190 



THE FOOD AND NUTRITION OF PLANTS. 



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

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

 accumulates them, as it does its most dilute inorganic food, by the 

 constant 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 expected (311), the 

 leaves contain a much larger amount of ashes, or earthy matter, 

 than the wood, but the trunk more than the branches (210). Her- 

 baceous plants also accumulate more than trees in proportion to 

 their weight when dry.* 



337. The ashes left after combustion are mostly composed of 

 the " alkaline chlorides, with the bases of potash and soda, earthy 

 and metallic phosphates, caustic or carbonate of lime and magne- 

 sia, silica, and oxides of iron and of manganese. Several other 

 substances 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 that are 



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



large component of granite, &c.) contains 



Clinkstone, .... 



Basalt, ..... 



Clay-slate, ..... 



Loam, ..... 



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 ac- 

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

 carbonate of potash, which is abundantly liberated during the natural decom- 

 position of these rocks. 



* The subjoined results, selected from Boussingault, exhibit in a tabular 

 form the relative quantities of organic arid inorganic constituents in several 

 kinds of herbage, compared, in several cases, with the root or grain. The 

 water was previously driven off by desiccation. 



. 1,152,000 Ibs. 



from 200,000 to 400,000 " 



" 47,500 " 75,000 " 



100,000 " 200,000 



" 87,000 " 300,000 " 



