REPRODUCTION AND SPORE FORMATION I43 



specialised group, most of which feed on liquid organic 

 substance, while retaining an exceedingly simple struc- 

 ture. Bacteria in the same forms shown by existing 

 species have been found preserved in fossils many 

 millions of years old. 



Reproduction. — The simpler bacteria multiply by 

 simple division of the cell and separation of the daughter 

 cells so formed. Sometimes the cell grows to a larger 

 size than the normal before division ; in other cases 

 the cell divides and each daughter cell subsequently 

 grows to the normal size. The daughter cells of a bacillus 

 frequently remain together for a time after division, so 

 that lines of cells having the form of jointed rods are 

 produced. 



In most bacteria growth and multiplication proceed 

 with great rapidity. Under very favourable condi- 

 tions a bacterium may reach maturity and divide in 

 twenty to thirty minutes. If division takes place only 

 once an hour, seventeen million individuals will have 

 arisen from a single cell in twenty-four hours. The chief 

 factors which control the rate of multiplication are 

 temperature and food supply. 



Spore Formation. — In certain species of the simpler 

 bacteria, chiefly bacUli, spores are produced under 

 certain conditions. These spores are formed in the 

 same sort of way as yeast spores, by the aggregation 

 of the protoplasm at a given spot in the cell, the proto- 

 plasm of the spore becoming surrounded by a dense wall. 

 As a rule only one spore is formed in each cell, either 

 in the centre or close to one end. When the spore 

 comes into conditions suitable for growth, the spore wall 

 is split, and a new vegetative cell protrudes, assuming 

 the form characteristic of the species. 



Bacterial spores are by far the most resistant form 



