470 RADIATION BIOLOGY 



almost a complete standstill (Went, 1928b; Beyer, 1928). This fact can- 

 not possibly be explained by differential auxin destruction. In the first 

 place, the growth along the rear is greater in light than in complete dark- 

 ness. In the second place, not more than 20 per cent destruction of auxin 

 has been reported, but still the growth of the front may cease altogether. 

 However, these facts can be brought in line with the Cholodny-Went 

 theory; a redistribution of auxin in the tip would increase the growth of 

 the rear as much as it would decrease the growth of the front. Most 

 investigators have not found a complete redistribution of auxin under 

 the influence of unilateral light, but this is probably due to the fact that 

 auxin redistributions were usually measured only during the first hour of 

 illumination. 



The facts presented thus far show a curious discrepancy when analyzed 

 in view of the findings of Thimann and Bonner (1933) and Went (1942). 

 These investigators found that extractable auxin in the lower zones of a 

 decapitated Avena coleoptile decreased in 2 hr to approximately 50 per 

 cent of that found in intact coleoptiles. This means that, if the extract- 

 able auxin were responsible for the growth of a coleoptile, the growth 

 rate of a unilaterally illuminated coleoptile could never drop to less than 

 half the normal rate within 2 hr after illumination. Since growth on the 

 illuminated side may cease altogether, it is obvious that the extractable 

 auxin is not responsible for growth. This is contrary to Went's con- 

 clusion (1942) but seems inescapable on the basis of the recently dis- 

 cussed facts. It also follows from Oppenoorth's work (1941), in which 

 a considerable decrease was found in ether-soluble auxin on the illumi- 

 nated side of an Avena coleoptile, but enough was left to cause growth. 



It seems possible to reconcile the opposing facts on the basis of the 

 following hypotheses: 



1. The auxin found in the coleoptile tip is auxin-a, as Kogl et al. (1934) 

 have shown to be very likely. The evidence of Wildman and Bonner 

 (1948), who showed that about 50 per cent of the auxin diffusing from 

 the tip of the coleoptile is indoleacetic acid, is inconclusive on the basis 

 of two facts: (a) The Salkowsky test is not specific for indoleacetic acid, 

 as they claim, but gives a color with many other indole derivatives that 

 are inactive in growth production (Gordon and Weber, 1951). Besides, 

 the color produced from their coleoptile diffusates was not typical of the 

 indoleacetic acid reaction, (h) A number of substances could have dif- 

 fused out of the cut cells and have given positive results with the Sal- 

 kowsky test. Also the diffusion-rate values obtained from coleoptile-tip 

 diffusates indicate the presence of a substance with a much greater 

 molecular weight than indoleacetic acid. 



2. The auxin-a produced in the tip can be redistributed under the 

 influence of light absorbed by carotenoids. The evidence of Haig (1935), 

 Wald and du Buy (1936), and Biinning (1937) indicates carotenoids to be 



