DAUBENY ON AMMONIA OF PLANTS. 267 



cultivated as articles of food should have access to vege- 

 table or animal manure from which thej may derive their 

 azote, but as this supply would soon be exhausted, were it 

 not at the same time regenerated from the atmosphere, we 

 see the advantage of intercalating a green fallow crop 

 ploughed into the ground with others ; as leguminous 

 plants, according to the experiments of Boussingault, have 

 the greatest power of absorbing nitrogen from the air. 



**On the same principle this chemist suggests the intro- 

 duction of the Jerusalem artichoke into light soils, which, 

 owing to the entire absence of mould, appear irreclaimably 

 barren; this vegetable, the tubers of which afford nourish- 

 ment to cattle almost equal to potatoes, having great power 

 of absorbing both carbon and nitrogen from the air, and 

 thus by degrees generating a certain amount of soil.^ 



**Ihave seen this vegetable very commonly cultivated 

 for the use of cattle, in the light lands of the Grand Duchy 

 of Baden, and in certain parts of Alsace. 



*' But if it be true, as Liebig has endeavored to establish, 

 that plants obtain every thing except their alkalies and 

 earthy constituents from the atmosphere, what, it may be 

 asked, becomes of the theory that attributes the unfitness 

 of a soil for yielding several successive crops of the same 

 plant to the excretions given out by its roots ? 



**For if plants receive the whole of their volatilizable 

 ingredients from the atmosphere, these excrementitious 

 matters, being composed chiefly of carbon, hydrogen, and 

 oxygen, will not be absorbed, and therefore cannot affect 

 the succeeding vegetation. 



*'The above inference would seem unavoidable, if it 

 were considered absolutely proved, that nothing but the 

 fixed ingredients of a plant were derived from the earth, 

 but this is not fully established, even with respect to the 

 humus, much less with respect to the more soluble matters 

 which the soil contains. 



*' These latter, there seems no reason for doubting, may 

 be taken up by the spongioles of the roots dissolved in 



" * It is to be observed, that Boussingault attributes to plants the 

 power of absorbing nitrogen from the air, but he alleges no proof that 

 they have that power, and his results may be just as well explained 

 by supposing them to have different powers of absorbing ammonia. 

 It is to be remarked, that the helianthus tuberosus belongs to a tribe 

 of plants remarkable for their power of absorbing and exhaling water, 

 and hence it is evident, that they will be brought into contact within a 

 given time with a larger amount of ammonia, than other plants, which 

 possess a less degree of energy in that respect." 



