CARBOHYDRATES. 43 



differing in form and size with different plants. The concentric rings 

 represent its gradual growth. Starch is scarcely changed at all by cold 

 water, but warm water makes the grains swell up and finally burst, form- 

 ing starch-paste. Starch-paste does not reduce metallic oxides in alkaline 

 solutions. A very rapid swelling is brought about at ordinary temper- 

 atures by means of concentrated solutions of metallic salts. Even with 

 dilute alkalies a starch-paste may be prepared in a short time. A well- 

 known test for starch is the indigo-blue coloration produced by iodine 

 solutions in the presence of hydriodic acid or an iodide. The color- 

 ation is not permanent on boiling, but reappears on cooling. The 

 presence of substances capable of being oxidized by iodine (caustic alkali, 

 sulphurous acid, arsenious acid, etc.) will prevent the appearance of this test, 

 the blue color only being obtained when such impurities have been oxidized. 

 All varieties of starch do not give a blue color with iodine; some of them 

 give a reddish-brown color, and with others the color is that of red wine. 



At present we do not know a great deal concerning the significance of 

 these different colorations; it is positively certain, however, that starch 

 cannot be regarded as a chemical individual. The conception " starch " 

 comprises a large group of substances of similar physical and chemical 

 properties, which form a unit on account of their common biological signifi- 

 cance. An attempt has been made to get an idea concerning the structure 

 of starches by studying their decomposition products. On boiling them 

 with dilute acids, glucose is obtained. If the acid is allowed to act in 

 the cold, or with only gentle heating, a hydration product is produced 

 which is known as " soluble starch." By the action of cold, dilute mineral 

 acids for several weeks, or by an hour's treatment with 4 per cent, sul- 

 phuric acid at 80 C., the so-called " amylodextrin " is obtained, and, 

 on further hydrolysis of the latter, dextrins are formed, while, as just 

 mentioned, the final product is grape-sugar. We stated in connection 

 with maltose that a similar breaking up of the starch molecule could be 

 effected by ferments, in this case diastatic ferments. It was also men- 

 tioned then, that at present we are not able to deduce a picture of the 

 starch formation from a study of the great number of intermediate products 

 obtained by partial hydrolysis and designated in the literature with par- 

 ticular names. We must be satisfied, for the time being, with the knowl- 

 edge that amylum contains a large number of anhydride-like grape-sugar 

 molecules, and by taking on water it is decomposed, step by step, into 

 smaller molecules, and finally into the basal component glucose. We 

 shall find, later on, that the proteins are quite similarly constituted. 

 Soluble starch, amylodextrin, dextrin, etc., correspond to the albumoses and 

 peptones, while glucose, the elementary building material, corresponds to 

 the amino-acids. A similar analogy is found with the fats, although here 

 the relations are much simpler. 



