CRYPTOGAMS 185 



state the spores of some species may live for years. They 

 are not necessarily killed by boiling. Only repeated or 

 greatly prolonged boiling will sterilize liquids, i.e. free 

 them from all Bacteria ; though a single boiling will kill 

 all active Bacteria present. Prevalence. Bacteria are 

 present in considerable numbers in ordinary air and in 

 most fresh waters. They are very abundant in most 

 soils. They abound in many milk supplies and are present 

 in butter, cheese, and other foods. Subsistence. Bacteria 

 are (1) saprophytic and (2) parasitic. The parasitic 

 species may cause deadly diseases in animals (including 

 man). 



443. Effects. Bacteria bring about chemical changes 

 in the substances in which they live. Such changes are : 

 the decay of the dead bodies of animals and plants ; the 

 fermentation (souring) of milk; the "ripening" of cream 

 and of cheese ; and the conversion of the alcohol in cider 

 into the acid of vinegar. In the manufacture of butter, 

 cheese, and vinegar, therefore, Bacteria play an important 

 part. Other instances of their usefulness in the arts 

 might be given. 



Among diseases known to be due to Bacteria are influ- 

 enza, erysipelas, scarlet fever, typhoid fever, consumption, 

 leprosy, lockjaw, and cholera. The principal source of 

 harm is the production of virulent poisons in the blood. 

 In spite, however, of the dangerous character of the para- 

 sitic species, the Bacteria are on the whole a highly bene- 

 ficial group of organisms. The dissolution of dead organic 

 bodies, and the enrichment and preparation of soils for 

 the uses of higher plants, effected by Bacteria, are very 

 important services. 



Yeasts 



444. If one examines microscopically a small portion of 

 yeast cake sold for raising bread, he finds (along with 

 starch grains from the potato used in making the cake) 

 numbers of small, colorless, unicellular plants, broadly 

 elliptical or somewhat ovate in outline, and of various 



