200 THE FOOD AND NUTRITION OF PLANTS. 



ation, contain, a few of them no oxygen at all (such as caoutchouc 

 and some oils), and all of them less oxygen than is requisite to con- 

 vert their hydrogen into water. In their direct formation, there- 

 fore, not only all the oxygen of the carbonic acid has been given 

 out, but also a portion belonging to the water. If formed by a 

 further deoxidation of neutral ternary products, as chlorophyll from 

 starch (87) and wax from sugar (86), the same result is attained 

 as respects the liberation of oxygen gas, but by two or more steps 

 instead of one. The Resins, doubtless, are riot direct vegetable 

 products, but originate from the alteration and partial oxidation of 

 the essential oils. Balsams, which exude from the bark of cer- 

 tain plants, are natural solutions of resins in their essential oils, as 

 rosin, or pine-resin, in the oil of turpentine. 



353. An opposite class, the Vegetable Acids (90), contain more 

 oxygen than is necessary for the conversion of their hydrogen into 

 water, but less than the amount which exists in carbonic acid and 

 water. Indeed the most general vegetable acid, the oxalic (which 

 is formed artificially by the action of nitric acid on starch), has no 

 hydrogen, except in the atom of water that is connected with it. 

 These acids are sometimes formed in the leaves, as in the Sorrel, 

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

 from the sap, as is probably the case in acid leaves, only a part of 

 the oxygen in the carbonic acid which contributes to their forma- 

 tion would be exhaled. But if they are formed from sugar, or any 

 other of the general products of the proper juice, the absorption of 

 a portion of oxygen from the air would be required for the conver- 

 sion ; 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 liberation of oxygen gas, though less 

 in quantity than in ordinary vegetation. 



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

 versal occurrence, and, although comparatively small in quantity, 

 of as high importance as those which constitute the permanent 

 fabric of the plant ; namely, the neutral quaternary organic com- 

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

 ally convertible bodies, related to each other as dextrine and sugar 

 to starch and cellulose, and playing the same part in the animal 

 economy that the neutral ternary products do in the vegetable. 

 To the basis or type of these azotized products Mulder has given 

 the name of Proteine (27) : hence they are sometimes collectively 



