46 FOOD OF PLANTS. 



substances pass off by the roots as excrementitious matter. This is a 

 chemical process. Elements arrange themselves in new combinations, 

 according to their reciprocal attraction ; and under the same condi- 

 tions, these are incapable of farther change ; but the products are as 

 variable as are the conditions. These transformations in plants are 

 curious and interesting, but they belong, more especially, to organic 

 chemistry in general. 



Each organ of a plant extracts from its food that which is necessary 

 for its own sustenance and other parts, not assimilated, are separated 

 as excrement. This, coming in contact with another organ in its cir- 

 culation, affords nutriment to it, and so on with a third, etc. ; and 

 when incapable of further transformation, it is separated from the 

 system by appropriate organs. Each part or organ is therefore fitted 

 for special functions and one may receive very different substances 

 from another. Thus man may receive carbonic acid into the stomach 

 with impunity, and even with advantage ; but to receive it into the 

 lungs, might, as it often does, produce death. So also with other 

 transformations in the animal economy ; the kidneys, for example, 

 separate from the body substances containing a large proportion of 

 nitrogen, the liver those with an excess of carbon and the lungs prin- 

 cipally those composed of oxygen and hydrogen. Volatile oils and 

 alcohol, which are incapable of being assimilated, are exhaled through 

 the lungs. Superabundant nitrogen is excreted as a liquid excrement 

 from the body and passes through the urinary ducts ; all gaseous 

 matter passing through the lungs and all incapable of further trans- 

 formation, through the intestinal canal. Transformations of the com- 

 pounds of plants are constantly taking place during their life ; and, as 

 a consequence, gaseous substances are eliminated by the leaves and 

 blossoms. Solid excrement is deposited in the bark. Soluble sub- 

 stances containing carbon, are excreted by the roots and are absorbed 

 by the soil, where they decay or putrefy and become nutriment, as 

 humus, for another generation of plants. Decayed leaves and old 

 roots of grass thus become humus. These excretions are most abun- 

 dant during blossoming and they diminish after the maturity of fruit. 



^5 plants decompose water, they assimilate its hydrogen with carbonic 

 acid and separate oxygen. Thus united, the first two form woody 

 fibre. An acre of ground, producing 10 cwt. of carbon, would annu- 

 ally give to the atmosphere 2.865 Ibs. of oxygen gas. For every 

 pound of hydrogen thus assimilated, it is calculated 547 cubic feet of 

 oxygen is received ; or, in the decomposition of water by plants, one 

 equivalent of the latter to one of the former imbibed. The hydrogen 

 of water decomposed by plants, also goes to the formation of wax, 

 fats, gums and volatile oils, which contain it in large quantities. 

 Vegetables likewise contain hydrogen in the form of water. 



In the formation of acids little oxygen is separated. Fruits are 



