BENTLEY GLASS 893 



are a cofactor. The en/yme is inactivated by chloroplasts in tlie light, 

 but reducing compounds will prevent this. It is reddish-brown in 

 color, has a molecular weight (derived irom sedimentation and dil- 

 fusion data) of about 1 1,000, absorbs at 278, 327, 420, and 460 m^i, and 

 upon being bleached loses its absorption at the two higher wave- 

 lengths. It does not appear to be either a flavin or heme protein, but 

 may well be identical with the methemoglobin-reducing protein re- 

 cently isolated from leaves by Davenport and Hill. 



Phototropism and Photoperiodism 



Kenneth V. Tliimann and G. M. Curry begin their review of 

 jihototropism by stating that it is not quite at the simplest level of 

 responses of organisms to light. Phototaxis, the orientation to light of 

 freely moving organisms, seems even simpler than phototropism, which 

 is properly a term limited to the light-directed curvature of fixed 

 organisms. It is not clear, however, why phototaxis is regarded as being 

 simpler — perhaps it is because no chemical photoreceptor or mediat- 

 ing substances have been isolated in the case of phototaxis as they 

 have in phototropisms. And perhaps Thimann woidd agree that there 

 are various levels of complexity in both phototaxis and phototropism. 



Recent studies (Briggs, Tocher, and Wilson) have refined the classic 

 experiments with agar blocks applied to oat or corn coleoptile tips. 

 Not only is it evident that more indole acetic acid (i.e. auxin) — about 

 twice as much — diffuses into the agar block from the shaded as from 

 the illuminated side of the coleoptile; but it is evident also that 

 neither the illumination of the tip nor slitting the base of the tip 

 modifies tlie total quantity of auxin produced, but that w'hen the 

 tip is slit all the way to the top no difference in amount of auxin 

 on the shaded and illuminated sides can arise. Hence it is clear 

 that the auxin is not destroyed by light, but must move away from the 

 lighted side to the dark side. 



The identity of the photoreceptor for the phototropic response has 

 been a subject of much interest, approached experimentally by analysis 

 of the action spectrum. Although red light wdll not evoke phototropic 

 curvatures, when dark-grown plants are exposed to red light before 

 being exposed to blue or white light, the growth rate near the tip 

 of the coleoptile is greatly enhanced and that at the junction of 

 coleoptile and mesocotyl is inhibited. Hence experiments are best 

 carried out with dark-grown plants exposed to monochromatic light. 

 The action spectrum for the positive curvature of oat coleoptiles has 



