116 



PLANTS WITHOUT CHLOROPHYLL 



Problem 94 : What are the conditions favorable for the growth 

 of yeast in bread ? ( Home work. ) 



Materials. Flour, water, sugar, salt, yeast cake, pans. 



Method. Make a dough by mixing flour, sugar, salt, and water 

 in proportions to make a thick paste. Knead with a little yeast 

 which has previously been mixed with water. Now place one lot 

 of dough in the ice box, one at the temperature of the room, and 

 one in a warm place (over 90 F.). Later bake each lot and use 

 in the laboratory. 



Observations. Which of the three lots has raised the most? 

 Which, after baking, has the best appearance? The best taste? 

 What makes the holes in the bread ? 



Conclusion. 1. What caused the bread to rise? 



2. Under what conditions does this best take place? 



Experiments with yeast may be continued almost indefinitely. 



For excellent suggestions, see 

 Conn's Bacteria, Yeasts, and 

 Molds in the Home, pp. 274-278. 



Problem 95: How we pro- 

 ceed to the study of bacteria. 



Materials. Dry sterilizer, 

 steam sterilizer, Petri dishes, 

 fermentation tubes, pipettes, 

 and compound microscope. 



Several pieces of apparatus 

 are useful though not indispen- 

 sable in the study of bacteria. 

 All of this apparatus should be 

 shown to the pupils and its use 

 explained. Older pupils should 

 be encouraged to assist in pre- 

 paring the culture media and 

 in the subsequent sterilization 

 of the material. Sterilizers may 

 be improvised by using two 

 pans, one of which fits closely 



% W&ZEQ,. A A _*ynz/^ 



Heat applied under the sterilizer turns 

 the water into steam, which circulates 

 through the holes in the shelves, es- 

 capes between the door and the inner 

 jacket, and returns as water after con- 

 densing between the inner and outer 

 jackets of the sterilizer! 



