372 
PHYSIOLOGY, 
BOOK II. 
CHAPTER XII. 
OF DIGESTION, RESPIRATION, AND SECRETION. 
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 fiuids of plants undergo in their 
leaves appears to consist in parting with superfluous water 
by evaporation; in decomposing w^ater 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 eva- 
poration and the suction of fluids by the roots are to be 
assigned. 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 circumstances; 
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. If, again, 
we take three similar plants, and, preventing their absorption 
