274 CONTENTS AND PRODUCTS OF CELLS 



of their starch grains. Detection of adulteration of various 

 products containing starch is accomplished by the aid of 

 the microscope. This method is now particularly important 

 in determining adulteration of stock foods. In potato starch 

 the grains are ovate, with a "nucleus" near one end, as 

 shown in Fig. 444. In poinsettia they are dumb-bell shaped, 

 with two nuclei. (Fig. 444.) In corn they have equal diame- 

 ters, with radial fissures. In Egyptian lotus they are forked 

 or branched. So far as known, all starch grains are marked 

 with rings, giving a striated appearance, due to the differ- 

 ence in density of the layers. When all 

 water is driven out of the starch, the rings 

 disappear. The layers are more or less 

 concentric, and are formed about a starch 

 nucleus. 



460. Starch grains may be simple, as 

 found in potato, wheat, arrow-root, corn, 



444. Starch grains. ' . 



a, potato; b, poinsettia; and many others; or they may be in 

 c> nce> groups called compound grains, as in 



oats, rice (Fig. 444), and many of the grasses. 



461. Starch may be found in all parts of the plant. It 

 is first formed in presence of chlorophyll in daytime, mostly 

 in the leaves, and at night it is converted into sugar and 

 then it is carried to some other part of the plant, as to 

 the roots or tubers, to be stored or to be used. When 

 found in the presence of chlorophyll, it is called transitory 

 starch, because it is soon converted into sugar to be trans- 

 ported to other parts of the plant. When deposited for future 

 use, as in twigs and tubers, it is stored starch. 



462. The composition of starch is represented by 

 C 6 HioO 5 . The grains are insoluble in cold water, but by 

 saliva they are changed to sugars, which are soluble. Great 

 heat converts them into dextrine, which is soluble in water. 

 Starch turns blue with iodin (76). The color may be de- 

 stroyed by heat, but will return as the temperature lowers. 



