THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 535 
There are several obvious inferences to be drawn from the figures in these Tables. 
To some we shall refer further on, in the proper order of the discussion. We here 
simply call attention to the very great increase of growth when an extraneous supply of 
combined Nitrogen was provided, as exhibited in the last two columns of Table XVI. 
2. Consideration of the Physiological Evidence as bearing upon the question of 
the assimilation of free Nitrogen. 
However directly the quantitative details given in the Tables may bear upon the 
question at issue, it is very important to consider them in connexion with the physio- 
logical details of the experiments. In order to estimate the value of the evidence 
afforded in this particular, the indications manifested from the earliest period of growth 
should be noticed. 
Reference to the Notes of the progress of the plants, given in the Appendix, will 
show that all the plants when they first came up looked green and vigorous, indicative 
_ of their being at that period in circumstances embracing all the conditions essential to 
healthy growth. As already pointed out, they at that time were probably supplied 
with an excess of combined Nitrogen in relation to their immediate wants. After some 
days, varying with the nature of the plants, they began to lose their deep-green colour, 
and to assume a lighter-green, or pale-yellow tint, indicative of a want of combined 
Nitrogen. We have already pointed out how favourable, probably, would be the con- 
ditions here afforded for the assimilation of free N itrogen, when the plant was passing 
from the state in which it had an excess to that in which it had a deficiency of com- 
bined Nitrogen for the demands of growth. The vigorous development of the plants 
grown in garden soil, but under. the same conditions as to atmosphere, &c. as the other 
experimental plants, indicates that the conditions of atmosphere provided in the experi- 
ments were not at fault (see Appendix, Experiments Nos. 12, 1857, and 15, 1858; also 
fig. 13, Plate XV.). In order to test whether the sum of all the conditions, excepting 
those connected with a sufficient supply of combined Nitrogen, were appropriate for 
vigorous growth, we have only to provide some combined Nitrogen when the plants 
show the declining vigour just described; and if this be all they require, they will 
resume their healthy green colour. Or if we add the combined Nitrogen before the 
plants arrive at the period in question, it will prevent them assuming the pale-green_ 
or yellow colour. We have had recourse to both of these expedients; and each, so far 
as the Cereals, buckwheat, and clover are concerned, has yielded a result indicating that 
all the conditions of the experiments, excepting those connected with a sufficient supply 
of combined Nitrogen, were adapted for healthy growth. 
The plants to which ammonia was given in 1857, were allowed to suffer more before 
they received it than those of 1858; yet in thirty-six hours after the addition of com- 
bined Nitrogen to the soil, in amount not exceeding 1} milligramme of the element to. 
each plant, they began to manifest an improved appearance. In two or three days the 
improvement was quite marked; but at the termination of periods varying from nine to 
MDCCCLXI. 4D 
