THEIR EARTHY CONSTITUENTS. 189 



derive an unaided subsistence directly from the inorganic world. 

 While it is strictly true, therefore, that all the organic elements 

 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 im- 

 mediately 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, slowly 

 absorbed by the water that percolates the soil, are imbibed by the 

 roots. Others suppose that a portion of the food which plants de- 

 rive from decaying vegetable matter may consist of soluble, still 

 organic compounds. The economy of the greenless parasitic 

 plants (135) is adduced in confirmation 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 products from inorganic materials; 

 and there is reason to think, that at least one Phsenogamous plant 

 (our Monotropa, 137) lives in much the same way. 



336. The Earthy Constituents, The mineral substances which form 

 the inorganic- constituents of plants (322) are furnished by the soil, 

 and are primarily derived from the slow disintegration and decom- 

 position of the rocks and earths that compose it.* These are dis- 

 solved, for the most part, in very minute proportions, in the water 

 which percolates the soil (aided, as to the more insoluble earthy 

 salts, by the carbonic acid which this water contains), and with this 



* According to Liebig, the quantity of potash contained in a layer of soil 

 formed by the disintegration of 40,000 square feet of the following rocks, &c., 



