EXERCISE 76 (Continued) 



7. What is the effect of temperature upon bacterial growth? 

 Prepare six dishes in such manner that they will be approximately 

 equally inoculated with bacteria. This may be done as follows : 

 Pour a few drops of liquid from the decaying beans used in the pre- 

 ceding exercise, or from some similar source, into a beaker of warm 

 water. Stir the water until it may be supposed that the bacteria 

 are evenly distributed through it. Melt the agar in six tubes, as 

 directed on page 95. When the tubes and water by which they 

 are surrounded have come to a temperature of about 45 C., add to 

 each tube a small measured quantity of the water from the beaker. 

 One or two drops from a medicine dropper to each tube will be suffi- 

 cient. Be sure to add the same number of drops to each tube. Pour 

 into petri dishes. Place two dishes in an ice box, two others in 

 a warm place, and let the other two remain on the laboratory desk. 

 Examine them daily and note the differences in size of colonies. 

 What is your conclusion ? 



8. Do chemical poisons prevent the growth of bacteria ? Prepare 

 tubes as above, but in each place small quantities of formalin, boracic 

 acid, or other preservatives. If several tubes are prepared contain- 

 ing varying quantities of the same preservative, it will be possible 

 to secure an indication of the quantity of each which is necessary. 



9. Are there bacteria in drinking-water ? Proceed as in Prob- 

 lem 7, but inoculate tubes with drinking-water, 5 to 10 drops, instead 

 of with water from the beaker. 



10. Are common drinking-cups contaminated? Place the edge 

 of a public drinking-cup in contact with the agar in a dish. Observe 

 results. 



References 



BERGEN and CALDWELL. Practical Botany, chap. xi. 

 BERGEN and CALDWELL. Introduction to Botany, chap. xiv. 



[97] 



