176 



PHYTOHORMONES 



would bend away from the place of application. Our own 

 experiments (u) have given similar results. 



H. Comparison of Phototropic and Geotropic 



Curvatures 



The similarities and differences between the different types 

 of curvature in Avena coleoptiles are difficult to appreciate 



Geotropic 



Geotropic Decap, 



Phototropic Auxin 



Fig. 51. The course of curvatures in Avena coleoptiles. Ordinate: extent 

 of curvature of each zone. Abscissae: towards right, time in minutes; towards 

 background left, successive 2 mm. zones. At left of each diagram is a coleoptile 

 corresponding with these zones, at the start of measurements. Top left: after 

 30 minutes horizontal; curvature already beginning in upper zones. Top right: 

 the same, but decapitated immediately afterwards; autotropism delayed until 

 after regeneration. Lower left: phototropic. Lower right: unilateral auxin 

 application, the top two zones being removed. (After Dolk, 1930.) 



from the somewhat detailed discussion above. Dolk (1930, 

 1936) has, however, made a comparative study of the in- 

 crease of curvatures with time and with distance from the 

 tip, which we are now in a position to interpret. Figure 51 

 shows a three-dimensional composite picture in which the 

 magnitude of curvature — represented by the ordinate — is 

 plotted against time for each 2 mm. zone of the coleoptile. 

 The plants were rotated on a clinostat throughout so that 

 gravity could not counteract the curvatures. 



