REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 59 



the anterior end therefore does not swing so far in the direction 

 away from the light as in the preceding phase it swung toward the 

 light. This is illustrated at 7-8 in Fig. 23. But as a result of such 

 swerving as does occur the anterior end is now (at 8) directed more 

 away from the source of light than before. There then follows a 

 new reaction, with increased swerving toward the dorsal side in the 

 next phase of the spiral (S-ii, Fig. 23), which carries the dorsal side 

 toward the source of light. Hence the anterior end swings still further 

 toward the position where the light shines directly upon it. This con- 

 tinues. As a result of this repeated swinging of the dorsal side slightly 

 away from the source of light and strongly toward the source of light 

 the organism gradually changes its course, continuing to swim in a 

 spiral and to swerve toward the dorsal side, until the axis of the spiral 

 is in line with the light rays and the anterior end is toward the source 

 of light. This method of reaction will best be understood by a study 

 of Figs. 23 and 24 and their explanation. 



Thus the orientation is gradual and for a certain stretch after the 

 light has begun to act the organism is not completely oriented. With 

 a fairly strong light, however, the period of time required for complete 

 orientation is very slight. Strasburger (1878, p. 24) noticed that when 

 Ha3matococcus is swimming toward a source of weak light and the 

 light is suddenly increased so as to reverse the orientation, there is a 

 period of ." verschiedenen Schwankungen " before the reverse orienta- 

 tion is attained. He paid little attention to the behavior of the 

 organisms during this period, however. 



Our account has been thus far purely descriptive ; we have attempted 

 to set forth the events as they may be observed, without trying to 

 indicate the causes at work. We must now inquire as to what is the 

 real stimulus and its method of action in producing orientation. 



First, we note that in becoming oriented Euglena does not turn 

 directly toward the source of light. As in the reaction to other stimuli, 

 the turning is throughout toward a structurally defined side. This 

 shows that the orientation of Euglena, like that of Stentor, cannot be 

 accounted for on the orthodox tropism theory. In other words, the 

 orientation is not due to the direct effect of the light on the motor 

 organs of the side on which it falls. As in Stentor, orientation may 

 be reached by turning either toward or away from the source of light, 

 or in any intermediate direction. The response is a u motor reaction " 

 of a definite type. 



Just what is the stimulus which produces this motor reaction? All 

 our experiments up to this point have shown clearly that this reaction 

 is produced by changes in the intensity of illumination, and that a change 

 in the illumination of the anterior end produces the reaction as well as 



