PROCURING OBJECTS. 



75 



iodine capable of acting on them while the membrane enclos- 

 ing its contents remains whole. If the granules be triturated 

 or immersed in hot water, the membrane will be rupturecl, and 

 iodine will then turn them blue. Starch is readily separated 

 from wheat, potato, arrow-root, &c., by repeated washings in 

 cold water. To obtain it from rice, the grains should be mace- 

 rated for a few days, and to prevent the decomposition of the 

 gluten, a little soda should be added to the macerating water. 

 Under the microscope, the surface of starch-grains often appears 

 corrugated, and each of them has one or two bright spots, 

 called the hilum, which is supposed to be the part where the 

 starch adheres to the cell. See Fig. 25. a, represents starch 

 cells of the pea, showing grains of starch in the interior; 6, 

 separate grains of starch, with strise and hilum ; c, granules 

 of wheat-starch ; d, sago meal ; e } rice-starch ; /, potato- 



starch ; g, isolated cells of rhubarb, containing starch-granules. 

 Under polarized light they present the beautiful phenomenon 

 of the black cross. They should be mounted dry, and protected 

 from the pressure of the upper glass by a rim of thin paper. 



