138 A TEXT-BOOK OF BOTANY 



sence of tubercles on the roots of leguminous plants or by 



the failure of such plants to grow at all. 



78. Yeasts. Yeasts are one-celled plants that reproduce 



by budding. This curious method consists in a cell's putting 

 out one or more projections which 

 gradually enlarge and finally become 

 pinched off. Often the cells thus pro- 

 duced cling together in short irregu- 

 lar chains (Fig. 124). The chief in- 

 terest in connection with yeasts is 

 the important part they play in the 

 fermentation of sugar solutions, " split- 

 ting" the sugar into alcohol and car- 

 bon dioxide, a process also induced by 

 certain bacteria ( 77), but chiefly by 

 the yeasts. Fermentation by yeasts 



FIG. 124! Yeast-ceils, re- is employed on a large scale in the 

 producing by budding, manufacture of beer, wine, and spirits, 



and forming chains. 



and in the making of bread. In the 

 last-named process, the dough is inoculated with yeast 

 plants and placed in a sufficiently warm temperature to 

 induce rapid growth. The plants begin to reproduce act- 

 ively by budding; the sugar in the dough is split into 

 alcohol and carbon dioxide; and the latter, being a gas, ex- 

 pands and puffs up the dough, making it light and porous, 

 that is, causing it to "rise." 



The yeasts commonly used have been cultivated for 

 centuries and are not known in the wild state. There are 

 also " wild yeasts " of many kinds, and many spores of the 

 higher Fungi behave like the yeasts in budding and induc- 

 ing fermentation. The " working " of yeast may be demon- 

 strated by introducing some of the yeast preparations into a 

 solution of sugar or sirup and setting it in a warm place. 

 After a few hours the bubbles of carbon dioxide should be 

 seen rising through the liquid. 



