162 BIOLOGY OF PLANTS. 



true. Thus, for example, it is asserted, that the quantity of 

 nitrogen removed from a well conducted farm, in the form of 

 cattle and grain, must be greater than that returned in the 

 excrements. But Dana has shown, by direct experiment, 

 that the quantity of nitrogen in the excrements of animals, is 

 nearly double* that found in the food, and hence the quanti- 

 ty returned to the soil is constantly increasing. 



The fact that ammonia is found in the atmosphere, that it 

 results from the putrefaction of animal bodies, and that it is 

 found in the sap of trees, does not prove that plants derive 

 all their nitrogen from it. 



But one of the strongest objections to this theory, is the fact, 

 that in warm climates, where vegetation is most flourishing, 

 the process of putrefaction in animal bodies, produces nitric 

 acid, instead of ammonia ; hence this latter substance will be 

 found in the least abundance, where the largest quantity is 

 needed, and where it is actually consumed, if this theory is true. 

 " No conclusion," says Liebig, '' can then have a better foun- 

 dation than this, that it is the ammonia of the atmosphere, 

 which funishes nitrogen to plants ;" and we may add, no con- 

 clusion is better established than this, that ammonia does not 

 furnish plants with the whole of th^ir nitrogen. 



Whatever reasons there may be for rejecting the theory 

 which derives all the carbon, oxygen and hydrogen of plants 

 from carbonic acid and water, we have equally good reasons 

 for the belief, that ammonia does not furnish plants with all 

 the nitrogen which they contain. 



" If it be true," says Daubeny, '' as Liebig has endeavored 

 to establish, that plants obtain everything, except their alka- 

 line and earthy constituents, from the atmosphere, what, it 

 may be asked, becomes of the theory that attributes the wifit- 

 ncss of a soil for yielding several successive crops of the 

 same plant, to the excretions given out by its roots ? For if 



* Dana's Muck Manual, p 136. 



