472 RADIATION BIOLOGY 



the decrease in light intensity from front to back. Rather complicated 

 experiments with unilateral illumination seemed to favor the light-direc- 

 tion hypothesis, but a simple experiment by Buder (1920) unambiguously 

 proved the light-intensity hypothesis to be correct. He inserted into an 

 isolated coleoptile a glass rod, the tip of which was bent at a right angle 

 and which was silvered except at the ends. Light introduced through 

 the base of this rod would thereby hit the inside surface of the coleoptile, 

 and Ught direction and hght diminution worked in opposite ways. The 

 phototropic curvatures then obtained were always according to the 

 decrease in light intensity. 



Several suggestions have been made concerning the mechanism of the 

 lateral transport of auxin under the influence of unilateral illumination. 

 Went (1932) suggested that the potential difference between the illumi- 

 nated and dark sides, which had been measured by Waller, was responsi- 

 ble for the lateral movement. Since the illuminated side of a plant part 

 becomes negative in comparison with the dark side, it seemed possible 

 that the anions of auxin could move within this potential gradient. The 

 experiments of Clark (1937, 1938) did not support this theory, because 

 he could not correlate the measurements of electrical polarity with the 

 polarity of auxin transport. Schrank (1950) has shown in more recent 

 work that phototropic curvatures are reduced in the same way as electri- 

 cally induced curvatures by filling the inside of the coleoptile with an 

 electric conductor, such as a nutrient solution. Introduction of non- 

 conductors into the coleoptile did not change their sensitivity. Schrank 

 also showed that an electrical potential could enhance or decrease a photo- 

 tropic curvature, depending on whether it increased or decreased the elec- 

 trical potential induced by unilateral light. Therefore his experiments 

 tend to support the idea that the lateral auxin transport after phototropic 

 stimu-lation is due to the induced electrical potential. Went (1936) calcu- 

 lated that approximately 0.001 erg is required for a lateral transport of 

 auxin in the tip sufficient to give phototropic curvature. The minimal 

 amount of Hght energy required for such a curvature is about 1 erg. This 

 fact makes it possible to view the induction of phototropic curvature as 

 normal physicochemical reaction, for which the energy is supphed by the 

 exciting Hght. 



There are some important experiments carried out by Koningsberger 

 and A^erkaaik (1938) which indicate that the base of the Avena coleoptile 

 is sensitive to light only when a gradient of auxin-a exists from tip to 

 base. When the bases of coleoptiles were illuminated from one side with 

 100 m-c, a phototropic curvature of 11° was obtained. Decapitated cole- 

 optiles with pure agar blocks applied to their cut surfaces produced a 

 curvature of only 1° or less. When agar blocks containing auxin-a were 

 applied, curvatures of 7°-8° appeared, whereas decapitated plants with 

 indoleacetic acid agar blocks produced curvatures of less than 1°. These 



