280 LEAVES 



Beans, etc. About 15 percent of the dry weight of Irish Potatoes 

 is starch, and in roots, stems, leaves, and in fruits it is present 

 in considerable amounts. Its abundance and wide distribution 

 is due to the fact that it is the chief form in which plants store 

 food. In leaves, when photosynthesis is active and sugar is 

 being formed more rapidly than carried away, the surplus sugar 

 is changed to starch and thus temporarily stored until the starch 

 is changed back to sugar to be transported to other parts of the 

 plant. In seeds starch is stored to serve as food for the young 

 plant during germination. In tubers, roots, and stems starch is 

 stored to be used in new growth. 



Unhke the sugars starch is insoluble in the cell sap. It occurs 

 as definitely shaped bodies, known as starch grains with size, shape 

 and markings different, in most cases, in 

 different species of plants {Fig. 250). So 

 characteristic are the size shape, and mark- 

 ings of the starch grains of many plants 

 that they are used in identifying adulter- 

 ants in ground vegetable products. Sugar 

 is transformed to starch by the chloroplasts 

 and plastids, and starch grains may be so 

 numerous within cells as to almost entirely 

 ^-^.y - I \ ' fill the cell cavities. 



° c Starch and cellulose have the same f orm- 



^ „^„ , , ula (C6Hio06)n, but the n in the starch 



Fig. 250. —a, starch i • j j. i, j-£c 4. 



grains of Irish Potato; formula is supposed to have a different 



fc, starch grains of Wheat; value, and possibly there is a different ar- 



c, starch grains of Corn, rangement of atoms, for the two substances 



have very different properties. Starch is readUy decomposed 



by enzymes and artificially by acids into dextrin, maltose, and 



glucose, the product depending upon the extent to which the 



starch molecule is decomposed. 



Starch and the sugars constitute the basic foods for animals. 



The value of cereals and most all seeds for food is chiefly due to the 



starch contained. In the digestive system it is changed to sugar 



in which form it is utilized. A number of commercial substances 



are made from starch, such as glucose, corn sirup, corn starch, 



tapioca, and dextrin. Dextrin is a gum-like substance and is 



used in photography, calico printing, making ink, and in the 



manufacture of paper. 



