660 LIGHT AND LIFE 



be obtained. It the lAA is left on only for 30 minutes the curvatures 

 are restricted to the upper 5 mm. Data from such a group of experi- 

 ments are summarized in Table 1; clearly lAA can reinstate the lost 

 phototropic sensitivity. This result is not due merely to reinstate- 

 ment of the growth of the decapitated coleoptile, for application of 

 the lAA after ultraviolet illumination did not cause curvature under 

 these conditions. (With very much larger doses of white light, total- 

 ling about 500,000 ergs/cm2, von Guttenberg (1959) has shown that 

 lAA can produce curvature if applied after the illumination, but 

 that is another phenomenon, to be discussed below) . It is concluded 

 that in the phototropism caused by ultraviolet light, and probably 

 therefore in the high-threshold response in general, there is a 



TABLE 1 



Reinstatement of Phototropic Curvature of Decapitated Coleoptiles 

 IN the Ultraviolet by Application of Indole Acetic Acid in Agar 



Exposure in Curvature 

 ergs/cm^ of in 



297 m/x degrees 



Intact plants, tips shielded 



Thrice decapitated controls 



Thrice decapitated, lAA on for 30 minutes 



Thrice decapitated, lAA on for 75 minutes 



strong possibility that lAA in some combined form is both photo- 

 receptor and effector. Whether lAA is inactivated or only trans- 

 located in this system remains unknown. 



These results provide an answer to the question as to whether 

 there is more than one type of phototropic response in the same 

 plant; there are at least two, making use of different photoreceptors. 



Phototropism in Phycomvces 



For a broader answer to the same question, one could study an- 

 other member of the grasses, such as barley, whose phototropism 

 may well differ in some respects from that of Avena, or Blaauw's 

 old object (I9I5), the sunflower seedling. However, a more com- 

 plete contrast seemed to be promised by leaving the flowering plants 

 altogether and studying a fungus. The sporangiophore of Phycomyces, 



