4IO General Botany 



The best known of the sac fungi are the yeasts, the green molds 

 {Penicillium and Aspergillus), the powdery mildews, the cup fungi 



{Peziza) and morels (Morchella), 

 and the lichens. 



The yeasts. The yeasts are 

 small, one-celled plants that mul- 

 tiply very rapidly. In the mak- 

 ing of bread, they are of primary 

 importance. When properly 

 mixed with flour and water they 



Fig. 251. Yeast (Saccharomyces): cells develop in all partS of the doUgh. 

 and branching filaments. Above are three The ycastS have withiu them 

 cells, each containing four resting spores. , . , . ,. • . . 



enzymes which oxidize part of 

 the sugar that is present into carbon dioxide and alcohol. 

 In this way the yeast obtains its energy. The carbon 

 dioxide accumulates in bubbles and causes the dough to 

 rise and become " light." When the dough is put into a hot 

 oven, the alcohol is vaporized, and together with the carbon 

 dioxide it is driven off into the air. The high temperature kills the 

 yeast, bakes the dough, and changes some of the starch into its 

 soluble form, dextrin, which makes it more readily digestible. 

 Sour bread is produced when the yeast that is added contains 

 acid-forming bacteria which change part of the alcohol into 

 acetic acid. 



Yeasts and bacteria are the organisms that change fruit juice 

 into " hard " cider and vinegar. Yeast first changes the sugar 

 in apple juice to carbon dioxide and alcohol, and bacteria further 

 oxidize the alcohol to acetic acid, thus forming vinegar. Yeasts 

 are also used in the manufacture of beer and wines. 



Yeast fungi may readily be grown by adding a bit of yeast 

 to a 5 per cent sugar solution in a test tube. The branching 

 groups of cells may then be examined under the microscope. 

 The manner of forming new cells among yeasts is unique, in that 

 the new cells start as small protuberances (buds) from the older 



