204 



LIGHT AND PROTOPLASM 



[Ch. VII 



of light, as in the figure. The spores now moved towards 

 the source of light, i.e. in the direction of the inf ailing rays 

 but constantly into a region of less intensity of light. 



d^> 



Fig. 59. — Diagram showing the position of apparatus and the direction of the rays 

 in an experiment in phototaxis. T, trough of water containing organisms, A and 

 B its two ends, M its middle. P, a prismatic box containing a solution of India 

 ink. S, screen to cut off extraneous light. L, gas-lamp having a Welsbach 

 burner. Drawn to scale. 



Loeb's ('90, p. 32) results were obtained by the use of quite 

 different methods. In one case he employed a chamber made 

 of two test tubes placed with their mouths together. One of 

 the tubes was darkened except for a clear streak at one end, c ; 

 and this darkened tube was pointed towards the light, so that 

 the rays fell through its axis. Although the clear chamber 

 was evidently the brighter, the Porthesia larvae with which 

 he experimented moved into the darkened chamber and thus 

 towards the source of light (Fig. 60). Again, a clear test tube 

 containing larvae (Fig. 61) was placed so that its closed end 

 b was directed towards the window FF. A bundle of sun's 

 rays SS struck nearly perpendicularly the mouth of the tube a, 

 when the larvas were aggregated at the beginning. Neverthe- 

 less the larvce, since their progress in the direction of the per- 

 pendicular rays was soon interrupted by the walls of the tube, 

 moved towards the window, from the region of greater intensity 

 of light in the direction of rays which passed more nearly in 

 the axis of the tube. That this is not negative photopathy to 

 strong light is indicated by the fact that the Porthesia larva is 

 attuned to a high intensity of light. The evidence would thus 

 seem satisfactory that the direction of migration of certain 



