Mast, Light Reactions in Lower Organisms. 151 



cited above cannot be valid, for it is based upon the supposition 

 that increase in speed in Volvox is due to increase in light intensity. 

 We shall refer to this question again (p. 153). 



If a colony which is not oriented turns toward the source of 

 light, it is clear that the stroke of the flagella on the shaded side 

 must be more effective in driving the organism forward than that 

 on the illuminated side. This may be conceived to be caused 

 directly by the difference in light intensity on opposite sides, or 

 indirectly in that a Volvox colony may possibly act as a lens and 

 thus cause the light on the side opposite that most highly illumi- 

 nated to become most intense; or, since the zooids are intimately 

 connected by protoplasmic strands, it is not impossible that 

 impulses produced by excessive photic stimulation may be trans- 

 mitted to the opposite side and result in action there. At any 

 rate, it is undoubtedly true that these strands serve to transmit 

 impulses from zooid to zooid, and thus bring about coordinate 

 action. 



It was found, as previously stated, that segments, e. g., halves 

 produced by cutting specimens parallel to the longitudinal axis, 

 orient essentially like normal colonies. Such segments, however, 

 cannot act as lenses, nor can impulses originating on one side be 

 transmitted to the opposite side. The last two of the possible 

 explanations suggested, therefore, must be abandoned, and it must 

 be concluded that the unequal effect of the stroke of the flagella 

 is due directly to difference in light intensity on opposite sides of 

 the organism. But this unequal effect of the stroke on opposite 

 sides may be caused, as Holmes pointed out, by an increase in 

 the backward phase of the stroke on the shaded side, or a decrease 

 in the same phase on the illuminated side or a decrease in the 

 forward phase on the shaded side, or an increase in this phase on 

 the illuminated side. Can it be ascertained which of these is the 

 cause of the difference between the effect of the stroke of the fla- 

 gella on the shaded sides and that of those on the illuminated side 

 of the colonies ^ 



If the light intensity of the field is suddenly decreased while 

 colonies of Volvox are swimming horizontally toward it, they stop 

 forward motion, the longitudinal axis takes a vertical position due 

 to the effect of gravity, and then the colonies swim slowly upward. 

 It is not at all difficult to find specimens in which this upward 

 swimming is just sufficient to overcome the effect of gravity, and 



