Dr Seller on the Nutrition of Plants. 59 



of the soil, all the advantage of the supply of carbon from 

 the atmosphere is lost ; for the organic matter of the soil, in 

 supplying the due proportion of hydrogen, must set free as 

 much carbon as would have sufficed for the growth of the 

 plant, were none afforded by the atmosphere — unless, indeed, 

 the organic compounds of the soil contained a less proportion 

 of carbon, as compared with their hydrogen, than the tissues 

 of plants ; which is known to be contrary to the truth. Thus, 

 were the organic matter of the soil the sole source of the 

 hydrogen of plants, there would be a continual accumulation 

 of carbon in the soil, unless it be supposed to be as constantly 

 added to the atmosphere, by combining with the oxygen ab- 

 sorbed into its porous substance. 



It is manifest, however, particularly on this last supposi- 

 tion, that the soil would be exhausted as rapidly as if no 

 carbon were afforded to plants by the atmosphei-e. 



On the contrary, the assumption adopted by Liebig — of 

 water being at once the source of the constituent hydrogen 

 and oxygen of plants — points to an inexhaustible supply of 

 these two elements, such as can keep pace with the fixation 

 of carbon from the air, however rapid that may be. 



One difficulty presents itself, namely, that some vegetable 

 compounds contain a considerably larger proportion of hy- 

 drogen, as compared to their oxygen, than water. And this 

 fact seems to point to the conclusion, that there are other 

 sources of the hydrogen of plants ; and these may be in- 

 organic, as ammonia and light carburetted hydrogen. This 

 subject I must relinquish for the present ; but I would re- 

 mark, that, in connexion with the supposition of ammonia 

 being a source of the hydrogen of the vegetable kingdom, the 

 evolution of nitrogen from the leaves of plants has not yet 

 obtained a sufficiently attentive examination. 



Liebig assumes that the source of the oxygen of plants is 

 the decomposition of water. And few difficulties pi'esent 

 themselves on this head not already adverted to. The quan- 

 tity of oxygen, however, absorbed by the soil in the prepar- 

 ation of the gi'ound for seed, and during the growth of plants, 

 is plainly greater than might be expected from the small 



