OF THE ASSIMILATION OF CARBON. 



which possess all the properties of metallic oxides, 

 and are known as " organic bases." 



Estimated by its proportional weight, nitrogen 

 forms only a very small part of plants, but it is 

 never entirely absent from any part of them. Even 

 when it does not absolutely enter into the compo- 

 sition of a particular part or organ, it is always to 

 be found in the fluids which pervade it. 



It follows from the facts thus far detailed, that 

 the development of a plant requires the presence, 

 first, of substances containing carbon and nitrogen, 

 and capable of yielding these elements to the grow- 

 ing organism ; secondly, of water and its elements ; 

 and lastly, of a soil to furnish the inorganic matters 

 which are likewise essential to vegetable life. 



OF THE ASSIMILATION OF CARBON. 



The fertility of every soil is generally supposed 

 by vegetable physiologists to depend on the presence 

 in it of a peculiar substance to which they have 

 given the name of humus. This substance, believed 

 to be the principal nutriment of plants, and to be 

 extracted by them from the soil in which they 

 grow, is itself the product of the decay of other 

 plants. 



Humus is described by chemists as a brown 

 substance, easily soluble in alkalies, but only 

 slightly soluble in water, and produced during 

 the decomposition of vegetable matters by the 

 action of acids or alkalies. It has, however, 



