144 BACTERIA 



of living protoplasm known. Many can withstand 

 for some time exposure to dry heat of considerably 

 over 100 C, while most vegetative forms are quickly 

 killed at 50 C. In some cases spores can even with- 

 stand immersion in boiling water for several minutes, 

 while they may remain alive in a dry and inactive 

 condition for years. The great difference between the 

 powers of resistance of vegetative cells and spores is 

 illustrated by the anthrax bacillus (B. anthracis). 

 The active vegetative cell is killed by two minutes' 

 exposure to I per cent, carbolic acid, while spores 

 resist immersion in the same strength in some cases 

 for fifteen days. Vegetative forms can, however, with- 

 stand extreme cold for a long time, for instance the 

 temperature of liquid air ( 190 C.) for twelve 

 months. 



Dispersal. Spore formation is not a means of multi- 

 plication in bacteria any more than it is in the case of 

 yeast. Only one spore is normally formed in the bac- 

 terial cell, and the growth and division of the organism 

 is completely arrested until the spore germinates. 

 The spore condition is the resting stage of the bacterium, 

 enabling it to withstand conditions unfavourable for 

 vegetative growth and multiplication. Spores are, of 

 course, an important means of dispersal for the forms 

 which produce them, since they are passively carried 

 about in currents of air without injury, whereas vegeta- 

 tive bacterial cells will sooner or later be killed by drying 

 up. But only comparatively few species of bacteria 

 are known to produce spores, most kinds being dis- 

 persed entirely by means of the ordinary cells. Bac- 

 teria often get into the air by detachment of minute 

 particles from the surface on which they are growing. 

 Such particles form part of the dust of the air, and prac- 



