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or the result of what is called capillary attraction. A plant, a 

 portion of whose roots were placed in lead water, or water in 

 which lead was suspended or dissolved, has been ascertained to 

 absorb a portion of it, which, being incapable of assimilation 

 by the vital process, has been afterwards excreted from the 

 roots of the plant. It is evident then that growing plants may 

 take up matters from the soil which are not congenial to them, 

 which are not altered by the vital action, and which, conse- 

 quently, must be again thrown off. Now these particular sub- 

 stances, though not congenial to the plants then growing, may 

 yet constitute the natural and proper food of another class, 

 which consequently will flourish in succession to them. There 

 is another species of excretions, which are properly the result 

 of the vital action. The substances of which the vegetable 

 organism consists, such as sugar, starch, gluten, albumen, are 

 never found in the soil in a formed or crude state ; or, if arti- 

 ficially presented to the plant, will either not be absorbed by it ; 

 or, if absorbed by it, will prove injurious or fatal to it, as the blood 

 of one animal transferred to the veins of another, but they are the 

 products of vital action in the plant upon the food presented to it. 

 But as in the animal so also in the vegetable life, that portion of 

 what has been received, which, after having passed through 

 the digestive process, is incapable of being assimilated, will be 

 excreted or passed off by the natural organs. But it is obvious 

 that this species of excretions is incapable, in an unaltered state, 

 of going to the nourishment of any vegetable substance. By 

 the operation of light and moisture and air, assisted by the 

 stirring of the ground, this presently becomes converted into 

 humus, or the enriching mould of the soil. 



But besides these effects of the cultivation of the soil in the 

 growth of plants, the earth becomes exhausted of particular 

 elements, which are essential to their growth, and form a sub- 

 stantial part of them, and which are carried off from the soil in 

 the form of seeds, or hay, or straw, or vegetables. These are 

 alkaline substances, such as potash, soda, and the phosphate 

 and carbonate of lime. These are found in very minute quan- 



