STUDIES OF SPOKE-PLANTS 283 



sition of foods. But the truth is that the half-hour or hour 

 rate of division is never maintained for long at a time. Soon 

 food-supply is exhausted, their own excretions accumulate 

 and exert a poisonous influence, or other unfavorable condi- 

 tions tend to check the rapid rate of growth. 



(c) Spore-formation occurs in some bacteria. Only a few 

 human diseases, and those not very common, are caused by 

 bacteria which produce spores. This is a fortunate circum- 

 stance, for as we have seen, spores are much more difficult to 

 kill than are the active bacteria. As a rule, one bacterium 

 produces only one spore, and this germinates and forms one 

 bacterium. There is therefore no multiplication, and spore- 

 formation is simply a device for adapting the organism to 

 unfavorable conditions. Spores taken from cattle dead from 

 the anthrax disease have been found capable of germinating 

 many years after the animals were buried. 



(d) Temperature. Some bacteria are able to multiply 

 near the freezing point, and some live in hot springs in water 

 at a temperature which will kill most kinds of bacteria and 

 other living things. Between these two extremes there are 

 all gradations. Contrary to the popular belief, freezing does 

 not kill all bacteria. Bacteria of several species, including 

 those which cause typhoid and diphtheria, have been kept sev- 

 eral days at the temperature of liquid air (about 190 C.), 

 and when thawed out, appeared to multiply normally. 

 However, a very large percentage of common bacteria die 

 when frozen in ice, and comparatively few are living after 

 the ice has been kept five or six months. A few years ago an 

 epidemic of typhoid fever was traced to ice which had been 

 stored seven months, so that all ice from waters contaminated 

 with sewage should be regarded with suspicion. While most 

 of the bacteria will probably die, the few which remain may 

 multiply rapidly when taken into the human body. 



The thermal death-point varies. Ten minutes' exposure 

 to a temperature of 70 C., or one minute at 100 C. (boil- 



