FUNGI, SAC FUNGI 297 



plant, however, is supposed to be a degenerate form, for some of 

 the other sac fungi as well as some of the spore-case fungi (or 

 alga-like fungi) under certain conditions degenerate into yeast 

 forms. No one, however, has ever succeeded in changing the 

 true yeasts into any of these better organized forms, though 

 some yeasts form a temporary mycelium which later breaks up 

 into yeast cells. 



446. Fermentation of yeast. * Yeast is used in bread- 

 making to produce the "rising" of the bread which causes it to 

 become spongy and light. It is a very inter- 

 esting plant, and the work it performs in bread 

 " rising" it very instructive in connection with 

 the subject of respiration (Chapter XV). 



Fermentation accompanies respiration of the 

 yeast plant, and is more rapid when respiration 

 takes place in the absence of oxygen (para- Fig. 245. 



graph I0<0. Yeast - . Saccharo- 



myces cenviseae. a, 



The tube and the lower part of the bulb of 

 a fermentation tube is filled with a solution of 

 grape sugar or an infusion of potato, f and a 2scu? f (StRe ) 

 small piece of yeast cake is put in the bulb. 

 This is kept in a warm place. As the yeast plant grows, bub- 

 bles of gas will rise in the tube and displace the solution which 

 is forced into the bulb. If some pieces of caustic potash are 

 placed in the bulb, the potash solution absorbs the gas in the 

 tube and the solution rises again. This shows that the gas is 

 carbon dioxide (paragraph 188) which is an index of respiration 

 of the yeast plant, or rather of the fermentation of the sugar in 

 the solution. 



This ferrrtentation has gone on largely in the absence of air, 

 but it is somewhat at the expense of growth and reproduction of 

 the yeast plant, for when air is present growth and multiplication 

 of the yeast is more rapid. During the fermentation of the 



* Saccharomyces ceriviseae. 



j" If possible it is well to sterilize the tube after filling with the grape sugar 

 or infusion. Also a second tube for a check is desirable. 



