ON VEGETABLE PHYSIOLOGY. 69 



for discovery, the sap in its simple state is ab- 

 sorbed from the earth by the roots, and conveyed 

 through the cells and the tubes of the wood into 

 the leaves ; where, by the processes of evapora- 

 tion and absorption (as just explained,) it acquires 

 new principles and becomes the true sap. It is 

 then taken up by the extreme vessels of the bark, 

 and by them conveyed back to the branches, 

 stem, and root; depositing in its passages through 

 the cortical vessels into the cavities of the cellular 

 tissue for elaboration, such portions of it as are 

 to be applied to the purposes of nutrition, or to 

 those secretions that are necessary for the pre- 

 servation of the plant. 



The functions of the leaves have been com- 

 pared to the respiration of animals, by which the 

 blood parts with its superfluous water, and ac- 

 quires new principles from the atmosphere; and 

 hence the leaves have been denominated the 

 lungs of the plant. But in this, as in every 

 other instance, a strong line of distinction may 

 be drawn. The change in the leaf is simply 

 effected by exudation and absorption, both of 

 which are varied according to the existing tem- 

 perature and the time of the day. That in the 

 lungs is accomplished by muscular action, and 

 is uniformly the same at all periods, and under 

 every atmospherical change. Animal respiration 

 destroys the purity of the atmosphere. Vege- 

 tation restores it; the deterioration of the night 



