Minnesota Plant Diseases. 119 



their nutrition runs low, yeast plants may prepare for unfavora- 

 ble seasons by forming sac-spores. A breeding act has been 

 described for at least two kinds of yeasts, preceding the forma- 

 tion of the sac-spores. The two plant cells which unite are 

 both similar, and inside of the united cell four spores are 

 formed. In most yeasts, however, no breeding act precedes 

 the formation of sac-spores. The sac-spores have thick walls, 

 are resistant and are often capable of resting for a long time 

 before resuming growth. 



Yeast plants are, in general, found growing most vigorous- 

 ly in liquid solutions of nutrient material for the budding habit 

 is of peculiar advantage in such an environment. The daughter 

 cells are easily separated from the mother cell and are carried 

 by convection currents to other parts of the liquid where they 

 get more nourishment. It is in sugar solutions or in closely 

 allied substances, as starch, that the yeasts thrive best. In 

 nature they are found in the juices exuding from ripe grapes or 

 other fruits. 



Many yeasts possess the power of breaking down the sugars 

 into carbonic acid gas which escapes in the form of bubbles 

 and into alcohol; i. e., they have the power of fermentation. 

 This process is made use of in bread-making and in beer and 

 wine making. In the former the carbonic acid gas is used 

 in the raising of the bread while in the latter the alcoholic 

 products are those sought for. Not all yeasts have the power 

 of fermentation and many, although possessing this power, are 

 not vigorous enough to be of commercial use. The common 

 beer and bread yeasts have been chosen because they are vig- 

 orous fermenters. Moreover, many yeasts can ferment only 

 certain kinds of sugars, as milk, or cane or grape sugar. One 

 may also find several kinds of yeasts which, as far as structure 

 and appearance is concerned, might be considered identical 

 but which show that they are different in their powers of fer- 

 mentation. Yeasts also play an important part in the produc- 

 tion of many drinks of far eastern peoples, as of Japanese 

 "saki," of kefir and kumys, though in these cases certain bac- 

 teria and blue molds may aid in the process. In the produc- 

 tion of by-products, singly and in combination, yeasts may 

 differ in the quality or tastes of the liquors thus produced, and 



