THE SOURCES OF THE, NITROGEN OF VEGETATION, ETC. 517 



on ssimilation, than in those of experiments the results of which showed no appreciable 

 formation of compounds of Nitrogen (Section II. Sub-sections A-C, pp. 484-497). 



5. The fact of the evolution of free Nitrogen during the decomposition of nitro- 

 genous organic matter has been confirmed by experiment ; but the circumstances of the 

 decomposition in which the evolution of free Nitrogen was observed, when compared 

 with those involved in an experiment on the question of assimilation, are not such 

 as to lead to the conclusion that there would be a loss of Nitrogen from this source in 

 experiments of the latter kind, unless in certain exceptional cases, in which it might be 

 presupposed (Section II. Sub-section D, pp. 497 -508). 



6. The forces, by virtue of which free Nitrogen is eliminated from its compounds in 

 organic matter, are of an oxidizing character ; they are not exercised in the absence of 

 oxygen. They are not likely to be operative in connexion with growing vegetable 

 matter (Section II. Sub-section E, pp. 950, 951). 



7. Although it is known that, under certain circumstances, nascent hydrogen may 

 combine with free Nitrogen and form ammonia, it is questionable whether the nascent 

 hydrogen eliminated during the decomposition of vegetable matter will be in the con- 

 ditions to effect such a combination ; nor are the circumstances of our experiments on 

 the question of the assimilation of free Nitrogen by plants such as to lead to the sup- 

 position, that an error in the results can arise from the formation of any ammonia under 

 the influence of the action supposed (Section II. Sub-section F, pp. 515, 516). 



Section III.— CONDITIONS OF GROWTH UNDER WHICH ASSIMILATION OF FREE 

 NITROGEN BY PLANTS IS MOST LIKELY TO TAKE PLACE; DIRECT EXPERI- 

 MENTS UPON THE QUESTION UNDER VARIOUS CIRCUMSTANCES OF GROWTH. 



A. — General consideration of conditions of growth. 



We have thus far discussed, in some detail, the arrangement adopted in our experi- 

 ments on the question of the assimilation of free Nitrogen by plants, and the colla- 

 teral points involved in the relation of gaseous Nitrogen to vegetation. In regard to 

 the latter, we have dwelt particularly on those which relate to the sources of avail- 

 able Nitrogen to plants, and which, therefore, may tend to influence the quantitative 

 results which we may obtain by the methods of experimenting followed. It remains 

 to consider what are the circumstances under which it is most probable that free 

 Nitrogen may be assimilated, provided the assimilation can take place at all. 



The demonstration of the fact, that the process of cell-development could go on in 

 the presence of free Nitrogen without the latter becoming incorporated into the cell 

 Avail, or into the contents of the cell, as a nitrogenous compound, would not carry with 

 it the demonstration that free Nitrogen could, under no conditions of growth, undergo 

 such change. Our aim should be, therefore, to seek the most probable circumstances 



