STUDIES OF SPORE-PLANTS 



269 



apples, and other fruits, commonly undergo a change called 

 fermentation. The result of this process is the production 

 of an invisible gas (carbon dioxide), which causes efferves- 

 cence of the fermenting liquid, and alcohol. No other method 

 of producing alcohol is known to occur in nature; and so 

 for thousands of years fermentation has been used in making 

 wines and other alcoholic beverages from fruit juices. It 

 was not until the nineteenth century that the microscopic 

 yeast plants were recognized as the cause of fermentation; 

 and the experiments performed by the great French naturalist, 

 Louis Pasteur, between 1855 and 1865 will always be famous 

 as having placed our knowledge of fermentation processes on 



a thoroughly scientific foundation. 

 i 



251. Study of Yeast Plants. (L) Scrape some small pieces from 

 a cake of compressed yeast, mix with a drop of water on an object- 

 slide, and gently lower a cover r glass 

 into position. Also, if brewer's or 

 baker's yeast in liquid form is avail- 

 able, mount a drop on another slide. 

 Prepare a third slide by mounting a 

 drop of molasses-solution ) water 10 

 parts, common dark-colored molasses 

 1 part) in which some scrapings of com- 

 pressed yeast or some drops of brewer's 

 yeast were placed on the previous day 

 and the liquid kept in a warm place 

 (70 to 90 F). It is best to draw up 

 with a rubber-bulbed pipette, or with a 

 dipping tube, some of the white sedi- 

 ment in the bottom of the bottle con- 

 taining the molasses-solution. 



Examine the slides with a high power 

 of microscope. Small oval translucent 

 bodies, some isolated, some united into 

 groups or chains, will be seen. Each 

 oval body is a yeast plant ; and it is a 



single cell. The chains are due to reproduction by formation of 

 buds. By comparing the sizes of the cells in a chain, it is possible to 

 determine the order in which the buds were formed (Fig. 83). Soon 



FIG. 83. Yeast cells, two cells 

 beginning to bud, and three 

 chains of cells, formed by 

 repeated budding. Clear 

 centers are cavities. Nuclei 

 not shown. (From Sedgwick 

 and Wilson.) 



