CHAPTER XXVIII. 



THE BACTERIA. 



The Bacteria include a vast number of minute saprophytic and 

 parasitic organisms which have multiplication by fission and the 

 absence of the photosynthetic pigments as common features. They 

 are among the simplest of organised plants and include the most 

 minute of living beings. They stand rather isolated from the other 

 Thallophytes. Most of them have more or less conspicuous gelatinous 

 walls, surrounding a protoplast in which, though granules of chromatin 

 may be detected, there is no fully organised nucleus. Sexuality is 

 absent. In contrast to this simplicity of structure and life history 

 they show the greatest possible variety of physiological activity : 

 in fact, therein lies their special interest and importance. Indeed 

 some which appear identical in form may have different physiological 

 powers, and are accordingly distinguished as " physiological species." 



Their cells may be spherical (Coccus), or rod-shaped (Bacillus), or 

 slightly spiral (Vibrio), or strongly spiral (Spirillum), or straight and 

 slender (Cladothrix), or grouped in cubical packets (Sarcina). They 

 have a superficial membrane, and protoplasmic body, sometimes with 

 chromatin-granules, but no definitely formed nucleus. Many of them 

 are motile, and bear cilia varying in number and position in dif- 

 ferent types. Their multiplication is by fission. Their mode of life is 

 best illustrated by an example. 



The Hay-Bacillus (B. subtilis) can be obtained in any decoction of 

 hay, in hot or even in boiling water. If the fluid is filtered and set 

 aside for 48 hours it will be found to be swarming with ciliated Bacilli, 

 while at the surface a scum is formed, which is the " zoogloea " 

 condition of the same plant. In old hay the Bacillus is in the resting 

 condition, as spores, the protoplasm having contracted away from 

 the wall, and being surrounded by a thick membrane (Fig. 346, c). 



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