96 Studies in Animal Behavior 



is rather more often found in the more primitive 

 members of the group and in larvae. Reversal in 

 strong light among the lower invertebrates is fairly 

 common and especially so in the phytoflagellates and 

 other forms which contain chlorophyll or some allied 

 photosensitive substance. The same trait, as Stras- 

 burger and others have shown, is frequently mani- 

 fested by the swarm spores of algae. In forms con- 

 taining chlorophyll, light is intimately concerned with 

 the general metabolism, and therefore has an un- 

 usually important influence upon organic welfare. 

 Adaptive behavior in relation to changes in light in- 

 tensity is, in these primitive forms, almost an es- 

 sential condition of existence. 



An excellent illustration of such adaptive behavior 

 is shown by the movements of the chloroplasts in 

 the cells of many plants. It is well known that 

 most green plants bend toward the light so that a 

 great part of their surface may be exposed to the 

 rays. Often the leaves move under the stimulus 

 of light so that the rays impinge upon them at right 

 angles (transverse heliotropism). But in addition 

 to these devices for securing a more effective ex- 

 posure, another adaptive feature is afforded by the 

 movements of the individual chloroplasts within the 

 cells. These chloroplasts are masses of protoplasm 

 containing chlorophyll. In weak light the chloro- 

 plasts are arranged along those sides of the cell at 

 right angles to the rays. When the light is intense, 

 these bodies move to the sides of the cell parallel 



