THE SOUKCES OF THE N1TKOGEN 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 Nitrogen, 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 



jjdccclxi. 4 D 



