138 MYCOLOGY 



4. Species which ferment dextrose and maltose, but not saccharose 

 Saccharomyces n. sp. obtained from stomach of bee by Klocker. 



5. Species which ferment neither maltose, dextrose nor saccharose: 

 Saccharomyces anomalus var belgicus, S. farinosus, S. hyalosporus, S. 

 membranifaciens . 



The general chemic phenomena associated with the formation of 

 alcohol by fermentation out of sugar may be expressed by the formula: 



CeHijOe = 2C2H6O + 2CO2 



Alcohol Carbon 



dioxide 



The carbon dioxide passes off in bubbles as a gas, while the alcohol 

 remains in solution. 



The most important yeast is the beer yeast Saccharomyces cerevi- 

 sicB which is a unicellular plant of spheric or elliptic shape 8 to i2ju 

 long and 8 to ion broad. Sometimes the cells formed by budding 

 remain connected to form a chain consisting of the mother, daughter, 

 granddaughter and great-granddaughter cells. Spore formation is 

 characteristic and the size of the spores varies from 2.5 to 6;u. There 

 are usually four spores in each cell. The following gives the tempera- 

 ture conditions of spore formation in this species; 



At 9°C. no spores develop. 



At 11° to i2°C. the first indications are seen after 10 days. 



At 30°C. the first indications are seen after 20 hours. 



At 36° to 37°C. the first indications are seen after 29 hours. 



At 37.5°C. no spores develop. 



The temperature limits for film formation are 33° to 34°C. and 6° 

 to 7°C. There are a number of races of the common beer yeast, which 

 may be separated into the bottom yeasts and the top yeasts. The bot- 

 tom yeasts are those which hve within the liquid and mostly at the 

 bottom even from the start. Some of these yeasts form spores with 

 diflB-Culty. The top fermentation yeasts are those which grow on the 

 surface of the liquid and cause a brisk fermentation with a large amount 

 of froth, or head, as exemplified by the Munich lager-beer yeasts. 

 Yeasts are among the oldest of cultivated plants, as in biblical times 

 leavened (yeast-raised) and unleavened bread were known. The 

 leaven was a lump of dough kept from one baking to the next. Un- 

 leavened bread was simply flour mixed with water and baked, and as 

 a result, a hard tough bread was obtained. The use of yeast as a 



