YEAST PLANTS . 223 
little cells are Yeast-plants, and bear the name of 
Saccharomyces cerevisiae. 
371. They reproduce by a kind of fission, called 
“budding.” Each cell pushes out a little projection 
which grows larger and larger, and finally a cell-wall 
forms between it and the old cell and these sooner or 
later separate from one another. Under 
favorable circumstances certain cells form 
spores internally, and these are now re- 
garded as asci, homologous with the asci 
of the higher sac-fungi. Yeast-plants are, 
therefore, to be considered as greatly sim- fic. 106.—Sac- 
plified Sac-fungi, and they are members of gaaten gs 
the family Saccharomycetaceae (of the Order HemrascaALEs) 
which has experienced what is probably the greatest 
reduction suffered by any plants of the Ascosporeae. 
372. Yeast-plants are saprophytes, and live upon the 
starch of flour. They break up the starch, and in the 
process liberate considerable quantities of carbon dioxide 
which appears as bubbles upon the surface of the yeast. 
Another result of the breaking up of the starch is the 
formation of alcohol; hence the growth of yeast-plants in 
a starchy substance is always accompanied by what is 
known as alcoholic fermentation. The housewife and 
baker use yeast-plants for the carbon dioxide gas which 
they evolve, to give lightness to the bread, while the 
brewer and distiller use the same plants for the alcohol 
produced by their activity. (See Chapter IV, paragraph 
139.) 
373. The Truffles (Order TuBERALEs) are well known 
from their large underground spore-fruits, which are 
edible. Internally there are narrow tortuous channels 
on whose walls asci develop, each containing a number of 
spores. Little is known of their round of life, and the 
