THE FUNGI 249 



ordinary leaf cells, wood-ray cells, or root cells. Among the 

 higher fungi, as we shall learn later, a great variety of digestive 

 and energy -forming ferments are secreted, which aid both para- 

 sitic and saprophytic fungi in securing their food. 



The use of fermentation to the yeast plant, beyond the diges- 

 tion of the small amount of food needed for reproduction and 

 growth, is not certainly known. It is supposed that it uses 

 some of the energy produced by fermentation for its life proc- 

 esses, as the higher plants and animals use the energy resulting 

 from respiration. This idea seems probable, for the reason that 

 certain yeasts can live and grow for a considerable time in a 

 sugar solution without the presence of oxygen for normal res- 

 piration. Other organisms, namely, bacteria and certain molds, 

 are able to produce energy by fermentation in the absence of 

 oxygen. This has led to the classification of organisms as 

 aerobic and anaerobic, according to their method of securing 

 energy for the life processes. 



Aerobic organisms secure this energy by means of oxygen 

 used in normal respiration, while anaerobic organisms are able 

 to secure it by fermentations in the absence of oxygen. Some of 

 the yeasts are able to accommodate themselves to either mode of 

 life, and are thus either aerobic or anaerobic, according to the 

 nature of the medium in which they are placed. 



Bread making. In the making of bread some active yeast, 

 composed of starch mixed with some sugar and water, is added 

 to bread sponge. The action of the yeast on the sponge is a 

 double one, caused by the secretion of a digestive cane-sugar 

 ferment and of an alcoholic energy-forming ferment, zymase. 

 The cane-sugar ferment transforms the cane into grape sugar, 

 as explained above, while the zymase completes the transforma- 

 tion of grape sugar into alcohol and carbon dioxide. The carbon 

 dioxide is liberated in the sponge and causes the rising of the 

 bread. The almost imperceptible amount of alcohol produced 

 disappears in the baking. The two formulae given above under 

 digestion would therefore indicate in general terms the action 

 of the yeast ferments in bread making. The cane sugar upon 

 which the yeast acts must be supplied in the sponge. 



