TROPISMS 



173 



It would be out of place to review the publications deal- 

 ing with the qualitative comparison of light-growth reaction 

 and phototropic curvature; our concern is with the auxin 

 side of the phenomenon. The simplest case is found in 

 hypocotyls of Raphanus (van Overbeek, 1933), which prob- 

 ably are comparable in their behavior to those of Helianthus 

 studied by Blaauw (1915). If Raphanus seedlings are ex- 

 posed to strong continuous light (1000-2000 meter-candles) 



20 



15 



10 



(I 



25 50 75 100 



Fig. 50. Auxin curvatures of Raphanus hj^pocotyls. Ordinate, curvature 

 in degrees; abscissa, concentration of auxin applied unilaterally. Curve D, in 

 darkness; L, in light of about 2000 meter-candles. (From van Overbeek, Rec. 

 trav. hot. neerl. SO: 537-626, 1933.) 



their growth is reduced by from 50 to 60 per cent. This 

 reduction in growth rate is not due to decreased auxin 

 production or transport, nor to increased destruction (see, 

 however, pp. 88, 175) ; it must therefore be due to a decrease 

 in the reacti\dty of the cells to auxin. The same effect may 

 be seen from the effect of light on curvatures produced by 

 one-sided application of auxin (see Figure 50). An exactly 

 similar effect of light in reducing straight growth from 

 applied auxin was found in Vicia Faba stems by Thimann 

 and Skoog (1934). This reduced reacti\'ity of plant tissues 

 in hght undoubtedly plays a part in phototropic curvatures. 

 Van Overbeek calculated that about half the phototropic 

 curvature of Raphanus hypocotyls is due to auxin redistribu- 



