226 BOTANY 



PAKT I 



large green leaves with which they are able to assimilate vigorously. These will 

 be discussed later (p. 229). 



B. Assimilation of Nitrogen 



Since a green plant obtains its carbon from carbon dioxide, which 

 is only present in a very small proportion in the air, it might be 

 assumed that the enormous supply of nitrogen in the air would form 

 the primary and the best source of this element of plant food. 

 Every water culture, however, shows clearly that atmospheric nitrogen 

 cannot be utilised by the typical green plant. If combined nitrogen 

 is omitted from the nutrient solution the plant will not grow. 



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, amine, etc., 

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

 are obtained with nitrates. It is still a matter of doubt to what 

 extent nitric acid affords a useful nitrogenous food substance. 



We 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 are not clear as to the products of assimilation. 

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

 than a carliohydrate, 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 pi"oducts 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 the artificial synthesis of albumen, which has been 

 practically accomplished, consists in 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, CHgNH^ * CO^H (which, it is true, is not of wide 

 occurrence in plants), is considered, it is evident that nitrogen is 

 combined with hydrogen as the NHg group. Nitric acid, HNOg, 

 must therefore be reduced when its nitrogen is to be employed 

 in the construction of proteid. Since the chloroplast M'ith the 

 help of sunlight reduces carbon dioxide, it might be assumed 

 that it could also effect the reduction of nitric acid. There is, 

 however, no ground on which this hypothesis can he supported ; on 

 the contrary it can hardly be doubted that all cells, even those without 

 chlorophyll, have tlie power of reducing nitric acid, and that sunlight is 

 not necessary in the process. 



