STARCH 45 



broad ends, and invested by several layers common to 

 both. 



I. Sections should also be cut from the Potato so as 

 to show the starch-grains in situ in the cells. The 

 razor should be wetted with water, and one section 

 (the thinnest cut) should be mounted in water : a 

 section which runs out to a thin edge will be found 

 to be best : examine under a high power, and 

 observe 



1. The numerous starch-grains as before. 



2. The thin cell-walls partitioning off the cells 



which are of considerable size, and each of them 

 may contain a large number of starch-grains. 



3. The protoplasm, which is so scanty as often to 



escape observation. 



c. Mount a small quantity of starch-grains in water 

 as before, and irrigate with iodine solution : the starch 

 granules will stain a more or less deep blue according 

 to the strength of the solution : this is the characteristic 

 reaction of starch. 



d. Treat another preparation of starch with strong 

 chlor-zinc-iodine : the starch-grains will as before 

 assume a blue colour, but they also swell, and lose 

 their bright, high refractive properties. This fact is to 

 be borne in mind when treating tissues containing 

 starch with this reagent. 



e. Mount a fresh slide of starch in water, and irri- 

 gate with solution of potash : observe that as the 

 reagent gains access to the granules they swell, and 

 at the same time assume a dull appearance, their high 

 refractive power being lost as they take up additional 

 water of imbibition under the influence of the reagent. 



