INTERRELATIONS OF PLANTS AND ANIMALS 125 



3. Why use yeast in wine making ? Beer making ? 



4. Are there yeast plants in the air ? How can you prove it ? 



Special Reports 



1. Bread making. 



2. The brewing industry. 



3. Some useful fungi. 



4. Some harmful fungi. 



5. The wheat rust. 



6. The corn smut. 



7. The chestnut fungus. 



References 



Hunter, Essentials of Biology. Chap. XIII. 

 Hunter, Elements of Biology. Chap. XI. 

 Conn, Bacteria, Yeast, and Molds in the Home. 



PROBLEM XXIII 



A study of bacteria, and of some of their relations to 

 man. 



a. Conditions of Growth 



Materials. One pint of fresh milk, box of cracked ice. 

 Nutrient gelatine in tubes as put up by Parke, Davis & Co. 

 Some sterilized Petri dishes with covers. Some sterilized 

 flasks and corks. (Sterilize them by boiling them in water 

 for fifteen minutes.) Alcohol lamp or Bunsen burner. 



Observations. 1. Pour equal amounts of milk into each of 

 3 flasks. Label them 1, 2, and 3. Cork No. 1 and set it aside 

 in a moderately warm room. Cork No. 2 and place it in an ice 

 box or in cracked ice. Boil the milk in No. 3 for half an hour, 

 plug the flask with sterilized cotton, and set it with flask No. 1. 

 Are there any differences in smell and taste in the course of a 

 few days ? 



2. Put some nutrient solution in each of three sterilized 

 Petri dishes. Label them 1, 2, and 3. Expose No. 1 to the 

 dusty air of a room for a few minutes. Cover it and set it 

 aside in a shady warm place. Examine it after a day or so. 

 Is there any noticeable effect on the gelatine ? 



