THE POOD 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/ood. 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 



