458 METABOLISM IN LIVING PLANTS. 



is obtained from nitric acid and ammonia, and sundry of their compounds, 

 especially calcium nitrate. These are absorbed by the plant and conveyed by 

 the crude sap to the place of consumption. The nitric acid must, of course, be 

 liberated from this salt, and this is brought about by the union of the calcium 

 with oxalic acid, derived from a portion of the carbohydrates, the two thus 

 forming insoluble crystals and crystalline masses of oxalate of lime (fig. 123). 

 The liberated nitric acid is now reduced in a manner analogous to that of the 

 carbonic acid in the formation of carbohydrates. It is supposed that the nitrogen 

 of the nitric acid then combines with a hydrocarbon, forming an amide 

 (asparagin, leucin, tyrosin), and that the albumen is formed by the union of this 

 with a carbohydrate. Sulphur is derived from the calcium sulphate, or from 

 some other sulphate, by the intervention of oxalic acid, in the same manner 

 as just described for nitrogen. The oxalic acid forms an insoluble salt with the 

 calcium or other base of the sulphate, which separates out in the cells in the 

 form of small crystals. The liberated sulphuric acid must then, in some way, 

 undergo a further reduction, in order that sulphur may enter into the molecule 

 of the albumen. Among the vegetable albumens are to be distinguished albumin, 

 casein, and fibrin. The glutin contained in corn, is a mixture of a casein and a 

 fibrin. All these albumens appear in soluble and insoluble forms. Thus, for 

 example, the conglutin contained in almonds is a soluble casein, and goes into 

 solution when milk of almonds is made by adding water to the almonds; while 

 the legumin contained in peas, beans, lentils, and other pulse seeds, is not soluble 

 in water, and can only be dissolved by pepsin in the presence of an acid. Al- 

 though all these albuminous compounds cannot be recognized by any definite 

 form, the aleurone grains and the so-called crystalloids have perfectly definite 

 shapes. The crystalloids are formed of albuminous substances, and have exactly 

 the appearance of crystals (fig. 123^"^^). 



Next to the albuminous substances, the most important building material 

 to be noticed is cellulose. This is a carbohydrate consisting of six atoms of 

 carbon, ten of hydrogen, and five of oxygen, and is produced from the primary 

 sugar-like carbohydrates. The transformation is effected by the living protoplasts, 

 which form a layer of cellulose on their periphery, called the cell-wall. At first 

 this cell-wall is mainly composed of pure cellulose; then, according to need, the 

 carbohydrate is changed by the protoplasm, either wholly or partially, into some 

 other carbohydrate, either into woody material (lignin) or cork (suberin), or the 

 cellulose becomes mucilaginous, as, for example, in the seed-coat of the Quince. 

 In the stems and branches of cherry, plum, almond, apricot, and peach trees, the 

 cellulose is generally hardened into a sticky, shapeless, amber-coloured substance, 

 which exudes from the fissures of the bark, and is known by the name of cherry- 

 gum (cerasin). In like manner gum-arabic (arabin) is formed from the cellulose 

 in the stems of some acacias, and gum-tragacanth in several tragacanth shrubs 

 (species of Astragalus). 



Protoplasm forms cellulose from a portion of the primary sugar -like carbo- 



