74 INFLUENCE OF PHYSICAL AGENTS 



cessation of growth, the minimum is the lowest temperature at which 

 growth even of the feeblest kind is possible. 



The table shows that wheat seedlings and free living metatrophic 

 bacteria like B. subtilis and B. liquefaciens need approximately the same 

 temperature, while the gourd plant, an inhabitant of warmer zones, 

 requires more, as do also the cholera and anthrax germs, 30 to 40 being 

 the optimum. The phosphorescent bacterium from our cold northern seas 

 has the lowest minimum of any, and at the other end of the scale we find 

 the representative of that curious group the thermophile bacteria. The 

 tubercle bacillus has the narrowest temperature range of all, only twelve 

 degrees separating maximum and minimum. It is, to use an expression 

 borrowed from animal physiology, stenothermic, as are all the strict parasites 

 on warm-blooded anjmals. The metatrophic bacteria, on the other hand, 

 are eurythermic, and can endure great variations of temperature with a 

 difference of as much as 30° between maximum and minimum. 



Bacteria which are pathogenic for warm-blooded animals, and at the 

 same time eurythermic (e.g. B. anthracis), are very probably metatrophic 

 in our climate. 



The thermophilic bacteria (48), singular as it may seem, are very widely 

 distributed even in our temperate climate, and a large number of species 

 have been isolated from the soil and from sewage. It is difficult to see 

 where these bacteria (mostly non-motile aerobic rods) find the necessary 

 conditions of existence. Strong sunshine sometimes heats the surface of the. 

 soil to 70 C. and might permit the development of such forms, but they 

 must be subject to long periods of quiescence. Manure and other substances 

 that become heated during fermentation are their most likely habitats. 



In B. thermophilus the optimum reaches the temperature of coagulation 

 of most proteids and the maximum exceeds this. We might be inclined to 

 regard the protoplasm for this reason as different from that of ordinary cells, 

 but we must not forget that in all proteid bodies the temperature of coagu- 

 lation varies very much according to the reaction of the protoplasm, the 

 amount of salts present and other factors, so that we are not justified in 

 looking upon the thermophile bacteria as being miracles of nature. Besides, 

 we know of other organisms that live at very high temperatures. In 

 the hot springs of Ischia and around the 'fumaroli' at Naples micro- 



