I 



PLANTS WHICH CAUSE DECAY 389 



process to that of tobacco -curing, but how far it is 

 produced by bacteria is not known. 



Another class of plants, very different from the 

 bacteria, which cause fermentation are the Yeasts. 

 Eub up a quarter of a yeast -cake in water to make a 

 paste; add this to a pint of water in which a table- 

 spoonful of honey or sugar has been dissolved. Fill 

 three good- sized bottles half full of the (well -stirred) 

 liquid, and stopper them by simply allowing the cork 

 to rest in the neck of the bottle without forcing it 

 down into it. 



Put one in a warm place in the dark, one in a cool 

 place (preferably on ice) in the dark, and one in a 

 warm place exposed to bright light. Observe every few 

 hours, noting the turbidity of the liquid, growth of the 

 Yeast, evolution of gas bubbles, change in taste, etc. 



Repeat the experiments shown in Figs. 30 and 31, 

 using Yeast instead of seeds. The experiment of lower- 

 ing a lighted match into a bottle in which Yeasts are 

 growing may also be tried: if it goes out it indicates 

 the presence of carbon dioxide. 



Examine under the microscope a little of the yeast- 

 cake rubbed up in water: notice the appearance of the 

 Yeast-cells: add a little iodine, and observe. How is 

 the yeast-cake prepared? 



Take some of the sediment from the bottom of the 

 yeasu culture and examine under the microscope. 

 Notice the appearance of the Yeast- cell (Fig. 213) 



