494 LIFE-HISTORIES 



ing its sugar into lactic acid. Such decomposition, in which 

 the products are not offensive, is distinguished as fermenta- 

 tion.'^ Both fermentation and putrefaction are regarded as 

 due to the action of enzyms (comparable to diastase) which 

 are secreted by the active organisms concerned. The tuber- 

 culosis bacterium is another species especially noteworthy 

 as it is the germ of " consumption" in man and other animals, 

 producing various forms of the disease according to the part 

 of the body in which it develops. An organism which 

 thus feeds upon the substance of another living thing is called 

 a parasite; ^ the organism which supports it being termed 

 the host. Bacteria capable of producing the disease often 

 occur abundantly in the sputum of tuberculous patients, and 

 if this dries small bits are readily detached and blown about. 

 A sufficient number drawn into the lungs or getting into the 

 blood of a susceptilile host give rise to the disease. Hence 

 the wisdom of isolating tuberculous patients to avoid con- 

 tagion, and the importance of enforcing the regulations of 

 Boards of Health against all spitting in public places. Direct 

 sunlight being soon fatal to the plant in all its stages, affords 

 a most valuable means of i^reventing infection, and often 

 of effecting a cure by killing the parasite. 



Nearly all contagious diseases are caused by fission fungi, 

 and to the micro-organisms or "microbes" of this same class 

 are due almost every sort of putrefaction, fermentation, and 

 decay. The discovery of this important truth has given a 

 new significance to cleanliness, and a knowledge of their 

 life-histories and peculiar properties affords a scientific basis 

 for methods of preventing or regulating the activities of 

 these excessively minute, yet exceedingly powerful agents 

 of change. While some forms of bacteria are a menace to 

 health others are useful in important ways as in the manu- 

 facture of butter and cheese, in the retting of flax and other 

 fibers, and as improving the soil for many farm-plants. 



The various forms assumed by the cells and colonies of 

 fission fungi may all be closely matched by forms of blue 



' Fer'-men-ta'tion < L. fcrvcrr, boil, be agitated. 

 - Par'a-sitc < (Ir. paraailon, one who eats at another's table, a hanger 

 on; < para, beside; sitos, food. 



