195 



We have now the position, the general appearance and the usual 

 test for starch. Let us examine it more carefully to see what its 

 structure is. For this purpose take a very little matter scraped 

 from the cut surface of the potato ; aim to get only starch and 

 none of the cell-structure. Place this on a clean glass slide, add 

 a drop of water and put on a thin glass cover as before. By care- 

 ful management of the light, we can probably now see the concen- 

 tric rings quite plainly. They have the shape of the outline of 

 the starch-grain. If it is egg-shaped, as is usually the case, so 

 are they. If it is almost triangular or linear, as sometimes hap- 

 pens, so are the concentric rings. These rings are sometimes 

 easily seen ; at others, only considerable care can bring them out. 

 I have sometimes found them plainer in the starch from a sprout- 

 ing potato than from others. In potatoes frozen and thawed, they 



appear distinct. If they can be brought out in no other way, the 

 application of dilute chromic acid will usually show them very 

 plainly. A few lines, scattered among the rest, are generally 

 plainest. 



Are these lines simple markings on the surface of the grains, or 

 are they edges of layers one inside the other? In order to ascer- 

 tain that we must roll them over and see how they look on the 

 edge or on the other side. This is easily clone. We have only to 

 incline the body of the microscope a little more, and sorne of the 

 grains will he carried down by the action of gravity, and will roll 

 over with more or less freedom. If this does not serve, we can 

 press on one edge of the thin cover with the point of a pencil, and 

 a great commotion will be caused among the grains. As this sub- 

 sides, we can watch some of them rolling over leisurely, now 

 stopping for a moment on the face to give us an opportunity to 

 examine that side, then rolling up on edge and hesitating there 



