ASSIMILATION OF CARBON 197 



will require constant supplies of carbon, hydrogen, oxygen, 

 nitrogen, sulphur, and phosphorus, all of which are required to 

 build up protoplasm. In addition, protoplasm contains water. 

 In flowering plants the living protoplasm can be studied only 

 by the aid of the compound microscope. It then reveals itself 

 as a colourless, transparent, viscid substance, often with small 

 granules in it : it is capable of growing, dividing, and moving 

 about. All the complex processes performed by plants are due 

 to the action of protoplasm. Therefore, in studying plants 

 without the aid of a compound microscope, we are largely 

 engaged in learning the properties of this living substance — 

 protoplasm. 



II. Carbohydrates are simpler bodies than proteids ; they 

 contain no nitrogen. Sugars are carbohydrates which are 

 soluble in water. The sugar used for domestic purposes is 

 Cane-sugar (C12 Hog On), which occurs in many plants 

 (notedly in Sugar-Maples, Beet, Sugar-Cane). Grape-sugars 

 (Cb H12 Oe) also are found in plants. The test for grape- 

 sugar is to warm Fehling's blue solution, which, on the 

 addition of grape-sugar, forms a yellow precipitate. Starch 

 (Ce Hio O5) is a solid body which is insoluble in water. 

 By the use of dilute acids, or of certain "ferments," starch 

 may be converted into sugar. Starch is easily recognised 

 by its character of becoming blue on the addition of iodine. 

 A drop of iodine placed on the cut surface of a potato- 

 tuber, on ground rice or corn, causes a deep blue spo't, thus 

 showing that these bodies contain starch. Cellulose. — The 

 solid framework or skeleton of a plant is mainly constituted 

 of cellulose, or bodies allied to cellulose. Cellulose is a 

 solid, colourless substance insoluble in water. When treated 

 with sulphuric acid, it swells up and forms a substance like 

 starch-paste, and then it will turn blue when treated with 

 iodine. More prolonged treatment with the dilute acid causes 

 the cellulose to change into sugar. Unchanged cellulose is 

 stained yellow by iodine. Wood and Cork may, for the present, 

 be regarded as peculiarly modified celluloses. Thus the 

 familiar carbohydrates are all sugars or substances easily 

 convertible into sugars. 



III. Fats and Oils. — These might be included under one 

 name, as a fatty-oil is merely a liquid fat. They are composed 

 of carbon, hydrogen, and oxygen, but contain no nitrogen. 



