Mast, Light Reactions in Loiver Organisms. 153 



sinking must then be due to a decrease in the effect of the backward 

 stroke of the flagella on all sides, and this decrease is due to an 

 increase in light intensity. But when the colonies turn toward the 

 source of light, and at the same time swim upward, it is evident 

 that the increase in light intensity must cause an increase in the 

 backward phase of the stroke of the flagella on all sides, for if this 

 were not true there could be no upward motion. The side nearest 

 the source of light, however, passes over a shorter distance than 

 the opposite side, as will readily be seen by referring to the dia- 

 gram, and therefore the increase in the effect of the backward 

 phase must be greater on the latter than on the former. But the 

 light intensity is greater on the former than on the latter (a 

 paradox). When the light intensity in the field is increased the 

 effect of the backward phase of the stroke of the flagella maybe 

 increased or decreased on all sides. If it is increased the effect is 

 most marked on the side in lowest light intensity. Furthermore, 

 if the light is strong the colonies turn toward its source more 

 rapidly and do not swim upward so far and thus make a sharper 

 curve than when it is weak; but the stronger the light the greater 

 the difference between the intensity on the shaded and that on the 

 illuminated side. It, therefore, follows that the greater the differ- 

 ence in intensity on these sides, the greater the difference in effect 

 of the backward phase of the stroke of the flagella, the effect being 

 greatest on the side least illuminated. These considerations sup- 

 port the conclusion arrived at above, i. e.^ that the factors which 

 regulate the activity of the colonies, as a whole, are different from 

 those which regulate the direction of motion. 



We have thus demonstrated that while orientation is due to 

 difference in light intensity on opposite sides of the colonies, it 

 is brought about in positive specimens by the flagella striking 

 backward with greater effect on the side in lowest light intensity 

 than elsewhere. I suggest the following explanation of this: 



First, it must be remembered that the organism constantly 

 rotates on its longitudinal axis. If then a colony is so situated 

 that one side is more highly illuminated than the opposite, it is 

 clear that the zooids will constantly be carried from a region of 

 higher to a region of lower light intensity, and vice versa. They 

 are thus subjected to constant changes in strength of illumination. 

 As stated above, the flagella strike backward with greater vigor 

 on the shaded side than on the opposite one and, therefore, it is 



