122 VEGETABLE PHYSIOLOGY 



and the fats, the carbohydrates needing to be supplied to it 

 as such, as we have seen. 



The difference between food and the crude 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 contain no chloroplasts can 

 do nothing with this gas. So long as a seed is in the 

 early stages of its germination, 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 certain conditions, of which ade- 

 quate illumination is one. 



Similar considerations apply to other constituents of 

 the materials from which the true food of the living sub- 

 stance is elaborated. They are absorbed in quantity, but 

 they do not become food until a considerable amount of 

 work has been done upon them by the plant itself. 



In the strict sense, it thus appears that the ordinary 

 green plant does not absorb its food from without. It takes 



