262 



Science of Plant Life 



Yeast first changes the sugar in apple juice to carbon dioxid 

 and alcohol, and bacteria further oxidize the alcohol to acetic 

 acid or vinegar. Yeasts are also used in the fermenting of 

 beer and wines. 



Yeast may readily be grown by adding a bit of yeast to a 

 5 per cent sugar solution in a test tube. The branching groups 

 of cells may then be examined under the microscope. The 

 manner of forming new cells among yeasts is unique, in that the 

 new cells start as small protuberances (buds) from the older 

 cells. These buds gradually enlarge until they attain their 

 complete growth and separate. The alcohol formed in the 

 test tube by the yeast may easily be detected by its odor. 



The molds. The molds are usually white, filamentous 

 plants that are of great economic importance because of the 

 damage that they do to foods during storage or shipment. 

 Like bacteria, the spores of molds are in the air and in the 

 dust everywhere. If the temperature is warm and the 

 food is moist, they germinate and, together with bacteria, soon 



Fig. 154. Bread mold, showing the rhizoids which penetrate the material on which 

 the mold grows and which absorb food, the horizontal filaments by which it spreads, 

 and the vertical filaments which bear the sporangia and spores. 



