CHE MOT AXIS 



109 



bend or move in the direction of the lines of diffusion." Why one 

 substance should act positive and another negative is at present 

 quite inexplicable. 



Chemotaxis can take place only in media which permit free 

 movement and its sphere of action is comparatively small. Different 

 kinds of bacteria by no means react in the same way to the same 

 substance. Furthermore, the action, whether positive or negative, 

 chemotaxis or neutral, varies with the chemical. The salts of potas- 

 sium are among the more active positive chemotactic substances, 

 followed by sodium and rubidium. The alkaline earths are less 

 effective. The influence of a salt is attributed mainly to its electro- 

 positive constituent; asparagin and peptone are strongly chemo- 

 tactic, whereas sugar and glycerin are inactive. 



Negative chemotaxis is noted when capillary tubes are filled 

 with free acids and alkalies or with alcohol. In some salts the action 

 of the acid radical and that of the base neutralize each other 

 (ammonium carbonate and monobasic potassium phosphate). In 

 this case the bacteria are neither attracted nor repelled by the sub- 

 stance. 



a B (7 



FIG. 17. Oxygen-loving bacteria infesting a thread of alga lying in the micro- 

 spectrum. The chlorophyll granules contained in the alga cells are not shown, but 

 the spectrum lines are given to denote the position of the spectrum. Mag. 200. (After 

 Engelmann.) 



Engelmann ingeniously made use of this phenomenon as a test 

 for oxygen and the effect exerted upon assimilation by the different 

 parts of the solar spectrum. If a thread of algse and some aerobic 

 bacteria are placed under an air-tight cover-glass, the bacteria are 

 active; but if the preparation is kept in the dark the action of the 

 bacteria will cease, showing that all the oxygen has been consumed. 

 If brought back to the light as the alg?e assimilate carbon dioxid 

 with the elimination of oxygen the bacteria again become active. 

 If exposed to the spectrum the greatest aggregation of bacteria 

 occurs at the red end of the spectrum, indicating that the maximum 

 assimilative activity of the algse protoplasm is proceeding at this 



