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 IJ. 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 IJ. 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 IJI.—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 
wall, 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 
