KENNETH V. T HIM ANN AND GEORGE M. CURRY 663 



15 



Fig. 10. Progress of negative curvatures of Phyconiyces continuously illuminated 

 from the left with 1.50 ergs/rm-/sec of 280 m^. Photographed at 5, 15, 25, 35, 45, 

 and 55 minutes after illumination. (From Curry and Gruen, 1957). 



the peak being near 280 m^ui, but minor differences prevent identifica- 

 tion of the two. 



Phototropism and Light-growth Reactions 

 The reversal of sign of these curvatures in the ultraviolet poses 

 an interesting question of mechanism. The effect of symmetrical 

 light on Phycomyces is to accelerate its growth, as was first shown by 

 Blaauw and has more recently been studied in detail by Delbriick 

 and Reichardt (1956). One would expect, therefore, that unilateral 

 illumination, by accelerating growth on the lighted side, would cause 

 negative cinvature. The fact that curvature is positive has for long 

 been explained in terms of a "lens effect", i.e., the clear cell contents 

 focus the light in a "bright stripe" on the opposite wall, and the 

 resulting greater acceleration there causes positive curvature. Evi- 

 dence for or against this interpretation need not be discussed here; 

 it is sufficient to note that this concept (following Blaauw) would 

 explain the curvature as simply due to the difference between the 

 growth responses of the two sides to the light. 



