3o8 General Botany 



and shrubs or with young trees. The order of succession is usu- 

 ally quite definite in a given region. By noting the seedling 

 trees in a forest one can often predict what the composition of 

 the forest will be 50 years from now, if it is left undisturbed. 



The vegetation of a continent like North America, then, is 

 made up of several great climatic plant formations, each of which 

 is composed of many local plant associations. The plant asso- 

 ciations are not permanent, but change as the habitats change, 

 and are succeeded by other plant associations. 



Plant realms. Taking the world as a whole, geographers dis- 

 tinguish three great realms that differ in their vegetation, mainly 

 because of differences in tehiperature. These are (i) the torrid 

 realm, where the temperatures are uniformly warm and frosts 

 are unknown ; (2) the temperate realm, where a warm growing sea- 

 son alternates with a cold period during which plant processes 

 are slowed down, or stopped, each year ; and (3) the frigid realm, 

 where the cold is either continuous or alternates with a short, 

 cool summer having almost uninterrupted light of low intensity. 

 Vegetation is markedly different in these three realms. Under 

 the most favorable conditions vegetation is densest and the 

 number and variety of plants are greatest in the torrid realm 

 and least in the frigid realm. Each of these realms, of course, is 

 occupied by several or by many climatic plant formations, de- 

 pending upon differences in climate. There are torrid forests, 

 grasslands, and deserts, just as there are temperate forests, grass- 

 lands, and deserts. 



Plant formations on mountains. High mountains occur in 

 all parts of the world. The vegetation of the summits differs 

 very materially from the vegetation at their bases. In polar 

 regions the summits may be continually hidden in ice and snow, 

 and the only plants that can grow there are microscopic ones 

 that live on the surface and cause the so-called " red snow." 

 Within the tropics the base of the mountain may be surrounded 

 by tropical forest ; higher up, temperate forests occur ; and then 



