YEASTS (SACCHAROMYCES) 



377 



Certain species 1 give desirable flavors to some kinds of cheese 

 and are quite useful in this connection. 



Naked-ascus Fungi (Protodiscales). This is a small group of 

 parasites which attack seed plants. They produce no ascocarp 

 and the asci are therefore borne exposed (Fig. 328}. So far as 

 known they have no sexual reproduction. They are regarded 

 as simple Ascomycetes. One common species is the Exoascus 

 deformans, which causes the disease known as Peach Curl. The 

 mycelium develops in the tissues of the host and forms on the 

 surface asci which appear as gray pow- 

 dery films. One species attacks the 

 young ovaries of Plums, causing the 

 malformation known as " Bladder 

 Plums/ 7 and one species causes Witches' 

 Brooms on some of our deciduous trees. 



Yeasts (Saccharomyces). The 

 Yeasts are very simple Ascomycetes. 

 In most Yeasts the hyphae are so short 

 and simple that they appear as single 

 globular cells The only reason for 

 calling them Ascomycetes is that under 

 certain conditions the cells form spores 

 and then resemble asci (Fig. 329). 



On account of their ability to fer- 

 ment sugars and produce carbon dioxide 

 and alcohol, they are useful in making 

 bread and in making alcohol, wine, 



beer, and other liquors which contain alcohol. When placed in 

 dough they grow and work rapidly, and the carbon dioxide pro- 

 duced causes the bread to rise. There are many kinds of Yeasts, 

 and each kind gives a different flavor to the fermented product. 

 For this reason brewers keep pure cultures of certain kinds of 

 Yeasts, which give the liquor the desired characteristics. 



Their main method of reproduction is by the rapid division of 

 cells, often called budding, in which small cells are apparently 

 pinched off from the parent cell. The cells often remain in 

 contact for some time after being budded off, forming chains of 

 cells. 



1 Cultural Studies of Species of Penicillium. Bulletin 148, Bureau of 

 Animal Industry, U. S. Dept. Agriculture, 1911. 



FIG. 329. Bread Yeast, 

 Saccharomyces cerevisiae. a, 

 single plant (X 600); 6, a 

 plant in the process of bud- 

 ding; c, plant which has 

 formed spores; d, plants re- 

 maining in contact and 

 forming chains as they are 

 multiplied by budding. 



