STARCH 29 



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 irrigate 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. Now wash out the potash thoroughly with water, and 

 irrigate the preparation with iodine solution : the swollen grains 

 will still stain blue, though much paler than before, showing 

 that the swelling with potash does not fundamentally alter the 

 nature of the starch. 



f. Mount some fresh starch in water, and heat it over a 

 spirit-lamp till it boils : on examining under the microscope, 

 the grains will be seen to have swollen and lost their high 

 refractive power, forming starch-paste : staining with iodine will 

 produce the blue colour, and show that they are only swollen, 

 not dissolved : compare the effect of potash. A temperature of 

 about 65 C. is sufficient to cause this swelling. 



g. Digest starch-grains in saliva for some hours at a 

 temperature of about 45-55 C. Examine them subsequently 



