PHYSIOLOGY OF MICROORGANISMS 



51 



filled with such a solution into water containing numerous bacteria. 

 These will be found to congregate in great numbers about the 

 mouth of the tube and to enter it. Probably chemotaxis accounts 

 for the flocking of the leukocytes or white blood corpuscles to 



Fig. 24. Chemotaxy: a, Spirilla attracted by a green algal cell which is 

 giving off oxygen, aerotaxis; 6, a leukocyte containing several bacteria which 

 it has engulfed; c, capillary pipette containing a solution of beef extract, and 

 at 2 an air bubble, placed in a drop of water containing motile bacteria. 

 The latter are attracted in large numbers to the mouth of the tube; d, an air 

 bubble surrounded by two concentric circles of organisms, the inner one 

 bacteria, the outer protozoa. Each remains in the concentration of oxygen 

 most favorable to its growth. 



any part of the body attacked by certain bacteria. Micro- 

 organisms are not always attracted by food stuffs and repelled 

 by harmful substances. A mixture of peptone and mercuric 

 chlorid will attract bacteria and then destroy them. 



Fig. 25. Chemotropism: a, 6, Mold hyphse and conidiophores, showing the 

 negative hydrotropism of the latter; c, an air bubble in a medium with four 

 germinating mold spores. The hyphae are growing toward the air, showing 

 positive aerotropism. 



Tropisms. Organisms which are not free to move in response 

 to a chemotactic stimulus may nevertheless be influenced in their 

 direction of growth. Such a response in the direction of growth 



