128 VEGETABLE PHYSIOLOGY 
of vegetable substance and the development of embryo, bud, 
or growing plant. 
In the case of the green plant this power of, constructing 
food extends to all the classes of foodstuffs ; m that of the 
saprophytic fungus it only applies to the proteins and the 
fats, the carbohydrates needing to be supplied to it as such, 
as we have seen. 
The difference between food and the raw 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 when 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 contam no chloroplasts—can do nothing with this 
gas. So long as a seed is in the early stages of its germina- 
tion, 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 particular conditions, of which 
~ adequate illumination is one. 
Similar considerations apply to other constituents of the 
materials from which the true food of the living substance 
is elaborated. They are absorbed in quantity, but they 
