KENNETH V. THIMANN AND GEORGE M. CURRY 665 



light. However, Brucker (1955) found that the sporangiophore con- 

 tains quantities of gallic acid, which, as Dcnnison (1959) has recently 

 shown, would suffice to render it virtually opaque to light of wave- 

 lengths around 280 ni/x. The negative curvature may therefore be 

 ascribed to the acceleration of growth oidy on the lighted side. 



There is at least some correlation between the light growth-reac- 

 tions and phototropism in Avena too. In 1925 Went showed that 

 there are two types of response to a brief symmetrical illumination. 

 When the tip alone is exposed, growth decreases slowly after 20-30 

 minutes and returns to normal in about 90 minutes; but when the 

 base alone is exposed the growth decrease occurs more quickly and 

 is smaller in extent; within 30 minutes the growth rate is again nor- 

 mal. In each case a slight acceleration follows. These data agree 

 roughly with the behavior of the "tip" and "base" curvatures. 



The Possible Mechanism of Phototropic Curvature 



The gap between the absorption of light by the photoreceptor and 

 its result in curvature is still largely unbridged. The following points, 

 concerned with Avena, represent only fragmentary contributions to a 

 consistent theory. 



In recent years the view has gained ground that phototropism in 

 higher plants is due to photo-catalyzed destruction of auxin. Not only 

 does Fig. 1 make this view very improbable, but further evidence 

 against it is provided by considerations of quantum yield. For the 

 extensive work with auxin has given a sound quantitative relation 

 between lAA concentration and curvature which can be brought to 

 bear on the presumed photo-destruction of lAA. 



To produce a 5° curvature in a decapitated Avena coleoptile an 

 agar block containing about 10-^ grams lAA per liter must be ap- 

 plied. The block has a volume of 10 mm^ and hence contains IG-^*' 

 grams of lAA. During the experiment it has been shown that about 

 one-fifth of the auxin leaves the block to enter the plant. The plant 

 therefore receives 2 X 10-^^ grams or 1 X 10~^^ moles of lAA. This 

 amounts to lO-^^ x 6 X 10"^ or <? X 1^^" molecules and represents 

 the difference in lAA between the convex and concave sides. 



The same 5° curvature can be produced by illuminating with about 

 2 ergs/cm2 of light (436 m/^; cf. Fig. 3) . One erg at 436 m^i — about 

 2 X 10^^ quanta, and the area of the tip is not more than 1/2 mm^, 

 so that about 2 X 10'^ quanta are received by the tip. Making the gen- 

 erous assumption that if 10 percent of these react with auxin molecules 

 in a one-quantum process, not more than 2 X ^^^ molecules of lAA 



