PHOTOTROPISM 



467 



more data on auxin redistribution an hour or more after illumination 

 rather than immediately afterward. 



Asana (1938) studied the auxin redistribution in Arena coleoptile tips 

 that were subjected to 14,000 m-c-sec, an amount of light that usuall}^ 

 produces a negative curvature. He established that the amount of dif- 

 fusing auxin was the same in the illuminated tips as it was in the dark 

 controls; therefore this amount of light does not alter the total auxin 

 content. But at this high intensity 58 per cent of the auxin diffused 

 from the light side, and 42 per cent came from the dark side. Asana's 

 results were based on individual auxin diffusions from about 200 plants, 

 and a statistical analysis showed that this difference was significant. The 

 work of Wilden (1939) confirms both Went's and Asana's results. She 

 determined the amount of auxin diffusing from either the illuminated or 



Table 9-2. Auxin Diffusing from Illuminated and Darkened Coleoptile Tips 



(Wilden, 1939.) 



the darkened side of coleoptile tips by placing them one-sidedly on Avena 

 test plants. Table 9-2 shows her results. 



Other investigators (Burkholder and Johnston, 1937) demonstrated a 

 lateral transport of auxin in Avena coleoptile tips when the amounts of 

 light that give the third positive curvature were used. 



In later work Oppenoorth (1941) studied the effect of light on the 

 auxin content of the illuminated and shaded sides of Avena coleoptiles, 

 using the ether-extraction method. Such extractions do not give direct 

 proof of the lateral transport of auxin, but the evidence can be most 

 readily explained by redistribution of auxin under the influence of light. 

 Oppenoorth, like Stewart and Went (1940), found that there is a certain 

 amount of auxin destruction in light, but that this destruction is inde- 

 pendent of the total amount of light applied. Therefore only a slight 

 difference in extractable auxin content of the light and dark sides could 

 be expected. It is clear that this auxin destruction cannot account for 

 the large phototropic curvatures produced with small amounts of light. 



Less direct but ecjually conclusive evidence of lateral auxin transport 

 was given by Boysen-Jensen (1928). He split Avena coleoptile tips and 



