88 PHYTOHORMONES 



controlling growth and the only one which as yet can be 

 experimentally investigated. Since the action of the genes 

 is through their control of internal factors, the intermediate 

 stages between growth genes and their effect can now be 

 approached through auxin. 



F. Radiation and Its Effects on Auxin 



It has often been observed that short wave-length radia- 

 tions have a markedly inhibiting effect on plant growth (see 

 Duggar, 1936, Chapters XXII, XXVI, and XXIX). In re- 

 gard to x-rays, the relation between this and their effect on 

 auxin has been investigated by Skoog (1935). He found 

 that the direct effect of hard x-radiation was on the auxin 

 itself, which is readily inactivated, both in vitro and in vivo. 

 The inactivation is almost certainly by oxidation. Neither 

 the sensitivity to auxin, nor its rate of transport, was 

 changed after the irradiation of Pisum, Avena, and Helian- 

 thus seedlings, but their auxin production was greatly de- 

 creased. For short periods after irradiation the diminished 

 growth could be accounted for by the auxin destruction. 

 Thereafter, a secondary effect of the x-rays on the auxin- 

 producing system became evident in Pisum. 



Ultra-violet light also inhibits growth, and correspond- 

 ingly Laibach and Maschmann (1933) found that unfiltered 

 ultra-violet inactivates auxin solutions almost completely. 

 According to Koningsberger (1936), ultra-violet of wave- 

 lengths between 230 and 330 m.jx inactivates auxin a lactone 

 solutions with great rapidity. 



Bright light also has a dwarfing effect on many plants. 

 This is due to the effect of light in decreasing sensitivity to 

 auxin. The experiments of van Overbeek (1933) with 

 Raphanus hypocotyls, and of Thimann and Skoog (1934) 

 with Vicia Faba stems, have shown that in light a given 

 amount of auxin produces much less elongation than in 

 darkness. Whether this reduced sensitivity is due to in- 

 creased- destruction of auxin in light is not clear. Van Over- 

 beek (1933) was not able to show (by transport experiments) 



