PHOTOTROPISM 



469 



light absorbed by riboflavin. The total amount of auxin destroyed is 

 small, on the order of 10-20 per cent, so that the differential growth 

 retardation cannot be very great. The curvatures produced by illumi- 

 nation of the base never exceed a few degrees; therefore they can be 

 explained by differential auxin destruction. The action spectrum 



Table 9-3. Phototropic and Growth Responses of Avena Coleoptiles 



Minimal effective amount of 

 light to cause phototropic 

 curvature, m-c-sec 



Time lapse between illumina- 

 tion and starting of curva- 

 ture 



Transmission of stimulus. . . . 



Spp.ctral sensitivity 



Pigment concentration 



Light-growth response after 

 illumination 



Auxin relations. 



Possibilities for photoinacti- 

 vation of auxin 



Type of auxin 



Possibilities for lateral trans- 

 port of auxin 



Phototropic theory applicable 



Illumination of tip 



20 



About 1 hr 



Transmission of stimulus 

 from tip to reacting cells 



Action spectrum resembles 

 most the absorption spec- 

 trum of carotene 



Highest concentration of 

 carotene in tip 



So-called "tip response," 

 maximum growth retar- 

 dation about 1 hr after 

 illumination 



Auxin continuously pro- 

 duced in tip; can be col- 

 lected by diffusion 



Auxin formed in tip and 

 diffusing downward ap- 

 parently not destroyed 

 by light 



Auxin-a 



Solid tip ; slight lateral dis- 

 placement will cause com- 

 plete redistribution lower 

 down in coleoptile 



Cholodny-Went theory 



Illumination of base 



50,000 



Less than 0.5 hr 



Response local, where illu- 

 minated; some transmis- 

 sion downward 



Action spectrum resembles 

 most the absorption spec- 

 trum of riboflavin 



Riboflavin distributed 

 evenly over whole coleop- 

 tile 



So-called "base response," 

 maximum growth retar- 

 dation within half an hour 

 after illumination 



Auxin present in extracta- 

 ble form and also supplied 

 by tip, but very little 

 obtainable by diffusion 



Some inactivation of ex- 

 tractable auxin by light 



Indoleacetic acid 



Hollow cylinder; auxin has 

 to move around periphery 

 before an effective auxin 

 gradient between dark 

 and light sides is produced 



Blaauw theory 



obtained by Haig (1935) for the phototropic response of the base closely 

 agrees with the riboflavin absorption spectrum. 



When the actual growth of the tip-illuminated coleoptile is measured 

 during the development of the phototropic curvature, it is found that the 

 rear may double its growth, whereas the growth of the front comes to 



