288 BOARD OF AGRICULTURE. 



variety of vegetable life. These diverse forms are, however, 

 limited in their distribution by the peculiarities of soil and 

 climate as well as by their specific characteristics. "While some 

 thrive only on rocky cliffs, deriving their sustenance from the 

 atmosphere, others require the rich vegetable mould of the 

 prairies or the fertile alluvion of the rivers. Some delight in 

 dry, sandy localities with abundance of light and heat, while 

 others are never found except in dark, dank forests. Many 

 aquatic plants grow in the cold, fresh water of mountain 

 swamps, and others amid the sulphurous vapors of hot springs 

 or the brackish waters of salt marshes and the sea. The succu- 

 lent cactus withstands the parching heat of the desert, while the 

 almost equally fleshy orchid sends out its aerial roots only in a 

 climate loaded with moisture. The most luxuriant vegetation 

 exists in equatorial regions, near the level of the sea, where 

 heat, light and moisture are most abundant and m*ost constant. 

 The boundless forests of Brazil exhibit the greatest number of 

 species, and probably the most enormous plant-growth per acre 

 to be seen anywhere, though possibly equalled in the latter 

 respect by the evergreen timber of the Sierra Nevada, which 

 often attains an average height of two hundred and fifty feet. 

 As we rrcede from the equator toward the north or south, we 

 notice a gradual change in the appearance of plants, dependent 

 principally upon the mean annual temperature, but often pecul- 

 iarly modified by those geographical features which affect the 

 time, manner and quantity of the rain-fall. 



Elevation above the sea level also produces the same result 

 upon the development of plants as a change in latitude. Lofty 

 mountains within the tropics furnish examples of vegetation 

 precisely analogous to that of the whole earth, their summits 

 often being crowned with the very species whose blossoms greet 

 the traveller in the brief summer of the polar regions. 



The surface of the earth has been divided by botanists into 

 eight zones, marked by certain isothermal lines, and character- 

 ized by the predominance of certain vegetable forms. Each of 

 these zones is represented, also, in the vegetation of mountains 

 near the equator, which rise above the snow line ; and these 

 zones of altitude are designated by the names of their peculiar 

 species of plants. Thus, the equatorial zone is called the region 

 of palms and bananas ; the tropical zone, the region of tree 



