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. 



