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CHAPTER X. 



OF DIGESTION, RESPIRATION, AND THE PECULIAR SECRETIONS 



OF PLANTS. 



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

 gradually conveyed into the leaves, where it becomes decom- 

 posed 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 evaporation ; in decomposing carbonic acid ; and in assi- 

 milating the various matters which are left behind. The 

 causes of these actions are believed to be light, and the atmo- 

 spheric dryness which light produces. 



According to De Candolle, it is lif^ht alone to which eva- 

 poration and the secretion of fluids by the roots are to be as- 

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

 same species, of the same size, and of the same sti^ength, and 

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

 the diflnsed 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 circumstances ; 

 but it uniformly happens that plants in the sun absoi'b more 

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

 darkness ; the last, however, pumping up something. If, again, 

 we take three similar plants, and, preventing their absorption 

 by the roots, after weighing them carefidly, place them in 



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