CoNGDON, Reactions to Light. 317 



the regarding of protozoan orientation as phototactic. There is a consideration, 

 however, that would do so, even though it were proven that a response to locaHzed 

 stimulus occurs. There are no paired bilaterally symmetrical sensory areas through 

 whose unsymmetrical stimulation orientation is accomplished. 



DIRECTION vs. INTENSITY. 



Radl ('03) makes the statement that from a physio-chemical point of view there 

 can be no question as to whether intensity or direction is the primary factor in the 

 action of light upon an animal. The amount of change produced in the protoplasm 

 by light is due to the amount of energy given up by the light, and that in turn is a 

 function of intensity, not of direction. That this view does not exhaust the question 

 is shown by a test which Holt and Lee ('01) applied to the protozoan Lyncaeus, 

 in imitation of the earlier experiments by Cohn. Their apparatus consists of a 

 wedge-shaped tank containing dilute india ink suspended over an aquarium. 

 Light from above produces in an imperfect way a field graded in intensity from one 

 end of the aquarium to the other. By varying the angle of incidence upon the prism 

 the light is given an oblique direction within the aquarium and the gradation of the 

 field is little changed. Lyncaeus aligns itself with the light, and goes slavishly into 

 either greater or less intensity according as the rays slant in the one or the other 

 direction. This kind of reaction had been previously used as an argument that 

 direction is the essential factor of light response. Holt and Lee applied in expla- 

 nation of the reaction Verworn's suggestion that if an animal always turned 

 toward the shaded side of its own body it would of necessity align with the light. 

 Thus Lyncaeus is forced into either light or dark areas while responding in an un- 

 varying way to the difference of intensity upon the sides of its body. Although 

 Holt and Lee have thus explained the behavior of the animal satisfactorily by 

 means of intensity changes, they did not settle the question of the relative merits 

 of intensity and direction. 



We owe to Mast ('07) the first conclusive proof that orientation is due primarily 

 to intensity. He used for his purpose the colonial protophyte Volvox. An individ- 

 ual was first illuminated by two like pencils of light. As a result it took a course 

 intermediate between them. Then without changing the direction of either beam 

 one of them was modified in intensity. The organism now changed its orientation, 

 bending its course somewhat toward the beam that had become relatively stronger. 

 Cole ('07) illustrated the same point in another way upon the two worms Allolo- 

 bophora and Bipalium. A partial shadow was cast upon the anterior end of a worm 

 which has been pointed toward the nearly horizontal light. The creature in spite 

 of the fact that it is negatively phototropic went into the shadow, thus moving almost 

 directly toward the source of light. Serpulid larvae, though phototactic, were found 

 by Zeleny ('05) to go into greater light intensity whether it led them in the direc- 

 tion of the light or not. 



It is not necessary to seek for further evidence that light produces stimulation 

 through variations of intensity. Direction plainly affects the intensity of light upon 

 the body or the retina by the casting of shadows or by the complications introduced 

 through eyes of ■varying position and visual angle. Torrey ('07) has recently 

 recalled to mind the view that light may possibly show an orienting effect dependent 

 upon direction in a way analogous to the action of an electric current. Such a 

 theory would not explain the orientations of Volvox and Allolobophora. 



