248 



CONTENTS AND PKODUCTS OF CELLS 



391. Starch grains. , 



a, potato; b, poinsettia; 



c, rice. 



abundant of the solid products of the cell. Starch grains 

 have a definite form for each group of plants, and groups 

 can be determined by the form of their starch grains. 

 Detection of adulteration of various products containing 

 starch is accomplished by the aid of the microscope. In 

 potato starch the grains are ovate, with a "nucleus" near 

 one end, as shown in Fig. 391. In poinsettia they are dumb- 

 bell-shaped, with two nuclei (Fig. 391). 

 In corn they have equal diameters, 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 difference 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. 



399. Starch grains may be simple, as found in potato, 

 wheat, arrow-root, corn, and many others; or they may be 

 in groups called compound grains, as in oats, rice (Fig. 

 391), and many of the grasses. 



400. Starch may be found in all parts of the plant. 

 It is first formed in presence of chlorophyll, mostly in 

 the leaves, and from there 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 

 liquid compounds to be transported to other parts of the 

 plant. When deposited for future use, as in twigs and 

 tubers, it is stored starch. 



401. The composition of starch is in the proportion of 

 CeHioOs. 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. 



