58 CELL-CONTENTS AND CELL-WALLS. 



better to use a weaker iodine solution than that used in 

 testing for proteins. 



(a) Pat a drop of water on a slide, dip into it a cut 

 piece of tuber, and note the small white starch-grains that 

 escape from the opened cells. With the microscope, note 

 that the grains show delicate lines, corresponding to the 



Fig. 21. Part of a Section of Potato Tuber, with Starch Grains. X 200. 



thin layers built up around the first-formed portion of the 

 grain, which appears as a clear spot (hilum) placed ex- 

 cent rically. 



(b) Cut thin sections from the tuber, and note that the 

 cells of the parenchyma have thin walls and are almost 

 filled up by the numerous grains, there being very little 

 protoplasm. Add iodine, and examine again ; to see the 

 stratification more clearly, use very weak iodine solution. 



(c) Mount some grains in water, and treat with chlor- 

 zinc-iodine ; the grains turn blue, as with iodine, but 

 swell and become less bright and refractive. 



