202 SULPHUR BACTERIA 



the bacteria is placed under a coverslip and left in 

 the dark, the organisms take up a position varying 

 from \ mm. to 2 mm. from the edge. If the drop is 

 now exposed to the light, the bacteria scatter in all 

 directions. This is consistent with the explanation 

 that they had produced oxygen in the light, so that 

 their movements were less restricted. 

 5. A fluid containing purple bacteria was placed in a glass 

 tube 5 cms. high, and exposed in a vertical position. 

 After five months the fluid was colourless except at 

 the bottom, where there was a layer of the bacteria 

 2 mm. high. By attaching the tube to a hydrogen- 

 generating apparatus the pressure of oxygen was 

 diminished. If this apparatus was now placed in 

 the dark the organisms scattered throughout the 

 field, but if in the light they collected at the bottom. 

 In this case the inference is that in the light so much 

 oxygen is developed that its concentration is greater 

 than the optimum for these bacteria, and so they 

 move as far as possible from the surface. The effect 

 in this experiment of introducing oxygen is to cause 

 repulsion of the bacteria, a result which is the 

 opposite of that obtained in the previous experiment. 

 It must, however, be borne in mind that the sulphur 

 bacteria are attracted to oxygen up to a certain 

 concentration, and therefore it must be presumed 

 that in the previous experiment the concentration 

 of oxygen was below the optimum, whilst in the 

 present experiment that point had been passed. 

 The facts, however, have been challenged by subsequent 

 investigators. Molisch (3), working with pure cultures of 

 purple bacteria found that oxygen was not liberated in 

 sufficient quantity to collect in fermentation tubes. Further, 

 he found that the purple bacteria had no noticeable effect on 

 motile bacteria sensitive to the presence of oxygen, when such 

 bacteria were cultivated in association with purple bacteria. 



He also stated that the oxygen which is necessary for the 

 manifestation of phosphorescence by phosphorescent bacteria 



