208 LIGHT AND PROTOPLASM [Ch. VII 



according to the form of the organism. In elongate, antero- 

 posteriorly differentiated animals the first visible phototactic 

 response is the orientation- of the organism's axis in the direc- 

 tion of the impinging ray, and with the head end directed 

 towards the source of light, or from that source, according as 

 the organism is positively or negatively phototactic. The 

 orientation is the more precise and the retention of the position 

 the more sure the nearer the light approaches the optimum 

 (attractive) or maximum (repellent) intensity, as the case may 

 be. If two rays of different intensities making an angle with 

 each other fall upon the organism, it apparently moves in the 

 direction of the intenser ray, if free to do so.* 



In Amoeba, without differentiated axes, the effect of the ray 

 of light is to determine the position of the centrifugal stream- 

 ing by which a pseudopod is thrown out away from the light ; 

 and the streaming continues in this single direction so long as 

 conditions do not change. Thus the locomotion is in a straight 

 line, lying in the ray of light. 



Light not merely determines the direction of the axis but 

 the position of the head end. As we have seen (p. 196) this 

 determination of the position of the head depends upon the 

 attunement of the organism, a quality which in turn varies 

 with certain internal and external conditions. Acting upon a 

 "highly attuned" protoplasmic mass, light will cause orienta- 

 tion in one sense ; upon "lowly-attuned" protoplasm, an orien- 

 tation in the opposite sense. 



Whether light has any other effect than that of orientation of 

 the body is a mooted question. Stbasbxjegbe ('78, p. 577) and 

 LOEB ('90, p. 109) recognize that migration from one point to 

 another is more rapid in strong light than in weak, but believe 

 this difference in rate of migration is wholly explicable upon 

 the ground that the orientation is more precise in the stronger 

 light, that there is less wandering from side to side. Some ex- 

 periments made by Mr. Cannon and me upon Daphnia seem to 

 confirm this view and at the same time afford quantitative data 

 upon the degree of hastening. Thus in 18 trials Daphnia 



* This statement is provisional only. It .seems to follow from the experiments 

 of LoEB made upon moths and described on page 197. The point is worthy of 

 detailed comparative study. 



