ORGANIC COMPOUNDS OF PLANTS 183 



value of n has been variously estimated from 2 to 200. 

 The different starch grains, as of wheat, corn and oats, 

 are all composed of the same elements, C, H, and O, and 

 differ in form because these elements are put together in 

 a different way in each. When acted upon by heat, as in 

 the popping of corn, the starch grains are ruptured. At a 

 temperature above 120 C., starch is changed to dextrin. 

 In the presence of water and dilute acids, starch gradually 

 undergoes hydration ; that is, water is chemically added 

 to the molecule. By the joint action of heat, ferments, 

 and various chemicals, starch is converted into a number 

 of products, as soluble starch and dextrose. In the 

 presence of iodin, starch is colored blue, different kinds 

 of starch giving different shades and tints. The nature 

 and mechanical form of the starch granules in a food, 

 determine, to a slight extent, its rapidity and ease of di- 

 gestion. Some starches are more easily digested than 

 others, and all undergo important chemical and physical 

 changes in the cooking and preparation of food. Since 

 starch makes up such a large portion of many human and 

 animal foods, its composition, properties, and food value 

 are of prime importance. 



237. Function and Value. In the seed, starch is a 

 reserve form of food for the use of the young plant before 

 it is able to obtain its own food ; in roots and tubers also, 

 it is stored up for that purpose. Many crops, as pota- 

 toes, corn, and sago, contain so much starch that they 

 are often cultivated for starch-making purposes. Starch 

 is obtained mechanically from potatoes by first pulping 

 to break the cells, and then washing the pulped mass 



