FUNCTION. "I PERSPIRATION. 287 



CHAPTER XIII. 



OF DIGESTION. 



AFTER food is received into the system of a plant, it is 

 gradually conveyed to the leaves, where it becomes decomposed 

 or digested. It is probable that, in its passage through the 

 stem, it undergoes some kind of decomposition, leaving a 

 portion of its water and carbon fixed among the tissue ; but 

 it is principally in the leaves that it is altered. By the time, 

 however, that it has arrived in these organs, it is by no means 

 in the same state as when it entered the roots ; but it becomes 

 altered in its nature, and in its specific gravity, by the addi- 

 tion of what soluble matter it meets with in its progress, as 

 has been proved experimentally by Knight. 



The alteration that the fluids of plants undergo in their 

 leaves appears to consist in parting with superfluous water by 

 perspiration ; in decomposing water, ammonia, and carbonic 

 acid ; and in assimilating the various matters which are left 

 behind. The causes of these actions are believed to be, light, 

 and the atmospheric dryness which light produces. 



According to De Candolle, it is light alone to which 

 PERSPIRATION and the suction of fluids by the roots are to be 

 ascribed. He says, " If you select three plants in leaf, of the 

 same species, of the same size, and of the same strength, and 

 place them in close vessels, one in total darkness, the other 

 in the diffused light of day, and the third in the sunshine, it 

 will be found that the first pumps up very little water, the 

 second much more, and the third a great deal more than 

 either. These results vary according to species and circum- 

 stances ; but it uniformly happens that plants in the sun absorb 

 more than those in diffused light, and the latter more than 

 those in darkness ; the last, however, pumping up something. 



