124 VEGETABLE PHYSIOLOGY 



CHAPTEK VIII 



THE FOOD OF PLANTS. INTKODUCTORY 



A good deal of misconception exists as to the nature of the 

 food of plants. The character of their environment, and 

 the absence in most cases of any means provided in their 

 structure for the taking in of any material having a com- 

 position at all approaching that of living substance, have 

 led to a not unnatural idea that they feed upon simple in- 

 organic compounds of comparatively very great simplicity. 

 This idea has found considerable support in the fact, which 

 is easily ascertained, that such bodies are those which are 

 absorbed in the first instance. By their roots when they 

 live fastened in the soil, or by their general surface when 

 they are inhabitants of water, comparatively simple inor- 

 ganic salts are found to enter them with the water which 

 they take up. By their green parts, and especially by 

 their leaves, carbon dioxide is absorbed, either from air or 

 water, according to their habitat. A study of the whole 

 vegetable kingdom, however, throws considerable doubt 

 upon the theory that these compounds are, in the strict 

 sense, to be called their food. Fungal and phanerogamic 

 parasites can make no use of such bodies as carbon dioxide, 

 but draw elaborated products from the bodies of their 

 hosts. Similarly those fungi which are saprophytic can only 

 live when supplied with organic compounds of some com- 

 plexity, which they derive from decaying animal or vegetable 

 matter. We have no reason to suppose that the living sub- 

 stance of these non-chlorophyllaceous plants is so radically 



