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 



