VARIETIES OF YEAST. 191 



are apt to harbor yeast-cells in the dust which lodges upon them. More 

 recently it has been shown that wild yeasts often live under apple-trees- 

 upon the surface of the earth. In a dry time the wind easily lifts the dust 

 containing them and conveys them over great distances (cf. Amwha, 

 Infusoria, etc.). The domesticated yeasts of to-day are probably the de- 

 scendants of similar wild yeasts. 



Red Yeast. One of the finest of the wild yeasts is the so-called "red 

 yeast," which is furthermore very easy to study. Red yeast, and many 

 others not red, grow luxuriantly upon a jelly, made by thickening beer- 

 wort with common gelatine. In this way "pure" cultures — that is, cul- 

 tures free from other species of yeasts, or bacteria, and consisting of one 

 kind only — can be easily made and studied. The microscope shows that 

 the cells of red yeast, which form red dots upon such jelly, are not them- 

 selves colored, but the pigment appears to lie between the cells, as in the 

 case of the " miracle germ " {Bacillus prodigiosus). 



Fermentation. To the processes where yeast is employed to- 

 produce chemical changes in various domestic, agricultural, and 

 industrial operations the term fermentation.^ or more often 

 alcoholic fermentation.^ is applied. In the '' raising" of bread 

 or cake, in brewing, cider-making, etc., yeast acting upon 

 sugar produces from it an abundance of alcohol and carbon 

 dioxide. Both products are sought for in brewing, and carbon 

 dioxide is especially desired in bread-making. 



But alcoholic fermentation is only one example of a large 

 class, and yeast is only one of many ferments. We may, there- 

 fore, postpone further consideration of fermentation to the next 

 chapter. 



Related Forms. It has been shown by the researches of Hansen that 

 ordinary commercial yeast is seldom one single species, as was formerly 

 supposed, but rather a mixture of several species. It is therefore no 

 longer safe to speak of commercial yeast as Saccliaromyces cerevisiop, unless 

 careful examination by the modern methods has shown it to be such ; and 

 to determine what species exist in any particular specimen is often a labori- 

 ous and difficult matter. 



Inasmuch as the natural position of yeast in the vegetal kingdc^m is 

 not established beyond all doubt, it is impossible to state precisely what 

 are its near relatives. There are numerous unicellular colorless plants, but 

 they are not necessarily closely related to yeast ; and the student must not 

 conclude for plants any more than for animals that because an organism 

 unicellular it is necessarily at the very bottom of the scale of life. 



