390 MICROORGANISMS 



scope that men were able to find out much about them. In 

 recent years we have learned many interesting things about 

 yeasts, but there are still many things that we do not under- 

 stand about them. You will doubtless be interested in learn- 

 ing some of the things that are known about these little plants 

 that play so large a part in our lives. Without them we should 

 have no leavened bread but we should also be without a 

 temperance problem, for they are responsible for all the 

 alcohol that is used in intoxicating drinks. 

 442. Study of Yeasts. 



Exercise 88. Growing Yeast 



Mix about 2 tablespoonfuls of molasses of some kind with a quart 

 of warm water in a suitable glass vessel. Stir the mixture thor- 

 oughly and then add about one-half of a cake of compressed yeast, 

 breaking the yeast cake into small particles. (If compressed yeast 

 is not available, dried yeast cake will answer, but it will be very 

 much slower in action.) If possible, keep the mixture at a temper- 

 ature of from 70 to 90F. Note the bubbles of gas that soon begin 

 to rise in the mixture. Mount a small drop of the mixture on a 

 microscope slide and study under the high power of the microscope. 

 Note the size, shape, abundance, and general character of the yeast 

 plants. In this fresh mixture, you will probably not find any bud- 

 ding cells for, in the yeast cake, the plants are more or less dormant. 

 If you study in the same way an older mixture of the same kind 

 which your instructor may have prepared a couple of hours or more 

 before class, you will find many budding plants. Recall the state- 

 ment previously made that it is this peculiar method of reproduction 

 which distinguishes the yeasts from other fungi and which gives 

 them the name, budding fungi. 



Exercise 89. Growing Yeast Produces Carbon Dioxide 



Fill a small bottle about two-thirds full with the yeast mixture. 

 Fill another small bottle up to the neck with clear lime water. Put 

 a perforated cork into the bottle containing the yeast mixture and 

 connect the two bottles by a U-shaped glass tube as shown in Fig. 

 268. As the gas which you have observed rising from the yeast 

 mixture accumulates above the liquid, it will be forced through the 

 glass tube over into the limewater. Observe the white milky 



