FERMENTATION 



199 



2. THE NATURE OF YEAST. 



A pennyworth of yeast is made up of many thousands of round or 

 oval cells, each one of which is an independent organism. Under the 

 microscope each has the appearance of a miniature ball (Fig. 118). 

 Within each ball are found : 



1. The protoplasm or living matter. 



2. A number of substances which are destined to build up fresh 



protoplasm or have been formed by the breaking down of 



the latter. 



In fact all the essential activities which we associate with living 

 organisms are all performed within the compass of each little 



FIG. 118. Group of cells of Saccharomyces cerevisiae. (After Hansen.) 



globule. Each one, during the process of multiplication, absorbs 

 food from the medium in which it is growing; it elaborates 

 this food into protoplasm ; it respires, as a result of which energy is 

 liberated and waste products excreted. Finally, each is able to 

 reproduce itself. This normally takes place by a process of budding. 

 A small wart-like protuberance appears on the surface of the cell. 

 This becomes larger and larger, until it is as large as the cell which gave 

 birth to it (Fig. 119). This daughter- 

 cell when mature has exactly the same 

 structure as the mother-cell. This 

 mode of formation of new cells is called 

 "budding. The daughter-cell in turn 

 buds off a cell by the same process, 

 whilst the mother-cell forms another 

 daughter-cell at another point, and so 

 the process goes on until a large number 

 of cells are formed. Sometimes each cell, as soon as formed, is 

 completely separated from the cell that bore it ; but often the process 

 of budding takes place more quickly than that of separation, so that 

 bunches of cells are seen clinging together, all originally derived from 

 one cell. The fermentation that is going on, the apparent " boiling," is 



FIG. 119. Stages in formation of new 

 yeast-cells by process of budding. 



