PHOTOTROPISM 



657 



half. Furthermore, the quantity of auxin from the shaded half was ap- 

 preciably more than the amount from half of an unilluminated tip. Went 

 concluded therefore, that one-sided exposure to light had caused some of the 

 auxin to migrate from the illuminated to the shaded side of the coleoptile. 



Boysen-Jensen (1928) came to a similar conclusion from a different kind 

 of an experiment. The tip of a coleoptile was split longitudinally, a thin 

 glass plate was inserted between the split halves and the coleoptile was ex- 

 posed to unilateral illumination of suitable intensity. 

 When the glass plate was parallel to the light 

 beam, normal phototropic curvatures occurred, but 

 when the glass plate was at right angles to the direc- 

 tion of the light very little curvature resulted. 

 When at right angles to the light beam the glass 

 plate apparently prevented the lateral migration of 

 auxin from the lighted to the shaded side of the 

 coleoptile so that the quantity of auxin was ap- 

 proximately the same on both sides. When the 

 glass plate was parallel to the light beam, however, 

 movement of auxin from the lighted side was not 

 prevented. This experiment also indicates that 

 under conditions of one sided illumination, auxin 

 migrates from the illuminated to the shaded side 

 of the oat coleoptile and that positive phototropic 

 curvature is correlated with the greater auxin con- 

 tent of the cells on the darker side of the coleoptile. 



Burkholder and Johnston (1937) have shown 

 that light of high intensity may cause a destruction 

 or inactivation of auxin in plant tissues. It is pos- 

 sible therefore that under certain conditions phototropic curvatures of oat 

 coleoptiles may be caused in part by an inactivation of auxin upon the illumi- 

 nated side and in part by a migration of auxin from the lighted to the shaded 

 side of the organ. 



All wave lengths of the visible spectrum are not equally effective in induc- 

 ing phototropic curvatures. The shorter wMve lengths are most effective and 

 the longer wave lengths at the red end of the spectrum evoke practically no 

 phototropic reaction. According to Johnston (1934) the most effective wave 

 lengths lie in the range from 440 mjx to 480 m,Li (Fig. 145). 



In the early years of the present century Blaauw demonstrated that a 

 certain minimum quantity of light was essential for a perceptible phototropic 

 curvature of an oat coleoptile. Since quantity is a product of intensity and 



Fig. 144. Diagram to 

 show method of demon- 

 strating results of uni- 

 lateral illumination upon 

 auxin distribution in co- 

 leoptile tips. {A) coleop- 

 tile tip, (B) and (C) 

 agar blocks, (D) metal 

 plate. Horizontal arrows 

 indicate direction of illu- 

 mination. Auxin is dis- 

 placed towards side of 

 coleoptile away from 

 light. 



