70 ORGANIC COMPOUNDS IN PLANTS. 



249. All these organic compounds are very nearly 

 related, and often change from one into another. Cellu 

 lose may turn into starch, gum or sugar. So may vege 

 table jelly. Gum or dextrine may be converted into sugar. 

 These substances appear to go successively through all 

 these forms, from sugar, the most soluble, to cellulose, 

 the most insoluble. All these substances, 241, taken into 

 the animal system, are supposed to aid in the process 

 of breathing, and keeping up the warmth of the body. 



250. There is another class of substances found in 

 plants, of which the cell-walls are not formed, and which 

 yet are essential to the simplest processes of vegetation. 

 They are composed of the elements of water, of carbon, 

 and also of nitrogen, to which are sometimes added phos 

 phorus and sulphur. From the nitrogen contained in 

 them, they are often called Nitrogenous Compounds. In 

 their simplest form they are composed of the four atmos 

 pheric elements only, and are found in a fluid, semi-fluid, 

 or solid state, within the cells ; and without their presence 

 in a liquid state no new cells can be formed. From their 

 great variety of appearance, and the readiness with which 

 they change, these substances have been called Protein, 

 from the name of an imaginary being, Proteus, who was 

 fabled to assume every variety of form, to conceal himself. 



251. Protein, in combination with sulphur, forms casein, 

 with still more sulphur and a little phosphorus, albumen, 

 and with more both of sulphur and phosphorus, Gluten 

 or Vegetable Fibrinc. These substances are of great 

 importance, and of the highest interest, from the fact that 

 though essential to the bodies of animals, constituting 

 the muscles and giving them strength, they are not, 

 according to some chemists,* formed in the animal 



* * Liebig, and others. 



