DIV. ii PHYSIOLOGY 257 



In the food -solution given above nitrogen was supplied as a 

 nitrate, and this form is most suitable for the higher plants. But 

 compounds of ammonia, so long as they are not injurious to the plant 

 owing to an alkaline reaction, can also be utilised. Organic com- 

 pounds of nitrogen also, such as amino-acids, acid amines, amines, etc., 

 Avill serve for food, though none of them lead to such good results as 

 are obtained with nitrates. Nitrites can also serve as a source of 

 nitrogen, but in too high concentrations are injurious. 



"\Ve are not nearly so well acquainted with the assimilation of 

 nitric acid and of ammonia as we are with that of carbon dioxide. 

 We do not know accurately the place in which the assimilation 

 takes place, we know less of the contributory external conditions, 

 and lastly, we jare not clear as to the products of assimilation. 

 Ultimately, of course, albumen is formed, a far more complex substance 

 than a carbohydrate, containing always, besides C, H, and 0, some 

 15-19 per cent of N, besides S and in some cases P. The methodical 

 study of the products of the breaking down of albumen gives some 

 insight into the structure of the proteid molecule. This shows that 

 in albumen a large number of amino-acids are combined with loss of 

 water. Since EMIL FISCHER has obtained albuminous substances 

 (polypeptides) by a union of amino-acids followed by polymerisation, 

 it is probable that in the plant also such amino-acids are first 

 formed and then unite further. If the simplest amino-acid, glycocoll, 

 XH CH 2 C0. 7 H (which, it is true, is not of wide occurrence in 

 plants), is considered, it is evident that this can be derived from 

 acetic acid by replacing an atom of H with the NH 2 'group. Nitric 

 acid, HN0 3 , must therefore be reduced when its nitrogen is to 

 be employed in the construction of proteid. This reduction is 

 independent both of sunlight and chlorophyll, so that nitric acid can 

 be assimilated in darkness and in colourless parts of the plant. 

 Indirectly, of course, chlorophyll and light are of importance in the 

 synthesis of proteids in so far as compounds containing carbon are 

 required, and these are formed in sunlight with the help of chlorophyll. 

 On account of their rich supply of carbohydrates the foliage leaves 

 are specially fitted for the production of proteid, but they are not 

 " organs of proteid formation " in the same degree as they are organs 

 for the formation of carbohydrates. Only in a few plants (nitrate 

 plants, e.g. Chenopodium, Amarantus, Urtica) can the nitric acid be 

 recognised in the leaves ; in most plants it appears to be transformed 

 soon after its absorption by the root. 



We know as little of the steps in the assimilation of ammonia as 

 of those of nitric acid. Since no preliminary reduction is required, 

 ammonia might be regarded as more readily assimilable than nitric 

 acid. When ammonia is found to be less favourable in a water 

 culture than nitrates, this may be due to certain subsidiary harmful 

 effects of the former substance. 



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