IRRITABILITY : INFLUENCE OF LIGHT 201 



Liberation of Oxygen by the Purple Bacteria on 

 Exposure to Light. 



Whilst direct proof of the Hberation of oxygen is still 

 not available, strong presumptive evidence of its liberation 

 is supplied by the following facts, which were elicited by 

 Engelmann. 



I. A colony of sulphur bacteria in the zoogloea condition 

 was introduced, together with a strongly aerobic 

 spirillum, into a drop of water. The spirilla collected 

 round the colony if the drop was poor in oxygen, 

 but did not do so if the drop was well aerated. If 

 the space surrounding the drop was filled with 

 hydrogen the spirilla showed a greater tendency to 

 collect around the zoogloea. 



Fig. 55. 



2. Variations in the pressure of oxygen were correlated with 



variations in the activity with which the movement 

 of the spirilla towards the zoogloea took place. 



3. In one observation of a drop of water which had been 



kept for some time in the dark and then brought 

 into the light, a number of spirilla were observed in 

 position round the circumference of an air bubble, 

 and near the bubble there was also a purple organism 

 [Monas vinosa) (marked a in diagram). When the 

 drop was rich in oxygen the purple organism had no 

 particular attraction for the spirilla (Fig. 5 5 A), 

 but when it was poor in oxygen they distributed 

 themselves as shown in the B position. 



4. Many of the actively motile sulphur bacteria are 



attracted by oxygen. If a drop of water containing 



