244 ESSENTIALS OF BOTANY 
that they are able to continue the life of the species on 
through very unfavorable conditions. 
.313. Economic Importance. — The great importance of 
the bacteria in relation to many of the affairs of everyday 
life has been suggested in several of the preceding sections. 
Farming, butter and cheese making, canning fruits and. 
vegetables, and a variety of other manufacturing opera- 
tions are largely dependent on employing certain useful 
bacteria or on warding off the attacks of injurious species. 
Such important diseases as influenza, consumption, ty- 
phoid fever, diphtheria, cholera, and the plague are caused 
by the attacks of parasitic bacteria, and a most important 
division of medical science is occupied with the detection 
of these parasites and the means of destroying them.! 
YEAST FUNGI 
314. Occurrence. — Yeasts are microscopic one-celled 
plants which are very widely distributed in the air and in 
the soil. On account of their minute size their presence 
is generally recognized only by the effects which their 
growth produces. Beer yeast is used in bread making and 
in fermenting various gruel-like preparations of ground 
malt, rye meal, corn meal, and so on in the manufacture of 
beer and many distilled liquors. This species of yeast is 
only known in a cultivated condition, but there are many 
wild yeasts which cause the fermentation of grape juice, 
cider, and canned fruits. 
315. Minute Structure and Reproduction. — The student 
can gather from Fig. 171 something of the structure of 
1 For general information about bacteria read Conn’s Bacteria, Yeasts,and, 
Molds in the Home, Ginn & Company. 
