FLAGELLUM 31 



Flagella are too fine and delicate to be seen on the living 

 organism or even on bacteria which have been colored 

 by the ordinary stains. They are rendered visible only by 

 certain methods which cause a precipitate on both bacteria 

 and flagella, which are thereby made thick enough to be 

 seen (Chapter XIX, page 207). The movement of liquid 

 around a bacterium caused by vibrations of flagella can 

 sometimes be observed with large forms and the use of 

 "dark-field" illumination. 



Flagella are very delicate and easily broken off from the 

 cell body. Slight changes in the density or reaction of the 

 medium frequently cause tliis breaking off, so that prepara- 

 tions made from actively motile bacteria frequently show 



Fig. 26.— a bacillus with peritrichic flagella. (Student preparation.) 



no flagella. For this reason and also on account of their 

 fineness the demonstration of flagella is not easy, and it is 

 not safe to say that a non-motile bacterium has no flagella 

 except after very careful study. 



The motion of bacteria is characteristic and a little prac- 

 tice in observing will enable the student to recognize it and 

 distinguish between motility and "Brownian" or molecular 

 motion. Dead and non-motile bacteria show the latter. 

 In fact, any finely divided particles suspended in a liquid 

 which is not too viscous and in which the particles are not 

 soluble show Brownian motion or "pedesis." This latter is 

 a dancing motion of the particle within a very small area 

 and without change of place, while motile bacteria move 



