AUXIN AND GROWTH 89 



any destruction of the free-moving auxin in Raphanus 

 hypocotyls. However, he later found (c/. X G) that in 

 light Arena coleoptiles react differently to auxin a and to 

 indole-acetic acid, and explains this by a greater destruction 

 of auxin a in the plant in Ught. On the other hand, in the 

 experiments of Thimann and Skoog (1934), Vicia Faba 

 stems, supplied, not with auxin a but with indole-acetic 

 acid, showed the characteristic decrease of sensiti\dty to 

 auxin in light. It is therefore probable that, while light 

 inactivation of auxin a may occur, there is also a direct 

 effect of light on sensitivity of cells to auxin. 



