162 THE PLANT CELL. 



i. Starch and Starch- formation. 



Starch, which has a composition represented by the general 

 formula 0,.Hj„0^,* is a carbohydrate belonging to the group of 

 polysaccharides. It occurs in plant-cells, either alone, or in chloro- 

 plasts or plastids, in the form of granules and grains of various 

 sizes and shapes. In order to examine starch, a small portion of a 

 thin slice of a potato-tuber should be placed in a small drop of 

 water on a slide, and gently squeezed. The slice is then 

 removed, and the now somewhat opalescent drop covered with 

 a cover-slip and examined under a low power. Numerous 

 starch-granules are then seen, which, when examined by trans- 

 mitted light, have a semi-translucent refractile appearance, but 

 by reflected light are white and opaque. The size of the indi- 

 vidual granules varies from a small circular particle to the large 

 oval grains many times the size of the former; and, by using 

 the ^-in. objective, cutting off the peripheral illuminating rays, 

 and using somewhat oblique illumination, one of the larger 

 granules may be seen to possess the following structure 

 (see Fig. Ill):— 



a. A dark spot situated somewhat eccentrically : this is the hllum 

 of the grain. 



b. Outside this alternating layers of light and dark lamellSB, ar- 

 ranged round the hilum, but, as a rule, thicker on one side of the hilum 

 than the other (see 4, Fig. 111). 



c. If the plane mirror of the microscope be used for illuminating, the 

 rays will be partially polarised ; and if, after these rays have passed 

 through the starch-granule, they be again passed through a NiCOl'S 

 prism (analyser) in the eye-piece of the microscope, they can be 

 analysed by rotating the prism (contained in the eye-piece) so that its 

 axis assumes different inclinations. The result of this analysis shows that 

 a granule of starch is made up of alternating zones of two substances 

 which rotate the plane of polaPisation in different directions, and 

 that one of these substances contains more wateP in its composition than 

 the other. The starch-grain is thus anlsotrople. 



In form, the larger granules in cells of potato-tubers are 

 oval, whilst the smallest are circular, no hilum being present in 

 these latter. In the cells of maize endosperm, the granules are 

 polyhedral, and in the rhizome of Iris, dumb-bell shaped. 



In potato, much of the reserve starch in the tuber-cells is 



* Usually found together with a certain amount of water of constitu- 

 tion. Cellulose is represented by the formula n(C6Hio05). 



