128 



with polar tufts of flagella, Spirochaete loosely 

 wound, long and filamentous). 



The form of one and the same individual, however, may 

 vary considerably at different stages of its life-history or ac- 

 cording to the medium in which it is living. The individual 

 bacterial cells are thus sometimes united into chains, or 

 masses of varying form, and owing to the partial dissolution 

 and swelling of the outer layers of the cell walls of the indivi- 

 dual cells such masses are often distinctly mucilaginous or 

 gelatinous. 



Plants which are thus capable of assuming several different 

 forms are termed polymorphic. 



So long as conditions are favourable, bacteria grow and 

 multiply very rapidly. Each tiny bacterial cell, whether 

 spherical, rod-shaped or curved, having grown to a certain 

 size, becomes divided by a cell- wall into two equal portions, 

 and, these segments separating from one another, two distinct 

 individuals are formed, each of which then continues to grow 

 until it has attained the normal size, when it also divides into 

 two, and so on. This mode of multiplication by cell-division 

 and the separation of the segments is very characteristic of 

 the bacteria which are in consequence often called the Fission 

 or Splitting-Fungi (Schizomycetes). When, from the exhaus- 

 tion of the substratum or some other reason, conditions become 

 unfavourable for growth, the preservation of the plant is 

 insured by the formation of resting spores which can exist for 

 long periods without further development. In the formation 

 of spores the protoplasm of the individual cells rounds itself oil 

 and becomes surrounded by a specially thickened membrane, 

 each cell thus becoming a spore. On germination, the protec- 

 tive coat disintegrates and the cell proceeds to grow and form 

 new individuals by division as before. All species are not known 

 to form spores. There is no sexual reproduction and, when 

 spores are formed, no special spore-bearing organ is developed. 

 Many bacteria which can be destroyed by boiling after ger- 

 mination are able to resist high temperatures in the spore 

 condition. Hence boiling a substance or liquid once is by 

 no means always sufficient to sterilise it, i.e. to absolutely 

 destroy all life in it. The boiling should be repeated several 

 times, an interval of a day being allowed to elapse between 

 each operation, so that some of the spores which might have 

 survived the previous operations may have time to germinate. 

 Direct sunlight is, as a rule, prejudicial to the growth and 

 development of bacteria. 



