The Food of Plants. 335 
in relation to food supply; the number, character and 
special functions of the elements appropriated; the re- 
Jations of food supply and nutrition to conditions of health 
and disease; the relations of food supply to improved 
qualities of plants for purposes of human food; the special 
capacity of the plant for digestion, and its relation to the 
character of food used, are all so intimately connected with 
the subject as a whole and with each other, that no com- 
plete statement can be made without taking some account 
of all these considerations. Concerning some of them, 
we are forced to admit that as yet, but little real progress 
has been made in the direction of their correct elucidation, 
nor can we look for a final solution until such time as 
chemistry shall make us more fully acquainted with the 
composition of plants in various stages of development, and 
,under widely different conditions of growth, and thus 
provide the key which shall unlock the door to those now 
mysterious physiological changes peculiar to nutrition. 
In the process of nutrition, certain substances enter 
directly into the composition of various parts of the plant, 
to the formation of which they are absolutely essential. 
There can, therefore, be no doubt that they are food sub- 
stances. Others, however, although taken into the plant, 
do not enter as an essential ingredient into the construction 
of parts. Nevertheless, it is found that their elimination 
from the food supply so disturbs the normal processes of 
growth, as to leave no doubt in our minds concerning their 
necessity in what are termed the metabolic processes, or 
the chemical changes incident to nutrition. It is therefore 
as proper to regard them as food substances as the former. 
In order to determine what elements may be properly 
regarded as plant food, we first of all resort to chemical 
analysis, and in the second place to special methods of 
cultivation. When a plant is burned, or when it suffers the 
slower oxidation of decay—the final results being the same 
in each case—we find that by far the greater part of the 
original structure disappears in the form of aqueous vapor, 
carbon dioxide gas and volatile acids, while a very small 
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