PLANTS WRICH CAUSE DEC AT 



367 



about in the liquid (Fig. 208). Add a drop of iodine 

 solution, and observe. 



We may get a good idea of the relative size of the 

 bacteria by comparing them with the starch - grains, 

 which latter are too small to 

 be distinctly visible to the 

 naked eye but are easily seen 

 with a hand- lens. 



Do you find that the spots 

 on the potato differ in color, 

 consistency, shape and gen- 

 eral appearance? If so, they 

 are, in all probability, due to 

 different kinds of bacteria. In 

 order to separate the various 

 kinds, we may proceed as 

 follows: Fill a vial half full of distilled water, and 

 sterilize for half an hour. Then pass the needle through 

 the flame two or three times ; when cool touch it to the 

 spot from which a culture is desired, and stir it about 

 in the vial. Remove the needle, sterilize in the flame, 

 and then dip it in the vial again and touch it to a pre- 

 viously sterilized slice of potato (arranged as before in 

 a tumbler), at various points a little distance apart, 

 and at once replace the cotton. ^ If the amount of water 

 used is sufficient, the bacteria will be so scattered that 



208. A scraping from a potato cnlttire, 

 showing stareh-grains and bacteria. 

 A good way to get an idea of the 

 size of bacteria is by comparing 

 them with some well-known object, 

 such as a starch-grain. 



1 During this operation the tumbler should be held nearly horizontal, to 

 keep bacteria from falling upon the potato from the air. 



