196 THE FOOD AND NUTKITION OF PLANTS. 



Grape-vine, &c., but usually in the fruit. If produced directly from 

 the sap, as they may be in acid leaves, only a part of the oxygen in 

 the carbonic acid which contributes to their formation would be ex- 

 haled. But if formed from sugar, or any other of the general pro- 

 ducts of the proper juice, the absorption of a portion of oxygen from 

 the air would be required for the conversion ; and this absorption 

 takes place (at least in some cases) when fruits acquire their acidity. 

 Even their formation by the plant, therefore, is attended by the lib- 

 eration of oxygen gas, though in less quantity than in ordinary vege- 

 tation. 



354. There is still another class of vegetable products of uni- 

 versal occurrence, and, although comparatively small in quantity in 

 plants, yet of as high importance as those which constitute their 

 permanent fabric ; namely, the neutral quaternary organic com- 

 pounds, of which nitrogen is a constituent (79). These, also, are 

 mutually convertible bodies, related to each other as dextrine and 

 sugar are to starch and cellulose, and playing the same part in the 

 animal economy that the neutral ternary products do in the vege- 

 table, i. e, forming the fabric of animals. The basis or type of 

 these azotized products has received the name of Proteine (27) : 

 hence they are sometimes collectively called proteine compounds. 

 In their production from the plant's food, the ammonia, or other 

 azotized matter it contains, plays an essential part ; and oxygen gas 

 is restored to the air from the decomposition of all the carbonic acid 

 concerned and of a part of the water.* 



355. In living cells the proteine forms the protoplasm, or vitally 

 active lining, which may be said to give origin to the vegetable 

 structure, since the cellulose is deposited under its influence to 

 form the permanent walls or fabric of the cells, as has already been 

 explained (26-36). When the cells have completed their growth 



96 76 6 266 96 76 6 266 



Besides, proteine either contains or is naturally combined with a small quan- 

 tity oi sulphur ani phosphorus (10). 



