branches of their taller neighbors; below 
these a still lower tier of small trees and shrubs receive upon 
their tender green foliage only the subdued light that reaches 
the interior of the forest. Barely rising above the ground in 
cool damp air, tender herbaceous plants and green velvety 
mosses subsist in the shadows of almost twilight depth. Hach 
eroup occupies a position and sustains a relation to light and 
other factors best suited to its needs. Many of these plants 
ot the deep shade, if placed in open, untempered light where 
eiasses thrive best, would soon die. 
Many of the lower forms of plant life, as mushrooms, 
slime-moulds, and saprophytes among flowering plants, thrive 
best in dark places. 
Soil.—_In temperate North America the relative amount 
of annual rainfall has determined three great vegetation 
formations: (1) Woodland Formation; (2) Grassland Forma- 
tion; (3) Desert Formation. In all the eastern parts of the 
continent with a rainfall in excess of 20 inches per annum, 
svoodland or forest growth of some kind, is the dominant 
feature; in the region of the grasslands and plains west of the 
Mississippi, with a rainfall between 20 and 10 inches per annum, 
cpen grasslands is the vegetational characteristic; in the arid 
regions still further west with a rainfall of 10 inches or less, 
Desert types of vegetation prevail and give character to the 
landseape. 
All the region east of the Mississippi River, including 
Mississippi, lies within a potential Woodland Formation, and 
yet it is well known that even within our state we not only 
have woodlands of different kinds, but much of the state does 
not support a forest growth at all. Nothing is more familiar 
knowledge than that even within restricted areas vegetation 
will vary decidedly. A single pasture or meadow will show 
one plant assemblage in one part and a different one in an- 
other part; a forest will show oaks and hickories in one part, 
