186 THE FOOD AND NUTRITION OF PLANTS. 



ducts from inorganic materials ; and there is reason to think, that 

 some Phajnogamous phmts (of which our Monotropa, or Indian Pipe 

 is one) are nourished in this Avay. 



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 

 water are taken ujj by the roots. However minute their proportion 

 in the water which the roots imbibe, the plant concentrates and 

 accumulates them, by the exhalation of the water from the leaves, 

 until they amount to an appreciable quantity, often to a pretty large 

 percentage, of the solid matter of the vegetable. As might be ex- 

 pected (312), the leaves contain a much larger amount of ashes, or 

 earthy matter, than the wood, and herbaceous plants more than trees, 

 in proportion to their weight when dry.f 



337. The ashes left after combustion are mostly composed of 

 the " alkahne cldorides, with the bases of potash and soda, earthy 

 and metallic phosphates, caustic or carbonate of hme and magnesia, 

 silica, and oxides of iron and of manganese. Several other sub- 

 stances are also met with there, but in quantities so small that they 

 may be neglected." Different species growing in the same soil 

 appear to take in some portion of all such materials as are natu- 



* 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., 

 to the depth of twenty inches, is as follows. This quantity of Felspar (a large 

 component of granite, &c.) contains . . . 1,152,000 lbs. 



Clinkstone, ..... from 200,000 to 400,000 " 



Basalt, ..... " 47,500 " 75,000 " 



Clay-slate, ....." 100,000 " 200,000 " 



Loam, . . . . . . " 87,000 " 300,000 " 



The silex yielded to the soil by the gradual decomposition of granite and 

 other rocks is in the form of a silicate of potash or other alkali, which, though 

 insoluble in pure water, is slowly acted upon and dissolved by the united action 

 of water and carbonic acid, or more largely by water impregnated with carbon- 

 ate of potash, which is abundantly liberated during the natural decomposition 

 of these rocks. 



t The subjoined results, selected from Boussingault, exhibit in a tabular form 

 the relative quantities of organic and inorganic constituents in several kinds of 



