254 GENERAL BOTANY 



mother cell takes place either at the end or in the middle of the 

 cell. As indicated above, one of the principal functions of the 

 spore is to carry bacteria over periods which endanger the life 

 of the ordinary vegetative bacterial cells. 



Sterilization. On account of the great resistance of the bac- 

 terial spores to extremes of temperature and drought the greatest 

 difficulty is experienced in destroying them in food or in cultures. 

 One method of killing the spores of bacteria is to subject the 

 material to be sterilized to flowing steam at 100 C. for a short 

 period on three successive days. This is the discontinuous 

 method of sterilization advanced by Tyndall. If milk or any 



o o .... O d l (X 



FIG. 137. Different methods of spore germination 



a, "direct conversion of a spore into a bacillus without the shedding of the spore 



wall " in B. leptosporus ; b, polar germination of a spore in B. anthracis ; c, equatorial 



spore germination in B. subtilis; d, equatorial germination in B. megatherium-, e t 



same with horseshoe appearance. After Novy. From Marshall's " Microbiology " 



other material is to be freed from bacterial spores by this method, 

 it is first heated in flowing steam at 100 C. for fifteen minutes, 

 which is sufficient to kill all living vegetative yeast, mold, or 

 bacterial cells in the substance, except certain resistant bacterial 

 spores. Any spores which may remain will germinate if left for 

 a few hours at the proper temperature. The material to be 

 sterilized is consequently allowed to stand after the first heating 

 until any spores which remained have had time to germinate 

 and form vegetative bacteria, when a second heating will kill 

 all of the bacterial cells thus produced. A third heating is 

 usually necessary after an interval, in order to kill any bacteria 

 which may have been produced by spores which had not yet 

 germinated at the time of the second heating. It is now thought 

 that the highly resistant spores are also gradually weakened 

 and finally killed during the prolonged period of heating. 



