278 LEAVES 



as cane sugar but differing in arrangement of atoms, occurs in 

 germinating seeds, and is especially abundant in germinating 

 Barley. It is formed when starch is broken into its components. 

 Maltose, when broken into its constituents, gives rise to two 

 molecules of glucose. 



The starches are the most abundant storage forms of food into 

 which the photosynthetic sugar is converted. They occur in all 

 parts of the plant, but are especially abundant in seeds, tubers, 

 and fleshy roots, where they are extensively used by man for food. 

 The food value of Corn, Wheat, Rice, and other Cereals, and of 

 Potatoes depends mainly upon the starch which they contain. 

 The starches, unlike the sugars, are in- 

 soluble and occur in the form of definitely 

 shaped bodies, known as starch grains, 

 which vary much in shape, size, and mark- 

 ings in different plants. {Fig. 249.) The 

 size, shape, and markings of the starch 

 grains are so characteristic of many plants 

 that by a study of the starch grains in a 

 mixture of ground vegetable products, the 

 constituents can often be determined and 

 , - the adulterants thereby detected. 



Fig 249 —a starch Starch and cellulose have the same for- 

 graiu of Irish Potato; mula (C6Hio05)n but they differ in the num- 



b, starch grain of Wheat; ber of combinations, CeHioOs, contained in 



c, starch grain of Corn. ^j^^-^. j^^^iecules. Their exact difference in 

 structure is not known, for the number of combinations, CeHioOs, 

 contained in a molecule of either starch or cellulose has not been 

 determined. They differ in physical properties as well as chemi- 

 cally. The starches are readily converted back into sugar, in 

 which form they are used by the plant. The starches are broken 

 into simpler compounds by a group of enzymes, and the prod- 

 ucts formed are dextrin, maltose, and glucose according to the 

 extent to which the starch molecules are broken up. 



The hemi-celluloses are quite prominent in some seeds, and 

 occur in many other places in the plant. They are much like 

 ordinary cellulose, but, being easily converted into sugar, they 

 are available sources of food and on this account are often called 

 reserve celluloses. They are usually very hard substances and 

 are deposited as extra layers on the cell walls. The hardness of 



