REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



35 



of the vessel is then cut off from the light by a screen (5), the shadow 

 of which passes across the middle of the vessel containing the Stentors. 

 One side of the vessel is thus in the light, the other in the shadow, 

 and these two regions are separated by a sharp line (Fig. 12, x y). 



The Stentors are soon all collected in the shaded side of the vessel. 

 Here they swim about freely in all directions, but do not cross the line 

 into the lighted portion. Now, by focusing the Braus-Driiner on this 

 line, the behavior of the individuals on reaching it may be observed. 



It is well to examine 



the conditions in this case > 



with care, as they present 

 opportunities for a pre- 

 cise and crucial test of 

 the theory that the reac- 

 tion to light is due to a di- -* 



rect orientation through 

 the falling of light on one 

 side of the organism (pho- 

 totaxis or phototropism 



in the strict sense, as de- * 



fined by Holt & Lee). 

 In the lighted portion of 

 the vessel the rays of light 

 come from a certain direc- 

 tion, as indicated by the " 



large arrows (Fig. 12). 



In the shaded region there ^ 



is not enough light to pro- 

 duce orientation, the ani- 

 mals swimming in every 

 direction. On passing P JG I2 * 



from the shaded region 



across the line x-y into the lighted region, the animal should (according 

 to the tropism theory) become oriented. According to the theory of 

 negative phototaxis by direct orientation due to differential action on 



*FiG. 12. Method of testing the manner of reaction to light in Stentor. The 

 large arrows show the direction from which the light rajs come. A screen (5) 

 cuts off the light from half the vessel, leaving a line (x-y) separating a shaded part 

 from a lighted part. The Stentors collect in the shaded part, here swimming 

 about without orientation. At a (i, 2, 3, 4) we see a diagram of the reaction 

 required by the tropism schema when the organism swims across the line x-y, 

 while at b (i, 2, 3, 4) we have a diagram of the reaction as actually given under 

 these conditions. 



