138 PLANTS WITHOUT CHLOROPHYLL 



the alcohol remaining in the fluid, and the carbon dioxide passing 

 off into the air. At the right time the beer is stored either in 

 bottles or casks, but fermentation slowly continues, forming car- 

 bon dioxide in the bottles. This : gives the sparkle to beer when it 

 is poured from the bottle. 



In wine making the wild yeasts growing on the^ skin of the grapes 

 set up a slow fermentation. It takes several weeks before the 

 wine is ready to bottle. In sparkling wines a second fermentation 

 in the bottles gives rise to carbon dioxide in such quantity as to 

 cause a decided frothing when the bottle is opened. 



Commercial Yeast. Cultivated yeasts are now supplied in 

 the home as compressed or dried yeast cakes. In both cases the 

 yeast plants are mixed with starch and other substances and 

 pressed into a cake. But the compressed yeast cake must be used 

 fresh, as the yeast plants begin to die rapidly after two or three 

 days. The dried yeast cake, while it contains a much smaller 

 number of yeast plants, is nevertheless probably more reliable if 

 the yeast cannot be obtained fresh. 



The cut illustrates 

 an experiment that 

 shows how yeast 

 plants depend upon 

 food in order to grow. 

 In each of three fer- 

 mentation tubes were 

 placed an equal 

 amount of a com- 

 pressed yeast cake. 

 Then tube a was 

 filled with distilled 

 water, tube b with a 

 solution of glucose 

 and water, and tube 

 c with a nutrient solution containing nitrogenous matter as well 

 as glucose. The quantity of gas (C02) in each tube is an index of 

 the amount of growth of the yeast cells. In which tube did the 

 greatest growth take place ? 



