PLANTS WITHOUT CHLOROPHYLL 139 



Bread Making. Most of us are familiar with the process of 

 bread making. The materials used are flour, milk or water or 

 both, salt, a little sugar to hasten the process of fermentation, or 

 " rising," as it is called, some butter or lard, and yeast. 



After mixing the materials thoroughly by a process called " knead- 

 ing," the bread is put aside in a warm place (about 75 Fahrenheit) 

 to " rise." If we examine the dough at this time, we find it filled 

 with holes, which give the mass a spongy appearance. The yeast 

 plants, owing to favorable conditions, have grown rapidly and filled 

 the cavities with carbon dioxide. Alcohol is present, too. but this 

 is evaporated when the dough is baked. The baking cooks the 

 starch of the bread, drives off the carbon dioxide and alcohol, and 

 kills the yeast plants, besides forming a protective crust on the loaf. 



Sour Bread. If yeast cakes are not fresh, sour bread may result 

 from their use. In such yeast cakes there are apt to be present 

 other tiny one-celled plants, known as bacteria. Certain of these 

 plants form acids after fermentation takes place. The sour taste 

 of the bread is usually due to this cause. The remedy would 

 be to have fresh yeast, to have good and fresh flour, and to have 

 clean vessels with which to work. 



Importance of Yeasts. Yeasts in their relation to man are 

 thus seen to be for the most part useful. They may get into 

 canned substances put up in sugar and cause them to "work," 

 giving them a peculiar flavor. But they can be easily killed by 

 heating to the temperature of boiling. On the other hand, yeast 

 plants are necessary for the existence of all the great industries 

 which depend upon fermentation. And best of all they give us 

 leavened bread, which has become a necessity to most of mankind. 



BACTERIA IN THEIR RELATION TO MAN 



What Bacteria do and Where They May be Found. A walk 

 through a crowded city street on any warm day makes one fully 

 alive to odors which pervade the atmosphere. Some of these un- 

 pleasant odors, if traced, are found to come from garbage pails, 

 from piles of decaying fruit or vegetables, or from some butcher 

 shop in which decayed meat is allowed to stand. This character- 

 istic phenomena of decay is one of the numerous ways in which 



