664 



LIGHT AND LIFE 



It should be said that from a biological viewpoint this may well 

 be a naive view. That opposite sides of a single cell should react 

 to an external stimulus quite independently of one another is es- 

 sentially improbable. It is true that different responses to chemical 

 stimuli by different parts of a single cell do occur, especially in the 

 siphonaceous algae. But to the writer it appears much more likely 

 that some interaction between the responses wotdd occur and that 

 the curvature would be the result of an integrated change in the cell, 

 rather than simply of the difference between two independent re- 

 actions. Indeed, the time relations give some support to this view, 

 for the effect of symmetrical continuous illumination is to produce 

 only a temporary growth acceleration, from which the growth rate 

 returns to normal after abotit 10 minvites. This is ascribed by Del- 

 briick and Reichardt (1956) to the onset of "adaptation." However, 

 if the illumination is unilateral, curvature begins after 4 or 5 minutes, 

 and continues to increase for 30 minutes or more; thus, long after the 

 light growth-reaction has died out curvature continues at a high rate. 

 This discrepancy certainly suggests that curvature is not the simple 

 result of a difference between light growth-reactions on opposite sides. 



However, from the viewpoint of the lens theory the ultraviolet 

 curvatures behave as expected. Brief symmetrical exposure of a 

 sporangiophore to ultraviolet causes a growth acceleration, which be- 

 gins 2 minutes after the onset of exposure and lasts only 7 to 10 

 minutes. As Fig. 11 shows, it can be elicited repeatedly. Such an 

 acceleration of growth by ultraviolet would, if the lens-theory is 

 valid, be expected to cause positive curvature just as with visible 



10 



20 



80 



90 



30 40 50 60 70 



TIME IN MINUTES 



Fig. 11. Light growth reactions of a Pliycomyces sporangiophore on brief illumi- 

 nation with ultraviolet liglit. Horizontal lines show tiie periods of iihunination. 

 Measurements made with a vertical travelling microscope. 



