IRRITABILITY ; IXFLUEXCE OF LIGHT 195 



of the light ; and, finally, light supplies the energy for_ the 

 carbon-assimilation which is characteristic of Volvox as of 

 all green plants. The photosynthetic effect of light is not 

 a phenomenon of irritability, and is discussed on page 204. 

 There have been no specific investigations into the tonic eftect 

 of light on the sulphur bacteria, but it may be presumed that 

 it operates in producing a state of well-being in the coloured 

 sulphur bacteria, for, so far as is known, no multiplication and 

 no pigment formation take place in the dark. It is impossible 

 to separate altogether the tonic from the photosynthetic effect 

 of light. The absence of colouring matter in the sulphur 

 bacteria from which light has been withheld may be the cause 

 of their lack of tone. It is responsible for their failure to 

 multiply, and so it is impossible in many cases to state whether 

 there is a tonic effect of light on these bacteria apart from 

 the deleterious efiect produced by the absence or loss of pig- 

 ment formation. In some cases, however, a distinct tonic 

 action of light may be noted. Thus it is shown in Engel- 

 mann's investigation that when motile coloured sulphur 

 bacteria lose their motility through the withdrawal of light, 

 they do not assume motility when they are once more exposed 

 to light if they have been kept too long in the dark. This 

 failure is to be attributed to the loss of tone by the withdrawal 

 of light, and to the same cause must be attributed the fact that 

 exposure to constant light intensity is injurious, for motile 

 organisms stop moving altogether unless slight changes are 

 made in the intensity of the light. 



The most notable investigation on the reaction of the 

 sulphur bacteria to light-intensity and colour is that of Engel- 

 mann (3). 



Engelmann's Investigation : Effects of Changes in the 

 Intensity of Light. 



Engelmann used for his experiments a number of species 

 which cannot now be identified. Among these was the very 

 sensitive Bac. photometricum (Engelmann), which was possibly 

 a pleomorphic form of Lankesteron. 



13 * 



