THE FOOD OF PLANTS 127 
protoplasm is dependent upon its receiving a supply of 
such materials, we are face to face with the fact that, with 
a few exceptions, the consideration of which may be deferred, 
they are not furnished at all from the environment to the 
ordinary green plant, and often only partially so to the 
saprophytic fungus, though they are freely obtained from 
their host-plants by parasites. On the contrary, we find 
the ordinary green plant taking in by ordinary physical 
processes carbon dioxide from the air, and water contain- 
ing a variety of salts from the soil. The saprophytic 
fungus may, and frequently does, obtain from its surround- 
ings certain compounds of ammonia, together with some 
carbohydrate bodies, such as sugar. We can ascertain that 
if these different compounds are supplied under suitable 
conditions to the groups of plants mentioned, the latter 
can flourish and develop. While we have the strongest 
grounds for holding that the protoplasm is essentially 
similar in all these cases, we see marked differences between 
them with regard to the materials which they absorb. 
The substances supplied to the green plant are utterly 
unlike what we have seen to be the actual food ; the sapro- 
phytic fungus can make use of the compounds of ammonia, 
but absorbs carbohydrates as such, while the parasite, 
whether fungus or phanerogam, obtains the materials 
which we see are directly capable of feeding it. 
If we say that the food of these various groups of plants 
varies in the degree of its complexity, we must carefully 
consider in what sense we use the term food. In the nutrition 
of the green plant there are clearly two very different 
processes combined, which should be kept carefully distinct. 
We have the absorption of the raw material of food rather 
than of food in the true sense, and we have, following such 
absorption, the expenditure of a considerable amount of 
energy upon these food materials, with the result that they 
are worked up into the complex compounds which we find 
protoplasm can assimilate. These are such as we see 
stored away in the substance of the plant for the nutrition 
