128 VEGETABLE PHYSIOLOGY 



of vegetable substance and the development of embryo, bud, 

 or growing plant. 



In the case of the green plant this power of ^constructing 

 food extends to all the classes of foodstuffs ; in that of the 

 saprophytic fungus it only applies to the proteins and the 

 fats, the carbohydrates needing to be supplied to it as such, 

 as we have seen. 



The difference between food and the raw 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 germina- 

 tion, 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 particular conditions, of which 

 adequate illumination is one. 



Similar considerations apply to other constituents of the 

 materials from which the true food of the living substance 

 is elaborated. They are absorbed in quantity, but they 



