44 



PHYTOHORMONES 



Gorter (1927). She found that whenever an extract causes 

 negative curvature, it does so within 23/^ hours after appH- 

 cation of the agar. Within this time she never observed 

 positive curvatures. More than 3 hours after decapitation 

 and apphcation of the block, however, pure agar or agar 

 containing inactive extracts caused positive curvature, 

 i.e. towards the apphed block. This she explained by the 



onset of regeneration, which 

 takes place about 2 ]/2 hours 

 after decapitation (Dolk, 

 1926; Zollikofer, 1928; Li, 

 1930). If a block of pure 

 agar be applied symmetri- 

 cally to the top of a decapi- 

 tated coleoptile, then for- 

 mation of auxin in the 

 regenerated tip is greatly 

 inhibited (Skoog, 1937). 

 This has the result that 

 when pure agar is placed 

 on one side of a decapi- 

 tated coleoptile, the new 

 production of auxin 2)^ 

 hours after decapitation 

 will be greater on the side 



100 200 300 min. 



Fig. 20. The course of curvatures after 

 application of auxin in agar (curve A) 

 and plain agar (curve B). Blocks applied 

 at time O, plants being decapitated 45 

 minutes earlier. Note development of 

 positive curvature in B 85 minutes after 

 application of plain agar, i.e. 130 minutes 

 after decapitation. This corresponds to 

 the regeneration time. 



without the block than on the side with it, and this will cause 

 a curvature towards the block (see Figure 20). Hence if agar 

 with a high auxin concentration is applied to one side, 

 the auxin supplied by it will exceed the auxin produced by 

 regeneration, and the negative curvature will continue to 

 increase after regeneration. With low applied auxin con- 

 centration, however, or with very small agar blocks, the 

 negative curvature which first develops may change after 

 regeneration into a positive curvature. In such cases the 

 positive curvatures are not due to any growth-retarding 

 substance. 



Whether there really are substances, other than toxic 



