PHOTOMECHANICAL RESPONSES 83 



Some relatively recent observations of importance upon 

 the phototropic responses of the catfish eye were made by 

 Welch and Osborn (1937). They found that the extreme 

 dark-adapted retina is obtained only if the eyes are prepared 

 during the night. If the animals are allowed to remain in 

 the dark, the visual cells and epithelial pigment may approach 

 the typical light conditions at the time of sunrise and tend 

 to return to the dark position at sunset. They found, further, 

 that with constant illumination, from 24 to 48 hours, the 

 rods and cones maintained throughout the typical light 

 condition. When kept in darkness, however, for a similar 

 period, the positions of the rods and cones are characteristic 

 of extreme dark adaptation during the night, but during the 

 day they approach the conditions characteristic of light 

 adaptation. If the existence of a similar persisting diurnal 

 rhythm should be demonstrated in the eyes of other forms, 

 it becomes apparent that many measurements which have 

 been made in the past on positional changes in the pigment 

 and visual cells may become subject to considerable modi- 

 fication. 



Arey and Mundt (1941) have shown that when Ameiurus 

 is kept constantly in the dark, the cones show greater elonga- 

 tion (extreme dark adaptation) at midnight than at noon. 

 This rhythmic alternation in total darkness imitates the 

 stronger diurnal rhythm in nature, but is ''independent of 

 actual alternating daytime light and nighttime dark." They 

 found that this rhythm will continue at least for four days. 

 They did not produce convincing evidence of a comparable 

 diurnal rhythm for rods and retinal pigment under conditions 

 of constant darkness. 



The existence of diurnal rhythms in the activities of 

 animals and plants is a field of biological investigation which 

 is relatively new and should yield many interesting and 

 fundamental facts in the physiology of the organism. 



In addition to the effects upon the epithelial pigment and 

 the positional changes in the visual cells, light has been 

 shown also to bring about changes in form, position, and 



