§4] PHOTOTAXIS AND PHOTOPATHY 187 



the microscope, no local sources of food or oxygen occurred in 

 the water between the plates of glass, chemotactic influences 

 were uniformly distributed. From the conditions of the ex- 

 periment already described, a difference in temperature or of 

 illumination at the two poles of the amoeba is scarcely conceiv- 

 able. The rays of radiant energy were the only directing agent. 

 Under these conditions the amosba nearly uniformly showed 

 itself negatively phototactic to light of an intensity varying 

 from strong diffuse light to direct sunlight. The absence of 

 uniformity is to be ascribed to the accidental presence of some 

 disturbing agent. The movements made by the amoeba were 

 represented graphically by making at intervals a camera drawing 

 of its outline. Two such graphic representations are repro- 

 duced in Figs. 53 and 54. It must be said that it is difficult to 

 get so extended a series of changes in light as is shown in Fig. 

 54, for the phenomenon of acclimatization comes in and the 

 responses become irregular. But, despite such irregularities, 

 my studies lead me unhesitatingly to conclude that Amoeba, 

 although not at all photopathic, is strongly phototactic. This 

 result is important, for, since Amoeba is responsive to light, it 

 may very well be that such responsiveness is a general property 

 of protoplasm. 



Ciliata. — A double action of light must be here taken into 

 account. Engelmann ('82°, pp. 391-395) states that those 

 Ciliata which contain chlorophyll (algee) — e.g. Paramecium 

 bursaria, Stentor viridis, Bursaria — move towards the light, 

 but only when the oxygen tension in the water is low. Also 

 when the water drop is illuminated by a miorospectrum, instead 

 of white light the organisms aggregate towards the red end. 

 Here are the rays by which most oxygen is produced from the 

 chlorophyll, since assimilation takes place fastest here. When 

 the organisms are placed in excessively oxidized water they 

 move from the light. The conclusions to which Engelmann 

 arrived from these and other facts were that these species have 

 a very delicate sensitiveness to variations in oxygen tension, and 

 that it is through this sensitiveness that light influences move- 

 ment. Accordingly, it would seem that the apparent photo- 

 taxis is truly a case of chemotaxis ; but this conclusion requires 

 better evidence. 



