CRYPTOGAMS 257 



220. YEAST 



Object. To demonstrate the result of desiccation on 

 yeast. 



Method. Mix a little fresh yeast with some corn meal 

 and a small quantity of sugar until it has the consistency of 

 stiff dough. 



Spread it on a board to dry. When it is thoroughly dried, 

 put it away in a jar and keep it dry. 



After a week or two, dissolve a little in a vessel of warm 

 sweetened water and keep it in a warm place. Does it 

 show signs of fermentation ? How do you account for this ? 

 Does drying kill or merely check the development of yeast 

 plants ? 



Try other portions after a longer interval. Result? Can 

 yeast be kept indefinitely in a dry state ? 



Why does the grocer sell dry yeast of any age, but only 

 fresh moist yeast ? 



Bacteria 



Bacteria are very minute plants, so minute that it requires 

 the highest power of the microscope to reveal them. Indeed 

 they cannot be seen clearly even with the microscope with- 

 out the use of various staining reagents. 



As individual plants, therefore, they are not to be con- 

 sidered in a work of this character. They may, however, 

 be studied in a broad way, since it is possible to cultivate 

 them in various nutrient solutions, where they develop so 

 rapidly as to form spots resembling mold, which are colonies 

 of bacteria. 



Culture Medium. A standard culture medium is made out 

 of the following: 



EXP. BOX. 17 



