4] PHOTOTAXIS AND PHOTOPATHY 189 



minated spot. The light in this case clearly did not act from 

 one side, and the test of phototaxis can therefore hardly be said 

 to have been critically made. Likewise, even with unilateral 

 illumination, VERWORN was unable to gain a phototactic re- 

 sponse with Stentor rceselii, St. coeruleus, Carchesium polypi- 

 num, and Uroleptus musculus. On the other hand, we have 

 often noticed here in Cambridge that our Stentor coeruleus is 

 (rather indefinitely) negatively phototactic to diffuse daylight. 

 Thus, an individual swimming free in a bit of glass tubing 

 pointing horizontally towards the window only very slowly 

 wanders away from the light. In conclusion, then, we must 

 admit that Ciliata are not markedly phototactic, but more 

 refined methods must be used before we can say of any of 

 them that they exhibit no trace of this response. 



Let us summarize briefly the results obtained from Protista. 

 Phototaxis is most marked among actively motile, chlorophyl- 

 laceous forms. Many colorless forms are, however, also photo- 

 tactic Beggiatoa, Amoeba, plasmodia of Myxomycetes, and 

 swarm-spores of Chytridium. The phenomenon is thus wide- 

 spread, if it is not universal. 



b. Cells and Cell-organs. Under this head will be consid- r* 

 ered, (a) the rearrangement of chlorophyll corpuscles, (/3) 

 the rearrangement of pigment in animal cells, and (7) the 

 migration of pigment cells in the metazoan body. 



a. That the chlorophyll bodies of the higher plants change 

 their position in the cell according to the intensity of the light 

 to which they are subjected has been made known chiefly 

 through the labors of FAMIXTZIN ('67), BORODIN ('69), 

 FRANK ('72), STAHL ('80), and MOORE ('87). If one fastens 

 a strip of black paper upon a leaf on which the sun's rays are 

 falling, one will find, upon removing the paper after a time, 

 that the darkened part is dark green whilst the brightly illu- 

 minated part is considerably lighter, so that an image of the 

 form of the dark paper is produced upon the leaf. This image 

 is, however, only temporary. A few hours after the removal 

 of the paper the leaf is of a uniform green again. Sections 

 through a leaf thus affected show that in the dark green 

 (shaded) part of the leaf the chlorophyll lies on those walls 

 of the cells which are perpendicular to the incoming rays, 



