122 VEGETABLE PHYSIOLOGY 
and the fats, the carbohydrates needing to be supplied to it 
as such, as we have seen. 
The difference between food and the crude materials 
from which it is constructed can be made clearer by 
inquiring whether such simple inorganic bodies as the 
green plant absorbs are capable of nourishing protoplasm 
whens freely supplied to it. If they are the true food, 
plants everywhere should be able to make use of them. 
But if we consider only one of them, the carbon dioxide of 
the air, we find this is not the case. The plants which 
are not green—that is, which contain no chloroplasts—can 
do nothing with this gas. So long as a seed is in the 
early stages of its germination, it is surrounded by carbon 
dioxide, which is given off by its own protoplasm. But it 
can make no use of it, and if the store of nourishment 
provided for it in the endosperm or cotyledons is cut off, it 
inevitably dies of starvation. A saprophytic fungus in 
like manner is dependent for its life upon the absorption 
of such a compound as sugar, and carbon dioxide cannot 
aid at all in its nutrition. 
Another fact throws a certain light upon the relation 
of carbon dioxide to the feeding of a green plant. If such 
an individual, in good health and endowed with ample 
vigour, is removed from light to darkness, though this gas 
be supplied in appropriate quantity, it can make no use of 
it. The gas is evidently useless for immediate nutrition, 
and its ultimate utility is dependent upon its being sub- 
mitted to the action of some mechanism in the plant which 
is called into play under certain conditions, of which ade- 
quate illumination is one. 
Similar considerations apply to other constituents of 
the materials from which the true food of the living sub- 
stance is elaborated. They are absorbed in quantity, but 
they do not become food until a considerable amount of 
work has been done upon them by the plant itself. 
In the strict sense, it thus appears that the ordinary 
green plant does not absorb its food from without. It takes 
