ASSIMILATION OF CARBON. 153 



by one organ, would furnish food for a second and a third. 

 The precise changes which take place, are not so easily detect- 

 ed, although some of them are easily deduced, from the known 

 character of the substances which meet ; thus, it is easy to 

 see, that woody fibre may be formed by the union of the car- 

 bon of the carbonic acid, with water, while the oxygen of the 

 acid, is separated by the leaves. This process may be ex- 

 pressed thus ; thirty-six equivalents of carbon derived from 

 thirty-six equivalents of carbonic acid, combined with twenty- 

 two equivalents of hydrogen and twenty-two of oxygen, de- 

 rived from twenty-two equivalents of water, form woody fibre, 

 with the separation of seventy-two equivalents of oxygen. 

 The oxygen which is separated, is probably derived fi-om the 

 carbonic acid, as there are just seventy-two equivalents in thir- 

 ty-six of acid, although a part may be derived from the water. 



Such transformations are constantly taking place, dur- 

 ing the growth of plants, and the consequence is, that ex- 

 crementitious matters, of different kinds, are thrown off, as 

 unfit to nourish the system. Some of these contain an ex- 

 cess of carbon, others of hydrogen, and others still of nitro- 

 gen and oxygen. Some of the matter is gaseous, and is giv- 

 en off by the leaves ; some of it is liquid, and is ejected at the 

 roots; while part of the effete matter is solid, and remains in 

 the form of the outer bark. In this respect, there is a strik- 

 ing analogy between animals and vegetables. The kidneys, 

 liver and lungs of animals are organs of excretion. The kid- 

 neys separate all those substances, which contain an excess 

 of nitrogen ; the liver, those in which carbon is in excess, 

 and the lungs, those in which oxygen and hydrogen are most 

 abundant. The latter also exhale alcohol and the volatile 

 oils, when taken into the system ; hence these substances are 

 incapable of assimilation. 



In the process of respiration, the oxygen of the inspired 

 air, does not enter into combination with the carbon in the 

 lungs, but combines with the hydrogen of the blood, while 



13* 



