fiO VITAL FUNCTIONS OP PLANTS. [LKCT. IK 



have thick succulent leaves, as the Aloes, and are 

 natives of a warm climate, and dry arid soil, per- 

 spire very little, but absorb powerfully by the 

 whole surface of the leaves. That these functions 

 depend on vitality is evident, for both cease when 

 the plant dies. 



Another function of vegetables depending on the 

 property we are treating of, is that which has been 

 termed their respiration. Air is as essential for the 

 existence of plants as of animals ; and, whether we 

 believe, with many philosophers, that they restore 

 the pure part of atmospheric air that has been 

 consumed by the breathing of animals, or join in 

 the opinion of others, who believe that they pro- 

 duce the same changes on it, to a certain extent, 

 as animals do, we know that it is during the life of 

 the plants only that any such effect is produced. 

 As the leaves are the principal organs by which 

 this function is performed, it is most vigorous 

 during the summer, while plants are clothed in 

 the full luxuriance of foliage ; and it is very much 

 diminished when the leaves drop in autumn, almost 

 ceasing in the state of torpidity of the plant in 

 winter. Still, however, as the vitality of the vege- 

 table then perpetuates its existence until the return 

 of spring ; the bare stem, and the branches, con- 

 tinue to operate the ordinary change on the sur- 

 rounding air, although in a very small degree ; 

 but when the leaves decay, and the plant 



