144 LIGHT AND THE BEHAVIOR OF ORGANISMS 



intensity the zooids are not carried from regions of higher 

 to regions of lower Ught intensity and vice versa as described 

 above. Under such conditions then the orienting reactions 

 must be due entirely to the shading of certain structures 

 within the zooids by other structures also within them, as 

 the surfaces turned toward the source of light change. 



Orientation in Volvox is, according to this account, due 

 to changes in light intensity on the zooids as a whole 

 together with changes of intensity on certain structures in 

 the zooids, made possible by difference of intensity on the 

 surface of the colony and rotation on the long axis. The 

 change of intensity is caused by the movement of shadows 

 of certain structures in the organism cast upon other struc- 

 tures. These shadows are present in a field of uniform 

 intensity. There is no evidence that the direction of the 

 rays in the field or through the body, or that the angle the 

 rays make with the surface of the body, is of importance 

 in orientation excepting in so far as it may affect difference 

 of intensity in the individual zooids or the colony as a 

 whole. Nor is the symmetry of the organism of prime 

 importance, for, as was stated above, asymmetrical seg- 

 ments of various forms orient nearly as accurately as entire 

 colonies. Orientation may take place in constant light 

 intensity quite as well as in a field having various intensities. 

 Difference of intensity in the field does however determine 

 the distribution of the colonies. They are negative in light 

 of high intensities, positive in that of low and neutral in 

 that of optimum intensity. Orientation then, whether posi- 

 tive or negative, tends to direct the colonies to the area of 

 optimum illumination. Light acts as a constant directive 

 stimulus on the colonies as a whole, but there is no evidence 

 that there is a directive stimulus without change of inten- 

 sity, for the reacting elements, the zooids, and especially 

 the structures within the zooids, are not subjected to con- 

 stant intensity. 



It is probable however that constant intensity affects the 

 activity of these organisms somewhat as temperature does. 



