200 PLANT LIFE. 



again. In some cases the division will take place in 

 20 minutes after the individual has been formed ; so 

 that, if circumstances are favourable, a single germ may 

 have 16 million descendants in eight hours. They are 

 exceedingly minute; it is said that 250,000,000 of the 

 Bubonic Plague germ can exist on a square inch of garden 

 soil. Generally their size varies from .2 to .15 micro- 

 millimetres. 



The shape varies; but only within fairly definite limits. 

 Most of them have no power of locomotion, but many 

 are able to move actively about by means of exceedingly 

 thin hairs, or whips, with which they lash the water or 

 other liquid. The most important kinds are the 

 following : 



Circular or Round forms : streptococcus, dividing 

 in only one direction ; Micrococcus, dividing in two 

 directions ; and Sarcina, dividing in three directions so 

 as to form peculiar oblong parcels. 



Rod-like or Stick-like forms: Bacterium motion- 

 less : Bacillus moving or swimming by scattered threads 

 attached all over the surface, and Pseudomonas, which 

 swim by lashes attached at one end. 



Curved or Corkscrew-shaped forms : Mikro- 

 spira, moving by 1-3 polar lashes ; Spirillum, with 5 to 

 20 lashes at one end, and Spirochaete, which wriggles 

 or bends about like a snake. How this last is able to 

 move does not seem to be understood. 



Thread-like forms: such as Crenothrix, and finally 

 Sulphur bacteria {Beggiatoa). 



The cells are very small, and generally almost trans- 

 parent. No nucleus similar to that found in the higher 

 plants has as yet been discovered ; and, though it is 

 always possible that they possess one, it seems very 

 improbable that they do so. Some forms are capable 



