190 THE PLANT WORLD 



Practical Exercise 3. What are the direct values of bacteria to (1) market 

 gardening; (2) fruit raising ; (3) manufacturing? 



Self-Testing Exercise 



Bacteria which cause (1) are useful. More and better 



crops are made possible through the (2) (3) bacteria. 



Bacteria are used in the processes of (4) fibers of plants, 



(5) of hides, (6) tobacco, and giving (7) 



to some animal products. 



PROBLEM IV. WHAT ARE YEASTS AND WHAT DO THEY DO? 



Fermentation. It is of common knowledge that the juice of 

 fresh apples, grapes, and some other fruits, if allowed to stand 

 exposed to the air for a short time, will ferment. That is, the 

 sweet juice will begin to taste sour and to have a peculiar odor, 

 which we recognize as that of alcohol. The fermenting juice 

 appears to be full of bubbles which rise to the surface. If we 

 collect enough of the gas in these bubbles to make a test, we find 

 it is carbon dioxide. 



Evidently something changed some part of the apple or grape, 

 namely, the sugar (C 6 Hi 2 6 ), into alcohol (C 2 H 5 OH) and carbon 

 dioxide (CO2). This chemical process is known as fermentation. 



Home Experiment. To determine the conditions favorable for the 

 growth of yeast. Label three pint fruit jars A, B ; and C. Add one 

 fourth of a compressed yeast cake to two cups of water containing two 

 tablespoonfuls of molasses or sugar. Stir the mixture well and di- 

 vide it into three equal parts and pour into the jars. Place covers 

 on the jars. Put jar A in the ice box on the ice and jar B over the 

 kitchen stove or near a radiator. Heat jar C by immersing in a pan 

 of boiling water, and then place it next to B. After forty-eight hours, 

 see if bubbles have made their appearance in any of the jars. 



Which jars, if any, show bubbles on the surface? Describe the 

 conditions which favor the growth of yeast. Explain how you know 

 that yeast has grown. 



Yeast. If a small piece of compressed yeast cake is shaken 

 up with some molasses and water and the mixture allowed to 

 stand overnight in a warm place, fermentation will take place. 

 Examination of a drop of the settlings from the mixture shows that 

 the common compressed yeast cake contains millions of tiny yeast 



