THE FOOD OF PLANTS 121 
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 containing a variety of salts from the soil. The 
saprophytic fungus may, and frequently does, obtain from 
its surroundings 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 saprophytic 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 care- 
fully distinct. We have the absorption of food materials 
rather than of food in the true sense, and we have, follow- 
ing 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, and which are 
those which are stored away in the substance of the plant 
for the nutrition of vegetable substance and the develop- 
ment of embryo, bud, or growing plant. 
In the case of the green plant this power of construct- 
ing food extends to all the classes of foodstuffs ; in that 
of the saprophytic fungus it only applies to the proteins 
