PHOTOTAXIS IN VOLVOX. 321 



That Volvox is negatively phototactic in strong light may be 

 determined by exposing it to direct sunlight, or to a beam from 

 a projection lantern after eliminating the heat rays by means of 

 an alum cell. In negative phototaxis the body is also definitely 

 oriented, but with the anterior end away from the light, and the 

 organism swims away in a very nearly straight course. When it 

 reaches a place where the light becomes less than the optimum 

 it stops and remains comparatively quiet, only moving about 

 slowly at intervals in an irregular manner. In very weak light 

 Volvox exhibits no pronounced phototactic movements, but either 

 lies quiet or rolls about sluggisly in various directions. With 

 stronger illumination it becomes more active and swims straight 

 towards the source of light, while in light of high intensity the 

 direction of the response is reversed. We may readily under- 

 stand, therefore, why in the experiments of Oltmanns the Vol- 

 vox formed groups in regions of a certain intensity of illumina- 

 tion. Collections would be formed if the Volvox moved about 

 irregularly without regard to the direction of the rays and came 

 to rest when they reached a region of a particular intensity of 

 light ; but it is clear that this is not the method pursued. There 

 arejew organisms in which the orientation to the direction of the 

 rays is more precise, or which travel to or from the light in more 

 nearly a straight line. 



How is the orientation of Volvox effected? It is practically 

 impossible to determine this by studying the movements of the 

 flagella of the individual cells, as any one who has attempted to 

 observe these movements will easily realize. We are safe in say- 

 ing that when Volvox changes its direction it is because the fla- 

 gella on the two sides of the organism beat unequally. Can we 

 explain the orientation, then, as a result of the fact that the 

 differences of intensity of light on the two sides of the body 

 cause the flagella to beat with unequal vigor so that the organism 

 is swung around into a position of equal bilateral stimulation ? 

 It is in this way that Holt and Lee 1 have attempted to explain 

 the orientation of } 7 olvox, but there are certain difficulties in the 

 way of such an interpretation. Let us consider a Volvox in a 

 region of suboptimal stimulation and lying obliquely to the rays 



1 American Journal of Physiology i Vol. IV. 



