ORIENTATION IN COLONIAL FORMS 141 



5 mm. immediately after the light is turned on, but while 

 they are sinking this short distance, they apparently become 

 acclimated and soon turn toward the light, and at the same 

 time swim upward, just as described above. During the 

 time in which these colonies sink they continue to rotate 

 in the same direction as before. The 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 back- 

 ward 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 may be 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 difference 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 support 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 



