-1:68 RADIATION BIOLOGY 



placed a thin slice of mica in the slit. When the light fell on the cole- 

 optile parallel with the mica, normal phototropic curvatures would 

 appear, but when the light source was perpendicular to the mica, there 

 would be hardly any curvature at all. Since mica does not interfere with 

 light, and consequently with auxin destruction, the only explanation that 

 will fit the evidence is that the mica interferes with the lateral transport 

 of auxin. 



We can now apply these data to the phototropic response of the Avena 

 coleoptile. When the coleoptile is illuminated with small amounts of 

 hght (under 1000 m-c-sec), only the tip is sensitive, and a curvature 

 results after the differential auxin distribution induced by light has been 

 transmitted from the tip to the growing zones. With strong illumination 

 the lower zones also become sensitive, and then the Blaauw mechanism 

 comes into action. 



In Table 9-3 all facts pertaining to phototropic curvature of Avena 

 coleoptiles are compared; it can be seen that a very clear-cut picture 

 emerges. The Cholodny-Went theory explains the curvature resulting 

 from low-intensity illumination; it is clearly based on phenomena occur- 

 ring in the extreme tip of the coleoptile. The Blaauw theory explains 

 at least part of the curvatures resulting from high-intensity illumination. 

 Basing these principal differences on basic differences in light-perceiving 

 processes is a very attractive proposition; this is actually supported by 

 all the facts presented here. 



Let us consider first the possibility of a differential destruction of auxin 

 under the influence of light. Numerous investigators have shown that 

 less auxin can be extracted after the coleoptiles have been illuminated 

 than before. This has been attributed to the destruction of auxin by 

 light. There are several mechanisms by which this destruction can take 

 place. Kogl ef al. (1936) have shown that both auxin-a and auxin-& are 

 rapidly inactivated by ultraviolet light. Schuringa (1941) showed that 

 this inactivation can occur in visible Hght when carotene is present. 

 Galston and Baker (1949) showed that indoleacetic acid is readily 

 destroyed by visible light in the presence of riboflavin. In view of the 

 only minor destruction of the diff'usible auxin by light and in consider- 

 ation of the fact that the over-all growth of the coleoptile is only slightly 

 decreased by illumination, it seems safe to conclude that neither auxin-a 

 nor indoleacetic acid photoinactivation can play a prominent part in 

 phototropism in Avena. Only curvatures of a few degrees could possi- 

 bly be attributed to differential auxin destruction on both sides of the 

 coleoptile. Therefore it is certain that the first positive curvature can- 

 not possibly be due to photoinactivation of auxin. On the other hand, 

 the weak second and especially third positive curvatures are of the proper 

 magnitude. It would seem, then, that the base response of the Avena 

 coleoptile can be satisfactorily explained by auxin destruction through 



