190 UNICELLULAR PLANTS. 



Yeast is a Plant. The superior constructive faculty of yeast, 

 just described, separates it fundamentally from all animals in 

 respect to its physiology, and allies it closely to all plants. Its 

 inferiority to the chlorophyll-bearing plants or parts of plants, on 

 the other hand, in no wise separates it fundamentally from 

 plants ; for it must not be forgotten that the power, even of 

 plant-cells to utilize mineral matters as raw materials and from 

 them to manufacture foods like starch, ordinarily resides exclu- 

 sively in the chlorophyll bodies, and is operative only in the 

 presence of light. It follows, therefore, that most of the cells, 

 even of the so-called green plants, and a considerable portion of 

 the contents of the so-called green cells, must be destitute of 

 this synthetic power. Considerations of this kind show how 

 exceedingly localized and special the starch -making function is, 

 even in the "green" plants; and yeast probably compares very 

 favorably in its synthetic powers with many of the colorless cells 

 of such plants, or even with the colorless protoplasmic portions 

 of chromatophore-bearing cells. 



But yeast is vegetal rather than animal, morphologically as 

 well as physiologically. Its structure more nearly resembles 

 that of some undoubted plants (fungi) than any animal. Its 

 wall is composed of a variety of cellulose, called fungus-cellulose ; 

 and cellulose, though occasionally occurring in animal structures, 

 is, broadly speaking, a vegetal compound. Finally, in its 

 methods of reproduction by budding, and by spores, yeast is 

 allied rather to plants than animals. 



Top Yeast. Bottom Yeast. In the process of brewing two well- 

 marked varieties of yeast occur, known as "top" and " bottom" yeast. 

 The former is used in the making of English ale, stout, and porter ; the 

 latter in the making of German or " lager " beer. The top yeast is culti- 

 vated at the ordinary summer temperature of a room, without special at- 

 tention to temperature ; the latter in rooms artificially cooled so that even 

 in summer, icicles often hang from the walls. The two yeasts also show 

 obvious differences in form, size, and structure ; and how much they must 

 differ in their function is plain from the very different products to which 

 they give rise. 



Wild Yeasts. Besides the commercial or cultivated yeasts there are 

 also wild yeasts, and to them are due in the main the fermentations of 

 apple-juice, of grape-juice, and other fruit juices. A drop of sweet cider 

 shows under the microscope a good example of one of these species ; and 

 Pasteur long ago proved that the outer skins of ripe grapes and other fruits 



