12 



PHYSIOLOGY 



Fig. 5.— A section of potato showing starch 

 grains within the cells. 



that it contains other substances besides the living sub- 

 stance. Such a cell may be obtained by cutting a very 

 thin section of a potato. (Fig. 5). When we look at 



the section with the 

 compound microscope 

 we see two things; the 

 walls of a series of ir- 

 regular boxes, and en- 

 closed by them a great 

 many transparent glob- 

 ules. 



Starch in the Potato. 

 — What are these glob- 

 ules ? Doubtless in the 

 beginning chemists tried many tests before they found 

 out, but now that we know the test to apply, it is 

 a very simple matter to prove that the globules are 

 made of starch. Iodine stains brown whatever it 

 touches except starch and this it stains blue. We may 

 test this by putting a few drops on a lump of starch and 

 on something which we know contains no starch. If we 

 put a drop of iodine on the potato cell the protoplasm and 

 the cell walls turn brown and the globules turn blue. 

 The globules are therefore made of starch. After this 

 we will refer to them as starch grains. 



Starch Used for Food. — What are the starch grains 

 in the potato for? The starch is made and stored for 

 the sake of furnishing the living substance of the potato 

 with something with which it can repair wear and tear. 

 It is one of the substances out of which new protoplasm 

 is made. 



