158 Frank W. Bancroft 



was mixed with the fluid in which the plants were swimming. 

 This made the determination of the direction of the eff'ective stroke 

 of the flagella an easy matter. The preparation was covered with 

 a coverglass resting on one or two thicknesses of filter paper, 

 according to whether the organisms were to be held fast by the 

 coverglass, or to be allowed to swim freely. The filter paper was 

 connected with the boot electrodes by means of a hanging drop. 

 The most useful modification of this method was to cut a hole in 

 the center of a circular piece of filter paper about 8 cm. in diameter. 

 This was placed upon a piece of glass, the volvox colonies were 

 put into a hole in the center, covered with a coverglass, and exam- 

 ined upon a revolving microscope stage. The boot electrodes 

 were connected as usual with the filter paper by means of drops. 

 By this means it was not only possible to move the preparation 

 about, and follow the movements of swimming individuals, but 

 the plants could also be turned through any desired angle, and thus 

 the angle which their long axes made with the current lines passing 

 through them could be varied to any extent without changing the 

 current, and while the organisms were kept continuously under 

 observation. The current density varied from about 20^ to about 

 2501J, from 40 to 100.? being the strength usually employed. The 

 plants were always studied in the water in which they had been 

 living when collected, or in tap water of similar composition. 



REVERSAL OF GALVANOTROPISM 



I have been able to confirm Terry's results concerning the direc- 

 tion of migration in all important respects. Plants that had been 

 kept in the dark or exposed to diffuse daylight all went to the 

 anode, while of those that had been exposed to direct sunlight for 

 half an hour or more, 30 per cent or more went to the cathode. 

 There appeared, however, to be considerable difference between 

 my results and Terry's as regards the intensities of light required 

 for the anodal and cathodal galvanotropism. Thus Terry found 

 that the usual response for volvox exposed to diffuse daylight was 

 cathodic, while sometimes days of exposure to darkness were 

 required to change the response to anodic. On the other hand, 



