H. MUNRO FOX 493 



mum oxygen concentration for the flagellates to move inwards. In 

 an atmosphere of hydrogen, on the other hand, the oxygen goes out 

 of solution again around the free edges of the liquid and the position 

 of optimum oxygen concentration for the flagellates again moves 

 outwards. 



Thus oxygen is the controlling factor in the position which the 

 flagellates take up: they collect into regions where the concentration 

 of dissolved oxygen is an optimum for them. The behavior of the 

 advancing band in the presence of an air bubble now receives its 

 explanation. The bubble acts at first like the edges of the preparation, 

 the water immediately around the bubble being saturated with oxygen 

 at the atmosphere partial pressure of the gas, so that the flagellates 

 are unable to move right up to its surface. But as the flagellates are 

 continually taking up oxygen, the amount of the latter present in 

 the bubble gradually becomes exhausted. The flagellates are thus 

 enabled to approach closer to the bubble until they touch its surface, 

 where they remain for a short time until nearly all the oxygen in the 

 bubble has gone into solution and been consumed by them. Then 

 the oxygen concentration at the surface of the bubble falls below the 

 optimum for the Bodo and they leave the region altogether. 



This phenomenon can be imitated as follows. A cover-slip with 

 wax legs is placed on a slide and some of the indigocarmine-glucose- 

 caustic potash mixture let in beneath it with a pipette. The central 

 area of the preparation soon becomes yellow, the indigocarmine being 

 reduced here in the absence of oxygen. This yellow area is square 

 with its sides parallel to the edges of the cover-glass. It is surrounded 

 by a purple band, the region between this purple band and the edge 

 of the liquid being blue. The purple band corresponds exactly to 

 the flagellate band. It is the region of equilibrium between oxygen 

 consumed at the center and oxygen diffusing in from the edge. Any 

 air bubble in the yellow area is surrounded at first by a narrow blue 

 ring, separated from the yellow by a circular purple band. The 

 purple band approaches slowly but continuously to the bubble until 

 it lies on the surface of the latter and then it gradually disappears. 

 This occurs when all the oxygen of the bubble has gone into solution 

 and been used up. We have here an exact parallel to the behavior of 

 a Bodo band surrounding an air bubble. 



