120 BOTANY. 



the accompanying diagram (Fig. 57), which represents a 

 "genealogical tree" of the Vegetable Kingdom. 



207. In the study of plants we now begin with the 

 simplest kinds, and pass to those which are more complex. 

 It follows from what has been said above that in enumer- 

 ating the groups of plants in the subsequent pages of this 

 book we are often compelled when we reach the end of 

 one group to return again to the common point of origin. 



208. Geographical Distribution of Plants. — 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 abun- 

 dant upon the lowlands than upon the tops of high moun- 

 tains. 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. 



209. In general, we may say that light, temperature, 

 and moisture are the chief controlling agents. Where 

 these are favorable, 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. 



210. In general, similar conditions have brought auout 

 similar vegetations. The North American Forest Eegion 

 (I) of the Western Hemisphere has its counterpart in the 

 Europseo-Siberian Forest Eegion (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 Eegion of Asia (II'), the Pampas Region 



