APPROPRIATION OF NITROGEN. 325 



854. Senebier i first demonstrated that plants obtain all their 

 carbon from carbonic acid gas. 



855. That definite quantitative relations exist between the 

 amounts of carbonic acid decomposed, carbon retained, and oxy- 

 gen evolved by the plant, was first pointed out by Saussure. 2 



APPROPRIATION OF NITROGEN. 



856. It has been shown that all land and many water plants 

 contain a variable amount of air in their tissues, chiefly in the 

 intercellular spaces and older modified cells (trachei'ds. tracheae, 

 etc.). When there is an active interchange of gases by any 

 plant, a portion of the nitrogen contained in its included air is 

 very likely to be eliminated. A trace of nitrogen is so generally 

 found with the oxygen evolved during assimilation proper, that 

 this has been regarded by some as a constant accompaniment 

 of the assimilative process. 



857. Amount of nitrogen in the plant. Besides the free nitro- 

 gen which constitutes a part of the included air of the plant, 

 there is a certain amount of combined nitrogen alwa} - s present 

 in active cells as an essential component of their living matter. 

 The protoplasmic matters in plants contain about 15 per cent of 

 nitrogen in combination. For all practical purposes they may 

 be regarded as having; chemically a common albuminous 3 basis 



c^ *, 



(roughly comparable to the white of egg), with which (as has 



1 Memoires Physico-chymiques, 1782. 



Many of Senebier's observations are almost identical with those of Ingen- 

 housz (as given in his "Nutrition of Plants"), and it has been thought by 

 some that the priority of the above discovery belongs rightfully to the latter. 

 It is to be remembered that at the date at which both of these experimenters 

 were working, chemists were just beginning to acquire, through the studies 

 of Lavoisier, clear notions in regard to the important part which oxygen 

 plays, and that in the early part of this transition period an obscure nomen- 

 clature renders it difficult to apportion to each of these observers his proper 

 share of credit. 



- Hecherches chimiques sur la vegetation, 1804. 



Some of the relations of light to the process of decomposition of carbonic 

 acid by green parts of plants were first indicated by Daubeny, and further ex- 

 amined by Draper. The subsequent histoiy of assimilation, to which Sachs, 

 Pfetfer, Engelmann, and many others have contributed, has been referred to 

 in the text and in citations in the notes. 



3 Attention may again be called to the various expressions employed to 

 designate the compounds in the plant which resemble albumin, and which 

 have been collectively termed albuminoids. Authors have made a distinction 



