100 BOTANY. 
205. Plants are distributed widely over the surface of 
the earth. They are most abundant in the hotter climates, 
and decrease in number toward the poles. Likewise, they 
are more abundant upon the lowlands than upon the tops 
of high mountains. The regularity and amount of rainfall 
has also a controlling influence upon land-vegetation, while 
for marine forms the direction and temperature of the 
ocean-currents largely determine their distribution. 
206. In general, we may say that light, temperature, and 
moisture are the chief controlling agents. Where these 
are favorable, there vegetation is abundant; where they are 
unfavorable, vegetation is scanty or wanting. The cold 
and poorly lighted polar regions (VI and VI’ of the map), 
the cold mountain-summits, the dry deserts of Africa and 
Australia (IX and IX’), and the dark depths of the oceans, 
are alike deficient in vegetation. 
207. In general, similar conditions have brought about 
similar vegetations. The North American Forest Region 
(I) of the Western Hemisphere has its counterpart in the 
Europeo-Siberian Forest Region (I’) of the east, in which 
approximately similar conditions prevail. So, too, the 
Prairie Region of North America (II) is to be compared 
with the Steppe Region of Asia (II’), the Pampas Region of 
South America (II’’), and the South African Region (II’”’). 
The Californian Region (IV) is in many respects similar 
to the Mediterranean Region (IV’) and the Chili-Andean 
Region of South America (IV”’). 
208. The accompanying map (Fig. 42) shows one of the 
ways of dividing the earth into botanical regions. Each 
region is capable of subdivision into districts. The plants 
of a region or district constitute a flora; thus we may 
speak of the Prairie Flora, or the flora of the Upper Mis- 
sissippi district, or the flora of Iowa. 
