458 



BOTANY OF THE LIVING PLANT 



BACTERIA. 



The relation of the Bacteria to the Cyanophyceae is obscure. But 

 they certainly offer many points of analogy in their mode of life. They 

 are unicellular or filamentous organisms, without chlorophyll, and they 

 lead a parasitic or saprophytic existence. 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, some- 

 times with chromatin-gran- 

 ules, but no definitely formed 

 nucleus. Many of them are 

 motile, and bear cilia varying 

 in number and position in 

 different types. Their multi- 

 plication is by fission. Their 

 mode of life is best illustrated 

 by an example. 



The Hay-Bacillus (B. sub- 

 tilis] 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. 391, c). 

 The spores can resist even the temperature of boiling water, and pass 

 still living into the decoction. There they germinate into active 

 Bacilli, motile in the fluid by cilia (Fig. 391, a, a). But those which 

 rise to the surface lose their motility (b), though continuing to divide ; 

 they form thick gelatinous walls, and so they remain associated 

 together as the scum of the zoogloea (e). If the supply of organic 

 material is exhausted they pass again into the resistant spore-stage. 

 It thus appears that a single boiling of the medium containing spores 

 of B. subtilis is not enough to sterilise it, for the spores can resist 



FIG. 391. 



Bacillus subtilis. a, d, motile cells and chains of cells ; 

 b, non-motile cells and chains of cells ; c, spores from the 

 zoogloeae; e, the zoogloea. (From A. Fischer, a-d x 

 1500 ; e x 500.) S. 



