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 



i 



STERILIZING 



CHAMBER. 



Heat applied under the sterilizer turns 

 the water into steam, which circulates 

 through the holes in the shelves, es- 

 capes between Che door and the inner 

 jacket, and returns as water after con- 

 densing between the inner and outer 

 jackets of the sterilizer. 



