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A TEXTBOOK OF BOTANY 



[Ch. X 



the cultivation of such colonies artificially upon nutrient 

 gelatine as an aid to the identification of the species. 



Bacteria are spread in diverse ways, partly by free loco- 

 motion, partly by movements of the animals with which 

 they are associated, partly by water currents, and partly 

 by winds. This latter method becomes possible through the 



formation from the 

 protoplasm in single 

 cells, when conditions 

 of life become unfa- 

 vorable, of very thick- 

 walled resting spores 

 (Fig. 277), so resistant 

 that in some cases they 

 can withstand boiling 

 water for hours, and so 

 persistent in vitality 

 as to remain alive for 

 months or years. With 

 these characteristics, 

 and a minuteness such 

 that the spores float 

 with the dust in the air 

 (page 360), it is little 

 wonder that Bacteria 

 penetrate into all kinds 

 of situations. In case 

 of some parasitic kinds the spores are released for dissem- 

 ination only on the death of the host ; and it is possible that 

 the production of the poisons which produce the host's 

 death has this adaptive significance. 



The Bacteria have most intimate and important relations 

 with man's economic interests, from which point of view we 

 may group them as follows : 



Decay Bacteria. These are typical saprophytes, and the 

 cause of nearly all organic decay. Thus the Hay Bacillus 



Fig. 277. — The Hay Bacillus, Bacillus 

 subtilis, in diverse states. 



The ciliate forms are motile, and the smooth 

 are non-motile, while the spores are shown 

 black; all X 1500. Below is the zooglcea; 

 X 250. (From Fischer.) 



