ii] HABITS OF CONVOLUTA 51 



ground co-operates with unilateral light to bring 

 C. roscoffensis to its proper light station its place 

 in the sun. Nevertheless background stimulus may, 

 under artificial conditions, act antagonistically to 

 that of lateral light and even dominate it. 



Thus if two half-black, half-white porcelain 

 troughs containing a little sea-water are so placed 

 that, in one, the white half, in the other, the black 

 half is directed towards the source of light, then, if 

 some fifty C. roscoffensis are placed in each of the 

 troughs, whereas in a very short time all the animals 

 are congregated on the white ground of the first 

 trough, only about forty per cent, manage to arrive 

 at and maintain themselves upon the black half of 

 the second trough (Fig. 10, B). 



When positive phototropism involves the passage 

 from black to white the movement is executed with 

 certainty and despatch ; but when it demands a 

 passage from white to black a movement against 

 the grain of habit there are hesitation, uncertainty, 

 and many failures. Under yet other conditions,- 

 when, for example, the intensity of the unilateral 

 light is lowered the stimulus of background may 

 prove more potent altogether than that of unilateral 

 light. In such circumstances, C. roscoffensis remains 

 on the black half of the dish, although the rays 

 of light signal to it to approach their source. It 

 looks as though the facts or illusions which, in 



42 



