THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 529 



place under any other circumstances. When supplied with an insufficient quantity of 

 nitrogenous matter, the vegetable organism might not decompose any of that matter ; 

 and yet, when an excess of combined Nitrogen was supplied, the decomposition might 

 occur. The results we have given, therefore, afford evidence against the fact of such 

 decomposition only Avithin a very limited range of circumstances of growth. In dis- 

 cussing the results of the experiments the consideration of which we are now about to 

 enter upon, we shall refer to this question again, in connexion with circumstances of 

 growth which we should suppose would be more favourable to an evolution of free 

 Nitrogen by the plant. 



II. — Experiments in which the plants had a known supply of combined Nitrogen beyond 



that contained in the Original seed. 



We have thus far considered the subject of the assimilation of free Nitrogen, by 

 reference to the results of experiments upon plants grown without any supply of com- 

 bined Nitrogen beyond that contained in the seed sown. We have found that, under 

 these conditions, we have only been able to study the results of growth of a very limited 

 character. The wheat, and barley, and oat plants, grown in 1858, did indeed progress 

 so far as to produce glumes and pales for seed ; but they did not afford the opportunity 

 of studying the results of growth during the period of the formation and the ripening 

 of seeds themselves. 



It yet remains to consider, therefore, what may take place under circumstances of 

 a more active and rigorous growth, and at a later stage of development of the plant. 

 When considering the conditions apparently the most favourable for the assimilation of 

 free Nitrogen by plants (p. 517 et seq.), we suggested the improbability of such an 

 assimilation taking place in the presence of an abundant supply of combined Nitrogen. 

 If the force of our remarks on this point be admitted, and it be still supposed that an 

 assimilation of free Nitrogen is possible with rigorous growth, only attainable by means 

 of a liberal supply of combined Nitrogen, we seem to be led to the following paradoxical 

 conclusions : — 



1. Healthy, active, and vigorous growth are favourable conditions for the assimilation 

 of free Nitrogen by plants. 



2. Healthy, active, and rigorous growth can only be attained by keeping within the 

 reach of the plant an excess of combined Nitrogen. 



3. Assimilation of free Nitrogen cannot take place in the presence of an excess of 

 combined Nitrogen. 



A priori conclusions with regard to the effect of molecular forces, and particularly of 

 those which give rise to vital phenomena, are, however, very unsafe ; and we have not 

 been satisfied to rely upon such evidence only, in reference to the question under investi- 

 gation, as could be afforded by experimenting with plants grown without an extraneous 

 supply of combined Nitrogen. We have found that active and rigorous growth cannot 

 be attained under the conditions provided, when no more combined Nitrogen than that 



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