THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 519 



of the living organism, it is only consistent with our established notions of force, that 

 the form which yields with the greatest ease will yield first, and that, if its supplies be 

 in sufficient quantity, it only will yield in an appreciable degree to the force applied. 



If, on the other hand, the forces involved in vegetable growth, tending to form nitro- 

 genous compounds, are capable of appropriating free Nitrogen only in the presence of a 

 certain amount of assimilable combined Nitrogen, then the question of deciding upon 

 the proper proportion of combined Nitrogen to effect the assimilation of that provided 

 in the free state would seem, a priori, to present serious difficulty. For if the plant 

 cannot assimilate free Nitrogen either in the presence of an excess of combined Nitro- 

 gen, or without the aid of a certain amount of it, it would, at first sight, appear that 

 there might be some difficulty in so arranging an experiment as to hit the proper 

 medium. 



But within a certain range of conditions this supposed difficulty would not occur. If 

 the assimilation of free Nitrogen be possible only as the result of the assimilating forces 

 acting upon it in the presence, or with the aid, of a certain amount of combined Nitro- 

 gen, then, when the quantity of combined Nitrogen has become too small, the point 

 must have been passed at which the maximum amount of free Nitrogen would be assi- 

 milated in relation to the then existing supply of combined Nitrogen. Hence, the 

 analysis of a. plant at the period at which its growth ceased in consequence of the falling 

 short of the relative supply of Nitrogen in the combined form, would show whether or 

 not an assimilation of free Nitrogen had taken place as the result of either of the con- 

 ditions referred to in the last paragraph. 



If, however, the plant cannot assimilate free Nitrogen under the conditions of the 

 supply of combined Nitrogen just referred to, unless it has attained a certain vigour of 

 growth, or reached a certain stage of its development, and the supply of combined 

 Nitrogen has been insufficient to bring it to the supposed requisite point, then no 

 assimilation of Nitrogen would take place, even though it might do so provided the 

 proper stage of growth had been passed. To the cases here supposed we shall recur 

 further on. 



If the assimilation of free Nitrogen can take place at all periods of the growth of the 

 plant, and in the absence of all sources of combined Nitrogen, the solution of our ques- 

 tion becomes much more simple than in either of the cases above referred to. 



In illustration of the fact that, within a certain range of other conditions, there can 

 be no difficulty in securing in an experiment those involved in the presence of an excess, 

 of a certain limited quantity, or of no combined Nitrogen, attention may be directed 

 to the phenomena of vegetable growth when seeds are grown in a soil and atmosphere 

 free from combined Nitrogen. 



Under the circumstances supposed, all the conditions with regard simply to the rela- 

 tive quantity of combined Nitrogen are afforded. Thus, when the seed is first sown, it 

 contains within itself an excess of combined Nitrogen, so far as the demands of the 

 plant at the time are concerned. The rapidity with which the Nitrogen of the seed can 



MDCCCLXI. 4 B 



