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 within 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. 
Tl.— 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 vigorous 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 e¢ seg.), 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 vigorous 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 vigorous 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 prioré 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 vigorous growth cannot 
be attained under the conditions provided, when no more combined Nitrogen than that 
4co2 
