THE FOOD OF PLANTS. 69 



of Sulphuric acid are now added.* A red coloration indicates 

 the presence of Nitric acid in the water. To determine the 

 presence of extremely small quantities of Nitric acid in water, 

 we evaporate down a few c.c. of the water, and then treat the 

 residue with brucine solution and Sulphuric acid. 1 



The presence of Nitric acid in water is also very conveniently 

 determined by evaporating down a few c.c. of the water in a 

 porcelain dish and adding to the residue, by means of a glass rod, 

 diphenylamine dissolved in Sulphuric acid (O05 gr. of dipheny- 

 lamine dissolved in 10 c.c. of pure concentrated Sulphuric acid). 

 A blue coloration is produced in presence of Nitric acid, owing 

 to the formation of aniline blue. 



Beetroot juice is fairly rich in nitrates. If we treat sections 

 of the root on the slide with the diphenylamine solution, they 

 become intensely blue. 2 A pretty lecture experiment for de- 

 monstrating the presence of nitrates in the root, is made by 

 cutting a root in two, and simply applying diphenylamine solution 

 to the cut surface. The blue coloration at once appears. 



The occurrence of nitrates in plants has been exhaustively 

 studied by Berthelot and Andre, 3 and by Schimper. 4 They found 

 nitrates very widely distributed in plants, and if we cut sections 

 of the stem of Helianthus, or e.g. leaves of Sambucus nigra, let 

 them dry to some extent on the slide, and then treat them with 

 a drop of the diphenylamine solution, the blue coloration in- 

 stantaneously appears. In most leaves, e.g. those of Sambucus 

 nigra, the nitrates are confined almost entirely to the nerve 

 parenchyma; in some cases, however (leaves of Tradescantia 

 Selloi), the mesophyll is also very rich in nitrates. Nitrites 

 which also give a blue colouration with diphenylamine and 

 Sulphuric acid, do not, as far as is at present known, occur in the 

 living plant. It is to be emphasised that the failure of reaction- 

 with diphenylamine is not conclusive evidence of ths absence of 

 nitrates from the tissues under investigation, since many sub- 

 stances occur in plant cells whose presence would interfere with 

 the reaction. 



It is of great interest to establish the fact that there exist 

 organisms which have the power of converting ammonia, occurring 



* The Sulphuric acid must, of course, be free from Nitric acid, and not of 

 itself give a red reaction with brucine solution. The acid may be freed from 

 Nitric acid, which may be present, by boiling with a very small quantity of 

 sulphur. 



