Mast, Light Reactions in Lower Organisms. 125 



the source of light, and if the colonies tend to travel parallel with 

 the axis, we should expect them to move parallel with the rays, 

 when the rays are vertical and the source of light is above. This 

 was found to be approximately true, as is shown by the following 

 experiment. 



On August 8, 1905, the plate glass aquarium was nearly filled 

 with filtered water and put upon the stage of the light grader which 

 was so arranged that the rays were vertical (see Fig. 5). A number 

 of colonies were then put into the aquarium with a pipette and 

 set free near the bottom in a beam of light, which was uniform in 

 intensity and two and one-half centimeters square in cross section. 

 After swimming upward to the surface of the water, some of the 

 colonies wandered out into the shaded region. These could 

 readily be forced to swim down again by reflecting the beam of 

 light upward through the aquarium slightly to one side of the 

 illuminated area produced by the light direct from the glower. 

 The reflected beam could be made vertical by tipping the light 

 grader so that the direct beam of the light made an angle of about 

 10 degrees with the vertical. In this way movements both upward 

 and downward were studied. 



In swimming up Volvox was found to travel very nearly parallel 

 with the light rays, taking a spiral course, which was in some 

 instances at least 2 mm. wide. In thus traveling upward, it could 

 be clearly seen that the anterior end described a larger circle 

 than the posterior, which in many colonies appeared to go almost 

 in a straight line. The anterior end appeared to swing about the 

 posterior as a pivot. While a large majority of the colonies trav- 

 eled nearly parallel with the rays, there were a few which deflected 

 considerably, some to such an extent that they passed out of the 

 beam of light before reaching the surface of the water. That the 

 movement parallel with the rays was due to the harmonious inter- 

 action of gravitation and light, and not to especially favorable 

 conditions of light intensity, was demonstrated by the course of 

 a certain colony in traveling upward toward the source of light 

 parallel with the direction of the force of gravity, and then again 

 in movement perpendicular to this force. When moving parallel 

 with the direction of the force of gravity, the colony observed did 

 not deflect more than one degree in making several trips up through 

 the water in the aquarium, but in moving perpendicular to this 

 force in the same aquarium and in the same light intensity, this 

 same colony deflected 30 degrees to the right. 



