206 LIGHT AND PROTOPLASM [Ch. VII 



of light a vertical screen was interposed. This screen formed 

 one side of a wooden box and consisted of two glass plates 

 making an angle of 2° with each other, the interspace being 

 filled with a solution of India ink in gelatine. When the sun- 

 light was let through this screen, the individuals in the trough 

 behind it sorted themselves into two groups ; the partheno- 

 genetic individuals, which collected opposite the clearer part 

 of the screen, and the female individuals, with fertilized eggs, 

 which collected behind the darker part of the screen, each suit- 

 ing itself to the intensity of light to which it. was attuned. 

 When the intensity of the light was changed, the organisms 

 also changed their positions. Finally, Lobb ('93, pp. 100-103) 



has found that fresh-water plana- 

 ^' "■ ' rians (Planaria torva) gradually 



accumulate in the darker parts of 

 the vessel, since the light con- 

 stantly stimulates them to move- 

 ment, and in their wanderings 

 they gain the dark places by 

 accident and there are at rest. 

 ^ ^„ „. , . .^. So it comes about that when 



Fig. 62. — Diagram showing position 



taken by Planaria torva in a shai- these Planaria are in a shaUow 

 low cylindrical glass vessel a, b, cylindrical vessel (Fig. 62, a, b, 



c, d, placed opposite a window ,- . ^ , r- • i ^ t^ 



AB. (LoEB, '93.) <'^ ^) in front of a window AB, 



they accumulate neither at the 

 side towards the window nor that away from it, but at c and 

 d, where the side walls of the vessel cut off much of the light. 

 All these cases, then, lead to one conclusion, that organisms 

 may move with reference to more or less intense light — that 

 there is such a thing as photopathy. 



Indeed, a phototactic and a photopathic response may be 

 exhibited by the same organism. Thus in Cannon's and my 

 experiments Daphnia was found to be phototactic, although 

 other observers have clearly shown it to be photopathic, a 

 result which we have not been able to disprove. We con- 

 clude, then, that some organisms have this double response 

 to light that they may move in the direction of its rays, and 

 that they may keep in a certain intensity of light to which 

 they are attuned. 



