4IO ELEMENTARY BIOLOGY. [CHAP. 



grown unfavourable. Either the spore may arise as a small 

 cell in the interior of certain of the vegetative cells, or the 

 latter may themselves become directly converted into 

 spores. In either case the spore is characterized by its 

 more granular protoplasm and firmer cell-wall. On germi- 

 nation, which may be delayed until after a long period 

 of rest, the hard membrane is thrown off, and the pro- 

 toplasm of the spore resumes the ordinary vegetative deve- 

 lopment. 



Bacteria grow and multiply in Pasteur's solution (without 

 sugar) with extreme rapidity, and, as they increase in 

 number, they render the fluid milky and opaque. Their 

 vital actions are arrested at the freezing point, but no tem- 

 perature has yet been reached low enough to kill them. 

 They thrive best in a temperature of about 30° C. but, in 

 most fluids, they are killed by a temperature of 60" C. 

 (140" F.). This, however, only applies to the vegetative 

 condition, for the spores can in many cases resist a tempera- 

 ture very considerably over the boiling point of water. 



In many points of their physiology Bacteria closely re- 

 semble Torulce; and a further resemblance lies in the cir- 

 cumstance that many of them excite specific fermentative 

 changes in substances contained in the fluid in which they 

 live, just as yeast excites such changes in sugar. 



All the forms of putrefaction which are undergone by 

 animal and vegetable matters are fermentations set up by 

 Bacteria of different kinds. Organic matters freely exposed 

 to the air are, in themselves, nowise unstable bodies, and 

 if due precautions have been taken to exclude Bacteria 

 they do not putrefy, so that, as has been well remarked, 

 " putrefaction is a concomitant not of death, but of life." 



Bacteria, like TorulcB and JProtococci, are not killed by 

 drying up, and from their excessive minuteness they must 



