431 

 CHAPTEE V. 



THE FOOD OF PLANTS. INTRODUCTORY. 



A GOOD deal of misconception exists as to the nature of the food 

 of plants. The character of their environment and the ahsence of 

 any means provided in their structure for taking in material of 

 a nature in any way approaching the composition of living sub- 

 stance, has led to a not unnatural idea that they feed upon simple 

 inorganic compounds of comparatively very great simplicity. 

 The 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 inorganic salts are found to enter 

 them with the water that they take up. By tlieir green parts, 

 and especially by their leaves, carbon dioxide is absorbed, either 

 from air or water, according to their habit. A study of the 

 whole vegetable kingdom, however, throws considerable doubt 

 upon the theory that these substances are, in the strict sense, to 

 be called their food. Fmigi and phanerogamic parasites can 

 make no use of such bodies as CO.,, but draw elaborated products 

 from the bodies of their hosts. Those fungi which are sapro- 

 phytic, in the same way can only live when furnished with 

 organic compounds of some complexity, which they derive from 

 decaying animal or vegetable matter. "We have no reason to 

 suppose that the hving substance of these non-chlorophyUaceous 

 plants is so radically different from that of their green relations 

 that it has a totally distinct mode of nutrition. 



In the higher plants we find a stage of their life in which 

 the nutritive processes approximate very closely to those of tlie 

 group last mentioned. When the young sporophyte first begins 

 its independent life — when, that is, it exists in the form of the 

 embryo in the seed — its living substance has no power to utilise 

 the simple inorganic bodies spoken of. The shoot which a 

 potato puts out derives its food from the interior of the tuber. 

 Fleshy roots, corms, bulbs, and all bodies which are capable of 

 renewed life after a period of quiescence, show us the same 

 thing ; the young shoot emerging from any of them is not fed 

 upon simple inorganic bodies, but upon substances of consider- 

 able complexity, which it derives from the tissues of Wie struc- 

 ture from which it springs. 



