PHOTOTAXIS 697 



natural difference of relative excitabilities, as between the 

 upper and lower halves of the anisotropic swimming organ 

 (p. 695). 



(b) Responsive movements positive, negative, or intermediate, 

 according to intensity of stimulation. — Turning now to the 

 question of different movements in the same organism as 

 modified by the varying intensity of illumination, examples 

 are furnished by the observations of Stahl and Strasburger. 

 These investigators find that the swarm- spores, generally 

 speaking, when the intensity of light is moderate, move 

 towards it, and when stronger, away. 



In the lateral leaflets of Desmodinm, again, under con- 

 tinuous illumination, we have observed recurrent reversals of 

 the direction of the more rapid of the two strokes which con- 

 stitute each individual pulsation. Even this phenomenon 

 finds a curiously exact parallel in the movements of certain 

 swarm-spores of Ulothrix under the continuous action of 

 light, as noticed by Strasburger. These he finds first to 

 retire from the light, then to remain stationary, and again to 

 return towards the light, only then to begin the whole 

 process over again, thus moving to and fro for some time 

 like a pendulum. 



Directive action of light. — The movement of the ciliated 

 organism, then, whether towards or away from it, is parallel 

 to the direction of incident light. But a difficult question 

 arises here as to how the organ perceives this direction as it 

 were, and by what mechanism it determines its own course 

 accordingly. At first sight, there appears no reason why 

 rhythmic beats caused by stimulus should not produce 

 propulsive movement in any direction. In this connection, 

 turning to the case of Ulothrix, we find the organism pro- 

 vided with symmetrical pairs of cilia. We also know that 

 the excitatory effect of stimulus of light depends upon its 

 angle of incidence. If, then, light strike the two cilia of a 

 given pair asymmetrically, they will undergo unequal excita- 

 tion, causing them to execute a turning movement. Thus a 

 stable condition can only be arrived at when excitation is 



