142 



GENERAL PHYSIOLOGY 



due to the chemotaxic influence of the oxygen of the air. 

 That it is the oxygen that attracts can be shown in another 

 way; where an algal cell is in the same solution with bacteria, 

 and is actively evolving oxygen as it manufactures its food, 

 the bacteria in the neighborhood are attracted to it. En- 



a B C 



FIG. 39. a, spectrum of the chromophyll of bacterio-purpurin, showing ab- 

 sorption bands at \ = 0.59 /a and A = 0.52 /*. An (invisible) absorption band 

 has been determined by means of the bolometer at A = 0.85 ju- b, the, bacteria 

 are seen aggregated chiefly in the regions of the absorption bands. The 

 accumulation of bacteria in these regions of absorbed energy seems due to the 

 fact that the moving bacteria cannot pass from a region of high energy to one 

 of low without a violent stimulus which impels them back again. (From 

 Engelmann, '83*.) 



gelmann has made use of this property to show the influence 

 of the different colors of the spectrum on a filament of algal 

 cells. The bacteria collect in the zones of greatest absorp- 

 tion, or, in other words, in that part of the filament where 

 oxygen is most rapidly evolving (Fig. 39). 



Numerous inorganic salts are capable of producing a chem- 

 otaxic influence on bacteria. Some chemical substances pro- 

 duce an attractive effect. Such substances are spoken of as 



