394 TEXTBOOK OF BOTANY 



2. Examine a rod that is not moving. Little can be seen of 

 its structure because of its small size. You will observe that it 

 does not change its shape, being kept rigid by the cell waU. Does 

 the protoplasm appear granular? Draw a cell. 



3. Study the rows or chains of cells. How are the chains formed ? 

 Are the ends of individual cells rounded or flattened? Draw a 

 chain in outline, showing the cell boundaries. 



4. Examine a bit of the scum that forms on the surface of the 

 infusion after a few days. In it will be found many bacteria held 

 together by a sticky substance. Are the cells moving or quiet? 

 Are they single or in colonies ? 



5. Look for bacteria of other forms in the infusion. They are 

 more likely to appear after some days. Draw in outline such 

 forms as you find, on the same scale as your previous drawings. 



6. In the scum from an old infusion, look for chains of cells in 

 which spores are being formed. Draw such a colony if you find 

 one. 



7. What causes the unpleasant odor that the infusion gives oflE? 



8. Boil a good-sized potato with its skin on for fifteen or twenty 

 minutes. Then, with a clean, sharp knife that has been sterilized 

 in a flame, cut the potato into thin slices, being careful not to touch 

 the cut surface with the fingers. Seven petri dishes and covers 

 must be prepared in advance. Each should be carefully washed 

 and then boiled in clean water while the potato is boiling. When 

 the potato is ready to slice, remove the petri dishes from the water 

 one at a time, pour out the water and place in the bottom a piece 

 of perfectly clean filter paper which, if possible, has been sterilized 

 by heat in a dry oven for a half- hour. Dip the paper into the 

 water in which the petri dishes are boiled. Handle dishes and 

 paper with stout forceps which have also been sterilized in a flame. 

 Place a slice of potato in each petri dish on the filter paper and 

 cover at once. All this work must be done as quickly as possible. 



9. Leave one dish covered. Uncover a second one and leave 

 it exposed to the air of the room for a half hour, then cover it again. 

 Uncover a third one long enough to make a scratch on the surface 

 of the potato with a needle that was first sterilized, then dipped 

 into a hay infusion. The fourth potato slice should be inoculated 

 in the same way with material from the floor of the room. 



