1264 Journal of Applied Microscopy 



GENERAL PHYSIOLOGY. 



Raymond Pearl. 



Books and papers for review should be sent to Raymond Pearl, Zoological 

 Laboratory, University of Michigan, Ann Arbor, Mich. 



Holt, E. B.,and Lee, F. S. The Theory of Pho- In the literature dealing with the effect 

 totactic Response. Amer. Jour. Physiol. 4: ^f jj ^t oil the movements of organisms 

 460-481, I90I. ° ° 



two modes of action of the stimulus 



have frequently been distinguished ; one through the direction of the rays, and 

 the other through the intensity of the light. It is the purpose of Holt and Lee 

 to determine to what extent the direction of ray per se is effective in producing 

 the orientation of an organism to light. The two leading theories of light 

 response, those of Loeb and Verworn, are carefully outlined, and that of Ver- 

 worn is " provisionally adopted," because it seems to the authors to be more 

 explicit and capable of being applied to all the facts than the other. Four typi- 

 cal cases of light reaction are examined. The first and simplest reaction con- 

 sidered is that described by Strasburger for swarm-spores. These organisms 

 move, away from the light in strong illumination and towards it in weak. This is 

 explained according to the Verworn theory as due, on the one hand to contrac- 

 tion phenomena induced by supra-optimal stimulation of one side in strong light, 

 and, on the other hand, to expansion phenomena induced by sub-optimal stimu- 

 lation of one side in weak light. This results in movement towards the optimum 

 intensity in any case. The second point considered is the reaction of an animal 

 exposed to light from two sources. The crustacean Lyiiceus was experimented 

 on and found to move away from two lights of equal intensity along a path which 

 equally divided the angle formed by the light rays striking the animal from the 

 two sources. This again is evidently what would be expected from Verworn "s 

 hypothesis, since the path taken is such as would cause the two sides to be illum- 

 inated by light of equal intensity. The third case of phototactic phenomena 

 treated is the response of an animal exposed to light rays coming vertically 

 through a prismatic screen. By such an arrangement one end of the vessel is 

 made darker than the other, independently of the direction of the rays. Lyiiceus 

 and Stentor were used for experimentation. Both went to the dark end of the 

 vessel along a more or less diagonal course. The explanation is that the ani- 

 mal shows contraction phenomena on the supra-optimally stimulated side until 

 the body is in such a position that both sides are stimulated with equal light 

 intensities. The simple diagonal path is in most cases moditied by the fact that 

 the animals strike the back wall of the containing vessel and are veered off by 

 it, but necessarily in the general direction of their previous course. The fourth 

 type of reaction considered is that shown by animals under the same experimen- 

 tal conditions as in the last case except that the light comes obliquely instead of 

 vertically through the prism. Under these conditions a negatively phototactic 

 animal will go into regions of brighter illumination, along a path more or less 

 parallel to the direction of the rays. The authors show that it is possible to 



