186 GROWTH HORMONES IN PLANTS 



be pointed out in explanation of this : (1) The phototropic stimukis 

 lasted only 10 seconds in the experiments cited, while the geotropic 

 stimulus lasted 30 minutes. It seems probable that the further 

 steps in the reaction (which take place after the stimulation) 

 probably are beginning during the lengthened period of geotropic 

 stimulation. (2) Since the geotropically sensitive zone is signifi- 

 cantly longer than the phototropic, the unequal distribution of 

 growth substance resulting from stimulation may set in more 

 rapidly in the former. In geotropic movement, the curvatures 

 reach a maximum and then decline; in phototropic stimulation, 

 the curvatures of the single zones increase gradually to a maxi- 

 mum and then disappear slowly. In order to explain this differ- 

 ence, it must be recalled that according to the growth-substance 

 explanation, the course of curvature is conditioned by the 

 duration and the magnitude of the unequal distribution of growth 

 substance. Since the first phases of the geotropic and the photo- 

 tropic stimulus processes are not identical, it is unreasonable to 

 expect a complete agreement in the course of the curvatures. It 

 is not known how long after the cessation of the stimulation the 

 displacement of growth substance continues ; possibly the unequal 

 distribution of growth substance is more rapidly equalized 

 when it is produced by gravity than when brought about by 

 light. From the curvatures in Figs. 46 and 54, it can be con- 

 cluded that a geotropic stimulation for 30 minutes is far stronger 

 than a phototropic stimulation obtained by illumination with 

 500 meter-candle seconds. This may cause a difference in the 

 course of the curvatures. 



Recovery from Geotropic Curvature Brings About Equilib- 

 rium.— Dolk (19296) investigated the opposing geotropic curva- 

 ture (the straightening of the tip) which starts when the rate of 

 growth of the concave side becomes greater than that of the 

 convex. The conditions necessary for the appearance of such a 

 response are that growth substance in the tip be again equally 

 distributed and that an unlocalized supply of it be furnished by 

 the tip to the basal region. With the extraction method, he 

 showed (1930) that more was present in the lower than in the 

 upper side of the tip after the coleoptile had been in a horizontal 

 position for 30 minutes. When the coleoptile was placed again 

 in an upright position, the amount of growth substance extracted 

 from the upper and under side was equal after 60 minutes (see 



