222 GENERAL CHARACTER OF PLANTS. 



root perform this selective office, but so involuntarily, 

 that poisons to the vegetable structure, if present, are 

 almost as readily absorbed as matters of nourishment. 

 The sap, at length, conveyed into the leaves and green 

 twigs is there exposed to the action of the light and the 

 air, admitted by cortical pores, as in the lungs or gills of 

 animals; and herein its descending course, it becomes 

 prepared to supply all the solids and other peculiar 

 products which characterize each particular species 

 of vegetable. 



The constitutions of plants are more variable than 

 those of animals, so that they are fitted, in great va- 

 riety, to occupy the whole surface of the earth. The 

 arctic regions have their particular tribes of plants, as 

 well as the luxurious region of the tropics, where frost 

 is unknown. At one extremity of the earth, or on the 

 snowy summits of the loftiest mountains, vegetation 

 only actively lives about two months in the year ; in this 

 short period the dwarf productions of this region of 

 ice, flower and perfect their seed, or prepare a new 

 generation of buds, and then again fall into a state of 

 dormancy, and commonly remain buried beneath their 

 congenial snows. Within the tropics, a region which 

 may truly be termed the paradise of plants, the utmost 

 variety prevails. Within the compass of a few leagues 

 thousands of species may be enumerated ; while the 

 whole Flora of Spitzbergen contains only about 30 

 species, and all of them dwarf herbs. In the tropics, 

 trees and shrubs are almost as numerous in species as 

 herbs. The trees attain the most gigantic magnitude, 

 and the forests, filled with evergreens, are nearly im- 

 pervious to the rays of the vertical sun ; here the vege- 

 tables continue throughout the year in a state of active 

 growth ; dormancy in many of these plants would be in- 

 stant death ; the stream of vitality continues without in- 

 terruption, and cold, before it attains the freezing tern- 



