SOURCE OF THEIR ORGANIC CONSTITUENTS. 185 



man and the higher animals, but which could never grow and per- 

 fect their fruit, if left, like their humble but indispensable predeces- 

 sors, to derive an unaided subsistence directly from the inorganic 

 world. While it is strictly true, therefore, that all the organic ele- 

 ments have been originally derived from the air, it is not true that 

 what is contained in almost any given plant, or in any one crop, is 

 immediately drawn from this source. A part of it is thus supplied, 

 but in proportions varying greatly in different species and under 

 different circumstances. Undisturbed vegetation consequently tends 

 always to enrich the soil. But in agriculture the crop is ordinarily 

 removed from the land, and with it not only what it has taken from 

 the earth, but also what it has drawn from the air ; and the soil is 

 accordingly impoverished. Hence the farmer finds it necessary 

 to follow the example of nature, and to restore to the land, in the 

 form of manure, an amount substantially equivalent to what he 

 takes away. 



335. The mode in which vegetable mould is turned to account 

 by growing plants has not yet been sufficiently investigated. Ac- 

 cording to Liebig, the decaying vegetable matter is not employed 

 until it has been resolved into its original inorganic elements, 

 namely, into water, carbonic acid, ammonia, &c. ; which are imbibed 

 by the roots both directly in the gaseous state, and when taken up by 

 the water as it percolates through the soil.* Others suppose that a 

 portion of the food which plants derive from decaying vegetable 

 matter may consist of soluble, still organic compounds. The econ- 

 omy of the greenless parasitic plants (152) is adduced in confirma- 

 tion of this view : but these are nourished by the foster plant just as 

 its own flowers are nourished. Decisive evidence to the point is 

 furnished by Fungi, the greater part of which live upon decaying 

 organic matter, and have not the power of forming organizable pi-o- 



* While it may be rightly said, that the proportion of carbonic acid in the 

 atmosphei'e is too minute directly to supply ordinary vegetation, especially 

 that of esculent plants, with sufficient carbon, this cannot be said of the air 

 contained in the pores and crevices of the soil, at least in any fertile soil. This 

 air in the soil contains a far larger proportion of carbonic acid than the atmos- 

 phere above ; the excess being derived partly by direct absorption or by the 

 action of rain, and in an enriched soil more largely from the decay of the mate- 

 rials of former generations of plants. In a recently manured soil, the carbonic 

 acid ordinarily amounts even to 10 or 20 per cent. See Boussingault and Lewy, 

 in Ann. Sci. Nat. ser. 3, Vol. 19, p. 13. 

 16* 



