ORIENTATION IN COLONIAL FORMS 143 



the light and away from it. Thus the zooids are carried 

 from regions of higher to regions of lower light intensity 

 and vice versa, and the motor reaction is induced just as it 

 is in entire colonies. 



" Orientation in negative colonies can be explained in 

 precisely the same way as that in positive ones, assuming 

 merely that in this condition the zooids respond with the 

 motor reaction when they pass from lower to higher light 

 intensity instead of when they pass from higher to lower 

 (as is true when the organisms are positive) . The backward 

 stroke then becomes most effective on the side most highly 

 illuminated." 



It is altogether likely that the orientation In Volvox Is 

 not entirely due to the change of light intensity on the 

 zooids caused by difference of intensity on the colonies as a 

 whole and rotation on the long axis as described above ; but 

 that as in Euglena and other forms discussed above, the 

 sensitiveness of the zooids depends upon the surface exposed. 

 When the zooids are carried from the Illuminated to the 

 shaded side It is clear that they are not only transferred 

 from a region of higher to a region of lower light intensity, 

 but the surface turned toward the light Is also changed. 

 This change in Itself may, as in various unicellular forms 

 studied, cause a reduction of Intensity on certain structures 

 within the zooids by the movement of shadows of other 

 structures, and consequently an orienting stimulus. The 

 orientation of segments Indicates that this factor plays a 

 very important part in the orientation of the colonies. 



Take for example a segment formed by cutting a colony 

 in half lengthwise. The zooids in this segment lie side by 

 side and are some distance apart, being connected by thin 

 strands of protoplasm. Most of the substance In the cavity 

 runs out when the colony Is cut, and what remains is trans- 

 parent. It is therefore evident that there is no more sub- 

 stance to shade the zooids when the Inner surface of the 

 segment faces the source of light than there is when the 

 outer faces it. If the segments are in a field of uniform 



