268 ORGANIC AGRICULTURAL CHEMISTRY 



Starch as Food. — Physiologically starch is a reserve food 

 material for the plant itself, as has already been discussed. 

 It occurs in the leaves and other green parts of all green plants, 

 where, as we have shown, it is a temporary reserve material, 

 being immediately converted into sugars and translocated. 

 In these parts of the plant it possesses food value for animals, 

 but as it never collects in any amount, its actual value is 

 slight, the chief carbohydrate constituent here being cellulose. 



Those portions of the plant in which starch is most abundant 

 are the strictly reserve organs, e.g. seeds, tubers, roots, etc. 

 In these organs the plant has stored large amounts of starch, 

 for the use of its offspring or for renewed growth, and it is these 

 storage houses of the plant which form the most valuable animal 

 foods. The three most important groups of plants in this respect 

 are: (i) cereal plants, in which the starch is stored in the seed, 

 i.e. the cereal grains, e.g. wheat, maize, oats, barley, rye, sorghum 

 grain, millet, etc. ; (2) tuber-forming plants, e.g. potato ; (3) roots 

 of certain plants, e.g. arrowroot, cassava, etc. ; (4) unripe fruits, 

 e.g. apples, pears, etc., which are not, however, important as a 

 source of starch because the fruit is never used in this state, 

 and on ripening the starch is converted into sugars. 



As was stated in the chapter on the chemistry of carbo- 

 hydrates, the grains of starch found in different plants have in 

 each case a distinctive microscopical structure. As a food 

 material there is no difference in these various forms, so that in 

 this respect starch is a single substance. Both human beings 

 and animals digest starch ; in the former case it is easily digested 

 only when first cooked, but in the latter raw starch is the 

 common form in which the food is eaten. Its common and 

 abundant occurrence in so many foods makes it in many in- 

 stances the particular constituent upon which the food value 

 of the material largely depends. Several forms of pure starch 

 are used as human food, e.g. ordinary cooking starch, usually 

 obtained from corn, and tapioca, which is a specially prepared 

 form of starch obtained from the cassava plant. 



Industrial Uses. — For industrial purposes, such as the manu- 



