154 BIOLOGY OF PLANTS. 



the carbonic acid is excreted or thrown off. The nitrogen- 

 ous substances are thrown out in a liquid form, by the urinary 

 organs, and the solid substances, by the intestinal canal. 

 Hence, nutrition in animal bodies is always attended by 

 excretions.* The same is true of vegetables. 



The doctrine of transformations, thus given, may serve to 

 illustrate the general nature of assimilation, so far at least as 

 it can be done on strictly chemical principles. Some, as 

 Liebig would seem to convey the idea, that the agency of the 

 vital poiuer is not required in those changes, and that the ef- 

 fect may he fully accounted for on chemical principles. But 

 what chemical force, or what law known to chemists, can 

 cause thirty-six equivalents of carbon, twenty-two equivalents 

 of hydrogen, and twenty-two equivalents of oxygen, to com- 

 bine and form woody fibre ? We never see such compounds 

 formed, unless it Idc in the vegetable or animal organs ; and 

 this single fact shows conclusively, that some other pov/er 

 than affinity is at work to form such combinations. 



We see no reason for rejecting the theory, that the vital power 

 of the plant may act by its catalytic force, and decompose bodies 

 which are external to the roots, causing one or more of their 

 elements to enter their organs, and to combine with substan- 

 ces already introduced, or previously formed. Such a view 

 is rendered highly probable, when we consider the fact, that 

 the living plant is a most powerful agent in decomposing the 

 soil, and obtaining the alkali which its wants may require. 

 Life is doubtless a powerful catalytic force in producing the 

 transformations which attend the process of assimilation. 



II. Source of the Hydrogen of Plants. The source of the 

 hydrogen of plants is easily determined, because there are 

 but few substances which contain it in sufficient quantities to 

 supply the wants of vegetation. The chief sources of the hy- 

 drogen are the following. 



* Some represent tlie clmnge to be a process of real combustion. 

 Others believe, that tlie absorbed oxygen unites with the carbon in the 

 course of tlie circulation. 



