REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



47 



Davenport's reference (Davenport, 1897, p. 189) to the negative 

 light reaction of Stentor makes no attempt to explain the mechanism of 

 the reaction. Holt & Lee (1901) have given an account of some fea- 

 tures of the light reaction of Stentor ccertileus. They did not attempt 

 to determine directly the mechanism of the reactions, by observation 

 of the exact movements of the organism. Specifically, they made no 

 observations to determine whether Stentor becomes oriented by turn- 

 ing directly aw^ay from the source of light or only indirectly through 

 a " motor reaction " such as I have described. They did attempt, how- 

 ever, to show that the gross phenomena observed might be interpreted 

 in accordance with the prevailing theory of tropisms set forth on page 

 7 of the present volume. It will be 

 well, therefore, to examine their obser- 

 vations in order to determine whether 

 they contain anything inconsistent with 

 the account set forth in the present 

 paper. 



Holt & Lee studied the behavior of 

 Stentor in an elongated trough which 

 was lighted from one side. The light 

 passed through a prism which con- 

 tained a translucent fluid (a weak solu- 

 tion of India ink), by means of which 

 a portion of the light was cut out (Figs. 

 18 and 19). 



At the thicker end of the prism more 

 light was cut out, hence this end of the 

 trough (Fig. 19, D) was darker than 

 the opposite end (/.). It was found 

 that when Stentors were placed in the trough close behind the prism 

 (ato. Fig. 19) they turned and swam away from the lighted side till the 

 back of the trough was reached (a to</. Fig. 19). This is of course ex- 

 actly what happens when no prism is interposed. Reaching the back of 

 the trough the animals give the motor reaction (by backing, then turning 

 toward the right aboral side), thus coming into either the position e or the 

 positiony* (Fig. 19). They then swim forward again, strike the wall. 



P"iG. 19.* 



* Fig. 19. — Reaction of such an infusorian as Stentor to light, under the con- 

 ditions shown in Fig. 18. After Holt & Lee- The animal in the position x-y^ 

 close behind the prism, turns and swims to the position d, where it comes against 

 the rear wall of the trough. It then turns either into the position e, toward the 

 darker end Z?, or into the position /", toward the lighter end L. In the latter case 

 it usually soon reacts again, and by repetition of the reaction it finally, as a rule, 

 becomes directed toward D. Thus, finally, most of the Stentors collect in the 

 dark end of the trough. 



