RELATION TO ENVIRONMENT. 



The distribution of various vegetative elements is treated in later 

 chapters. 



I. Relation of Plants to Earth's Surface as a Whole. 



962. Northern hemisphere. In considering the relation of 

 plants to the earth's surface as a whole, it is at once evident that 

 the northern hemisphere possesses a much larger area for 

 plant distribution than the southern hemisphere. The former 

 comprises the great continents of Europe, Asia, North America, 

 with a number of large islands in the West Indies, with Green- 

 land and numerous smaller islands, and in addition the northern 

 parts of the two southern continents, South America and Africa. 

 The distribution of plant life is mainly on land, and for a limited 

 distance in the waters along the ocean and lake shores. The 

 great part of the oceans and of the larger lakes is not inhabited 

 by plants, chiefly because of the depth of the water where the 

 plants cannot get a foothold, and because of oceanic currents 

 which would in some cases carry floating plants into water of such 

 extremes of temperature that they could not exist. In some of 

 the more quiet oceanic waters, as in the Atlantic, northeast of the 

 West Indies, there are large areas of floating sargassum, known as 

 the Sargasso Sea. . Areas on land, devoid of vegetation, are the 

 regions of perpetual ice and snow, as in alpine heights or in the 

 farther arctic regions, or in the extreme deserts. The greater 

 habitable land area of the northern hemisphere, together with 

 the greater extremes of climate and topography, not only assures 

 a larger number of individuals, but also a larger number of 

 species of plants. The plant population is therefore more 

 greatly diversified than in the southern hemisphere. 



963. Southern hemisphere. This includes a fraction of the 

 habitable land area of the globe, the larger part of the continents 

 of South America and Africa, the large island of Australia, the 

 East Indies, and Madagascar, with numerous smaller islands. 

 Besides the smaller land area, the climatic and land variations 

 are not so great as in the northern hemisphere. The actual 



