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 
a the yeasts. Fermentation by yeasts 
Fic. Lal ivisusicesite re is employed on a large scale in the 
producing by budding, manufacture of beer, wine, and spirits, 
and forming chains. 7 . 
andin 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. 
