54 PLANT-ANIMALS [CH. 



the results of a series of experiments with unilateral, 

 monochromatic light. As the illustration shows, 

 the animals represented by dots mass themselves 

 toward the source of light when that light is 

 white or green. In blue light, they remain distri- 

 buted with fair uniformity around the periphery of 

 the containing vessel. In red light they show some 

 sign of a negative reaction. 



The most probable explanation of this re- 

 sponse to green light is that the orange eye-spots 

 and pigment glands are the organs of light-per- 

 ception ; and that the pigment contained in the eye- 

 spots and glands absorbs principally the green light. 

 From observations on other animals it would appear 

 probable that green light not infrequently serves 

 marine organisms for purposes of perception : nor, 

 when we reflect upon the green-blue colour of sea- 

 water, will this appear surprising. 



In the third place, C. roscoflfensis, with its well- 

 defined tropisms, is an admirable subject in which to 

 study what without a great violation of language we 

 may call the problem of the parallelogram of physio- 

 logical forces : in other words, the problem of the 

 mode of response of an organism to two directive 

 stimuli, simultaneously applied and acting along dif- 

 ferent lines. As might be expected, when two stimuli 

 act on C. roscoiFensis one not infrequently dominates 

 the other, so that the resulting reaction is that which 



