132 NATURE STUDY 



on a little powdered starch, or in water in which starch paste has 

 been mixed. It shows a blue color. If a drop of iodine is placed 

 on a fresh-cut slice of a potato or with the scrapings from the 

 slice, it gives the same color. 



If a potato has been allowed to sprout and the sprouts grow 

 until the potato becomes watery and shrunken, it will not give 

 the blue color. The starch has disappeared. 



With a microscope one can show the starch grains in the 

 cells of the potato by examining a thin slice. This, stained 

 with iodine, will show the starch grains blue. In the sprouted 

 potato, the cells will be seen empty of starch grains. 



The change from starch to sugar may be shown well with 

 barley grains. Crush grains of barley and with the iodine show 

 that they contain starch. Sprout other barley grains by keeping 

 moist and warm. When just beginning to sprout, stop the pro- 

 cess and dry thoroly with gentle heat (do not scorch); these grains 

 will be quite sweet to the taste showing presence of sugar. 



In many fruits, starch is first formed in the green fruit which 

 is changed to sugar on ripening. A green apple will show by 

 iodine the presence of starch in abundance, while the ripe apple 

 will show the absence of starch and the sweet taste shows that 

 sugar is present. 



It may be taught in this connection that the digestion of 

 starch in the human body by the saliva and by the pancreatic juice 

 is a process exactly similar to that in the potato, grains and fruit. 

 In the plant, the starch is changed to sugar by a substance in 

 the plant's juices, and in the body, by a substance in saliva or 

 pancreatic juice. 



In the plant, the sugar is food for growing cells in the sprout- 

 ing tuber or grains; in the body, sugar is food for the action or 

 growth of the cells of the body. 



