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 

 Europseo-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 (11'), 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. 



