406 RADIATION BIOLOGY 



coleoptile and therefore makes all sides grow at the same rate. The 

 Cholodny-Went theory states that unilateral illumination causes a lateral 

 shift in the downward-moving auxin, causing unequal distribution of 

 auxin in the growing zones, which in turn causes unequal growth and 

 curvature. The main difference between this and Blaauw's theory is 

 that in one case the coleoptile reacts as a whole, because the front grows 

 as much less as the back grows more, and in the other case each cell 

 responds independently of the others. Another advantage of this theory 

 is that it is applicable as well to geotropism, for which it has been proved 

 by many investigators (Dolk, 1936). Without going into all the differ- 

 ent experiments that purportedly differentiated between the two theories, 

 let us first see whether an integration is possible. 



Van Overbeek (1933) showed that the phototropic curvature in Rapha- 

 nus hypocotyls could be explained quantitatively by a combination of 

 Blaauw's theory and the auxin-redistribution principle. About 50 per 

 cent of the phototropic curvature was due to the lateral transport of 

 auxin, and the other 50 per cent was due to a change in the reactivity 

 of the cells to auxin under the influence of light. 



A number of experiments have shown that the auxin in Avena cole- 

 optiles actually becomes redistributed under the influence of unilateral 

 illumination. Went (1928a) placed coleoptile tips astride two agar 

 blocks, separated by mica, and thus collected separately the auxin dif- 

 fusing from the two halves of the tips. He found that, during the first 

 1.5 hr after unilateral illumination with 1000 m-c-sec, the total amount 

 of auxin diffusing from the coleoptile tip amounted to 84 per cent of that 

 diffusing from the tips of coleoptiles kept in darkness. But in the lighted 

 coleoptiles 27 per cent of the total auxin (in darkness) was reaching the 

 side facing the light, and 57 per cent arrived at the side away from the 

 light. This means that the dark side received more auxin than it would 

 have if the whole coleoptile had been kept in total darkness. In other 

 experiments it was found that the difference in auxin diffusion from the 

 light and dark sides increases during the second or third hour after illumi- 

 nation. In an experiment in which coleoptile tips were unilaterally 

 illuminated with 100 m-c-sec, during the first 75 min 41 per cent of the 

 auxin diffused from the light side and 59 per cent from the dark side. 

 During the next 75-min period exactly the same amount of auxin dif- 

 fused from the coleoptile tips, but only 1 1 per cent went to the light side 

 and 89 per cent to the dark side. The data of Arisz (1915) show that 

 this behavior of lateral auxin transport is in complete agreement with 

 the rate of phototropic curvature. The rate of curving of the coleoptile 

 reaches its maximum after 1.5 hr of illumination; from then on it con- 

 tinues to bend at the same rate for several hours. Therefore the main 

 auxin differential induced by light should persist from the first to the 

 fourth hour after illumination, particularly. This indicates the need for 



