THE BIONOMICS OF CONVOLUTA ROSCOFFENSIS. 399 



maintained above a certain degree, Convoluta passes into 

 a condition of liglit-rigor — that is, into a state akin to that 

 induced by darkness. At this high light-intensity the photo- 

 tonic influence of light is withdrawn or greatly disturbed. 



Further, we have to distinguish between the constant 

 tonic effect of light, which can only be estimated indirectly 

 by noting the effects of prolonged darkness, and the direc- 

 tive influence of lateral light, which induces phototropic 

 movements. These two modes of influence of light are 

 known to operate among plants, and it is interesting to 

 recognise them at work on Convoluta. 



Before passing on to describe how another light-factor 

 serves as a stimulus to Convoluta, we may mention that 

 prolonged darkness produces no appreciable destruction of 

 the chlorophyll of the mature green cells, nor does it impair 

 their photosynthetic powers. After a fortnight in complete 

 darkness, all starch may disappear from the green cells. 

 Then, exposure to bright light induces starch-formation 

 within five or ten minutes. 



We have now to refer to the influence of background on the 

 movements of Convoluta, and to show that this light-factor 

 may materially modify the phototropic response. In our work 

 on the physiology of the higher Crustacea we have shown that 

 background plays an important part in determining direction 

 of movements.^ This is true also of Convoluta. To demon- 

 strate the influence of background, we use two shallow porce- 

 lain troughs, the bottom of each of which is half black and 

 half white. We add to each trough some fifty Convoluta 

 and a little water, distributing the animals evenly over the 

 bottom. I£ the troughs are so orientated that the two halves 

 are equally illuminated, all the Convoluta rapidly pass to 

 the white half. If the troughs are so placed that in one the 

 white half and in the other the black half is the more 

 illuminated, and if equal numbers of Convoluta are placed 

 on the half of each distant from the light, then within two 

 minutes the proportion of animals on the white to those on 



1 1902 (abstract). 



