REACTIONS TO LIGHT IX CILIATES AND FLAGELLATES. 51 



The reaction occurs at a decrease in illumination not only when the 

 organisms are oriented and swimming toward the source of light, but 

 also when they are not oriented and are merely scattered in a weakly 

 lighted area. Further, in cases where most of the Euglenae are oriented 

 and swimming toward a source of light, a number of specimens will 

 always be found that are not oriented at all, or are swimming away 

 from the source of light. Such individuals react to a sudden decrease 

 in illumination in the same manner as do the specimens that are oriented 

 with the anterior end toward the source of light. This result may be 

 observed in a curious way as a consequence of the fact that it requires 

 some time for the light to produce its orienting effect. Thus, if the 

 Euglenae are placed between a weak and a strong light they swim toward 

 the strong light. If, now, the strong light is cut off, they react in the 

 usual way and swim toward the weak light. Now the strong light 

 maybe restored ; the Euglenae continue for a few seconds to swim toward 

 the weak light, thus away from the strong light. If while they are 

 swimming in this manner the strong light is cut off, the Euglenae, swim- 

 ming away from it, react in the usual manner, by turning strongly 

 toward the dorsal side. 



The usual reaction may be produced by a decrease in illumination 

 that is not sufficient to cause a permanent change in orientation. Thus 

 the Euglenae on a slide or in a shallow dish may be lighted from a 

 window at one side. By passing a small screen in front of the window 

 at some distance from the preparation a portion of the light is cut off; 

 the Euglenae then respond in the usual way, by swerving toward the 

 dorsal side. The movement thus becomes very irregular. Since the 

 Euglenae continue to revolve on their long axes the dorsal side may lie 

 first to the (observer's) right, then to the left. The Euglena? all seem, 

 therefore, to vibrate from side to side. This is the " Erschiitterung " 

 or trembling described by Strasburger (1878) as occurring in swarm- 

 spores when the illumination is changed ; it will be understood better 

 when we have considered more in detail the mechanism of the reactions. 

 Meanwhile the screen retains its position, but still admits more light 

 from the direction of the window than from any other direction. The 

 reaction of the Euglenae, therefore, soon ceases ; their orientation is 

 restored in the way to be described later, and they continue to swim 

 toward the window. 



This experiment is an important one. It shows that the typical reac- 

 tion may be produced by a decrease in light that is not sufficient to 

 permanently destroy the orientation. Thus it is clearly the decrease 

 in illumination to which the organisms react ; not to a change in the 

 direction of the light rays. The experiment shows further that it is 

 not the absolute amount of light that determines the reaction. Some 



