I40 METABOLISM 



I. Belonging to the aliphatic series : 



1. Guanidin residue — CNH.NHj. 



2. Amino-acids. 



a. Monamino-acids : leucin, glycocoll, alanin, aspartic, and gin- 



taminic acids. 



b. Diamino-acids : ornithin (united with guanidin to form 

 arginin), lysin, histidin. 



3. Carbohydrate groups. 



II. Belonging to the aromatic series : 



1. Tyrosin. 



2. Phenylalanin. 



III. Heterocyclic groups : 



1. Pyrrol group. 



2. Indol group. 



3. Pyridin group. 



From this general summary we may conclude that, among the proteid 

 reactions, the xanthoproteic test and Millon's test for the tyrosin group, 

 Molisch's test for carbohydrate, the lead-sulphide reaction for sulphur groups, 

 are the most characteristic; the biuret test alone applies to the complete proteid 

 molecule. 



The classification of proteids given above is provisional ; it is based more 

 on solubility, coagulability, &c., than on constitution. For our purpose the 

 following summary will suffice : — 



I. True Proteids : 



1. Albumins. These bodies are soluble in pure water and can often 



be crystallized. 



2. Globulins. Insoluble in water, soluble in dilute solutions of neutral 



salts, from which they may be precipitated unaltered by removal 

 of the salts. 



3. Nucleo-albumins. Distinguished by containing phosphorus. 



II. Proteids ; compounds of albumin with other bodies, and more com- 



plicated than true proteids : 



1. Nucleo-proteid. Compounds of proteid and nuclein ; occurring 



especially in the nucleus. 



2. Haemoglobins. Compounds of proteid and haematin ; a decom- 



position product of haematin is haematoporphyrin (referred to at 

 p. 109). 



III. Glutinoids. Bodies of simpler composition than typical proteid, in 

 which individual proteid groups are wanting. 



So far as we know the true proteids occurring in plants belong especially 

 to the globulins and nucleo-albumins ; albumins proper occur only occasionally. 

 Owing to the sparing solubility of vegetable proteids, a fact which has been 

 drawn attention to by Winterstein (1901), it has come about that many pro- 

 teids in the plant have been quite overlooked, e. g. in Vaucheria (Reinke, 

 1883). We imagine that extraction with baryta water, 20 per cent, hydro- 

 chloric acid, &c., as tried by Winterstein, would result in the discovery of 

 proteids in such cases. 



Let us now return to our main problem, where and how are nitrates of 

 potash and ammonia assimilated in the green plant ? 



The nitric acid present in the soil obviously penetrates the proto- 

 plasm and is easily absorbed by the root in dilute solution. In many plants 

 nitric acid occurs in such quantities that its determination presents no 

 difficulty. Although micro-chemical methods fail to demonstrate its presence 

 in other plants we must not assume its instantaneous alteration in the root- 

 cells, since many secondary conditions may interfere with the ordinary tests 

 (e. g. diphenylarnin) for the presence of nitric acid. Tobacco, turnips, sunflowers, 



