PHOTOTROPISM 473 



experiments do not agree with the suggestion that the base response is 

 due to the destruction of indoleacetic acid by Hght. According to these 

 experiments, both base and tip response can be due only to lateral trans- 

 port or inactivation of auxin-a. 



The previous analysis shows that most of the phototropic response of 

 Avena coleoptiles is due to the lateral transport of auxin, particularly in 

 the extreme tip. Van Overbeek showed that in Raphanus only 50 per 

 cent of the curvature could be accounted for by lateral redistribution of 

 auxin. Blaauw's calculations (1915) indicate that almost all the photo- 

 tropic response of Helianthus hypocotyls can be explained by the differ- 

 ential response of the sides of the hypocotyl to light. When he calcu- 

 lated the amount of curvature produced with 512 m-c from the growth 

 response of the light and shaded sides of the hypocotyl, he obtained a 

 value of 11°. The actual curvature was 10°. Therefore it must be con- 

 cluded that several different mechanisms exist in different plants, or 

 coexist in one plant, v/hich are all responsible for phototropic curvature. 

 We might now try to summarize the various processes that are known or 

 can be surmised to intervene between light absorption and phototropic 

 response. 



The action spectrum of phototropism, as determined by Blaauw (1909), 

 Haig (1935), Johnston (1934), Bunning (1937), Galston and Baker (1949), 

 and others, indicates that phototropically active light is absorbed by a 

 pigment that has an absorption spectrum like those of the carotenes or 

 riboflavin. As Galston (1950) has pointed out, an unequivocal decision 

 between the activity of these pigm.ents is difficult to reach. Yet the 

 actual data agree best with the assumption that carotene-absorbed light 

 is effective in inducing the tip reaction and the phototropic response of 

 the coleoptile tip, whereas riboflavin-absorbed light is effective in causing 

 phototropism in the base of the coleoptile. 



From the auxin-extraction data and from the periodic nature of the 

 degree of curvature in relation to light energy, we can say with certainty 

 that auxin destruction plays at most a very minor role in the establish- 

 ment of phototropic curvatures. Stewart and Went (1940), confirming 

 du Buy's (1933) measurements, showed that the auxin diffusing through 

 Avena coleoptiles is not destroyed by light at all. They found, as was 

 confirmed by Oppenoorth (1941), that the ether-extractable auxin from 

 Avena coleoptiles is reduced in amount by light, but that this decrease is 

 not related to the amount of light energy and thus cannot be the basis 

 for phototropic curvatures. It is very likely that this small amount of 

 auxin destruction in light is responsible for the light-growth response of 

 the base of the coleoptile, as measured by Dillewijn (1927) and Went 

 (1941). It consists in a growth retardation, reaching its maximum extent 

 after 25 min at 20°C, followed by a growth acceleration at 40 min. This 

 I'esponse does not bear a quantitative relation to the amount of light 



