2o6 SULPHUR BACTERIA 



in organisms that were hitherto colourless or in which the 

 colour was feebly developed. In such a case there would be 

 a development of colour without mass movements of the 

 organism in any particular direction. Hence colour may 

 develop on the side of the culture vessel towards the light 

 because the individuals on that side have been stimulated 

 by the light to develop the pigment to a stronger degree. 

 Winogradsky also attributed the shock movements to the 

 directive effects of light, but the statement was made from 

 general observation, and the experimental data given above 

 do not support this view. Beijerinck investigated a species 

 of Chromatiuni and found that it swam towards the point of 

 maximum light intensity. This observation is not correct, 

 for, as already stated, in bright light the bacteria collect on 

 the side of the culture vessel which is away from the light. 

 Beijerinck's statement would be true only if he experimented 

 in light of low intensity. Molisch (3) came to the conclusion 

 that light exercised no directive influence on the purple 

 bacteria. 



The whole subject needs reinvestigation. 



Summary of the Effects of Light on the Purple Bacteria. 



The ionic effect is indicated by the necessity of light for the 

 development of colour, and for the growth of the bacteria, and 

 by the fact that the resumption of movement in the light is 

 not possible if the organisms are previously kept too long in 

 the dark. Also, the need of a slight change in the intensity 

 of light to keep the bacteria at their highest speed is indicative 

 of this tonic effect. 



The directive or phototactic eft'ect of light on organisms in 

 general is of two kinds. There is a directive effect which 

 makes the organisms change the inclinations of their direc- 

 tions so as to bring them either nearer or farther away from 

 the source of light. There is no evidence that the purple 

 bacteria respond in this way. On the other hand, there is 

 ample evidence that changes in the intensity or in the colour 

 of light produce the second effect, namely, a complete 



