SOURCE OF THE NITROGEN. 163 



plants receive the whole of their volatizable ingredients from 

 the atmosphere, these excrementitious matters, being com- 

 posed chiefly of carbon, hydrogen and oxygen, will not be 

 absorbed, and therefore cannot affect the succeeding crop.^"* 



If the theory is true, which derives all the organic constitu- 

 ents from carbonic acid, ammonia and water, a plant ought 

 to grow in a purely earthy soil, when supplied with ammonia. 

 But no instance has been produced, and it is yet doubtful, 

 whether the experiment would succeed if tried. 



The forms in which ammonia enters the organs of pi ants, 

 are probably various. It may enter uncombined, simply dis- 

 solved in water, and be assimilated in a manner similar to oxy- 

 gen, carbon and hydrogen, p. 156. But it probably enters as a 

 salt, that is, in combination with acids. Dr. C. T. Jackson 

 supposes, that " the carbonate of ammonia acts upon the or- 

 ganic matters of the soil, and renders the organic acids neu- 

 tral and soluble ; decomposes and renders inert, noxious, 

 metallic salts and other compounds." Dr. Dana supposes, 

 that ammonia combines with the geine to form a soluble 

 compound, and also acts by its presence to convert vegetable 

 matters into geine. In either case, it would be introduced 

 into the organs of plants, and its nitrogen assimilated. 



It has been supposed by some, that the powerful influence 

 of ammonia was due to its stimulating properties, but others 

 have doubted such influence; among the latter is Liebig, 

 and among the former, Berzelius. The influence of light, 

 heat and electricity would lead to the opinion, that the vital 

 power of plants is capable of being excited, in a manner 

 analogous to that of animals. 



If plants do not derive all their nitrogen from ammonia, 

 what other sources are there from which it can be derived ? 

 We have already observed, that the decomposition of vegeta- 

 ble matters forms, 



3. Geine or humus, which may be a further source of nitro- 

 gen. Humus consists of humic, crenic and apocrenic acids. 



