PHYSIOLOGY. 



pand greatly. This causes the bread to "rise" more, and baked in 

 this condition it is "light." There are two special processes accom- 

 panying the fermentation by yeast: ist, the evolution of carbon dioxide 

 as shown above; and, 2d, the formation of alcohol. The best illus- 

 tration of this second process is the brewing of beer, where a form of 

 the same organism which is employed in "bread rising" is used to "brew 

 beer." 



237. The yeast plant. Before the caustic potash is placed in the tube 

 some of the fermented liquid should be taken for study of the yeast plant, 

 unless separate cultures are made for this pur- 

 pose. Place a drop of the fermented liquid 

 on a glass slip, place on this a cover-glass, and 

 examine with the microscope. Note the min- 

 ute oval cells with granular protoplasm. These 

 are the yeast plant. Note in some a small 

 "bud" at one side of the end. These buds 

 increase in size and separate from the parent 

 plant. The yeast plant is one celled, and 

 multiplies by "budding" 

 or "sprouting." It is a 

 fungus, and some species 

 of yeast like the present 

 one do not form any my- 

 celium. Under certain 

 conditions, which are not 



very favorable for growth 

 Fig. 1080. 



Yeast. Saccharo- ( exam P le > when the yeast is 

 myces ceriviseae. a, grown in a weak nutrient 

 small colony; b, single . 



cell budding; c, single substance on a thin layer 

 Fig. 1 08. cell forming an ascus Q t 



Fermentation tube filled ""^ *~" " " * 



with CO 2 from actipn of 

 yeast in a sugar solution. 



spores free from the several spores are formed 

 ascus. (After Rees.) . 



in many of the yeast cells. 



After a period of rest these spores, will sprout and produce the yeast plant 

 again. Because of this peculiar spore formation some place the yeast 

 among the sac fungi. (See classification of the fungi.) 



238. Organized ferments and unorganized ferments. An organism 

 like the yeast plant which produces a fermentation of a liquid with evo- 

 lution of gas and alcohol is sometimes called a ferment, or ferment or- 

 ganism, or an organized ferment. On the other hand the diastatic fer- 

 ments or enzymes like diastase, taka diastase, animal diastase (ptyalin in 

 the saliva), cytase, etc., are unorganized ferments. In the case of these 

 it is better to say enzyme and leave the word ferment for the ferment 

 organisms. 



