272 



FUNGI 



of them are inhaled. Fortunately, however, most of them 

 are harmless, unless inhaled in very great numbers or 





o 





Q90 oo„V° ° oO°o 



518 519 



518, 519. — Milk (highly magnified) : 518, pure, fresh milk; 519, milk that has 

 stood for hours in a warm room in a dirty dish, showing fat globules and many 

 forms of bacteria. 



under certain unhealthful conditions, while a few, such as 

 the yeast fungus and the bacteria concerned in the pro- 

 cesses of decomposition, are very useful. The presence of 



520 521 522 



520-523. — Forms of bacteria : 520, bacteria of consumption {Bacillus tubercu- 

 losis) ; 521, cholera bacillus ; 522, bacilli of anthrax, showing spores ; 523, typhoid 

 bacillus. 



bacteria in the soil is also of importance sometimes, since 

 through their agency the nitrogen compounds are rendered 

 soluble by the roots of plants (Sec. 198). 



386. Difficulty of Classification. — The life history of 

 fungi in general is very obscure and difficult to trace, both 

 on account of the microscopic size of the great majority of 

 them, and of the curious habit of polymorphism exhibited 

 by many species ; that is, the same individual appears 

 under entirely different forms at different stages of its ex- 



