TROPISMS 179 



Avena coleoptiles, curvatures could be induced if the wound 

 was close to the base. This is doubtless due to interference 

 with the upward-moving stream of substance from the seed 

 (c/. YD). In general, all positive traumatotropic curvatures 

 appearing above the incision (Stark, 1917; Beyer, 1925; 

 Biinning, 1927) may probably be ascribed to interference 

 with the upward-moving stream of food factor. 



Tendeloo (1927) has shown that while Avena normally 

 gives positive traumatotropic curvatures, regeneration of 

 auxin production at the lower cut surface of the incision may 

 subsequently give rise to a negative curvature. The nega- 

 tive curvatures obtained by Weimann (1929) may also be 

 due to this cause. 



Keeble and Nelson (1935) have explained the traumato- 

 tropic curvatures of roots by interference with the distribu- 

 tion of auxin, and there seems no reason to doubt that this 

 is one of the principal factors operating in traumatrotropism. 



The effects of wounding, however, consist not only of 

 interference with the transport, but also involve destruction 

 of auxin by enzymes freed from the cut cells (Thimann, 

 1934). This phenomenon doubtless explains the results of 

 Gradmann and Cholodny with split stems, discussed in X C. 

 In Cholodny's experiments spHt halves of stems, placed 

 horizontally with the cut surface downward, curve much 

 less than those with the cut surface upward. We may tenta- 

 tively explain this by saying that the auxin which accumu- 

 lates under the influence of gravity on the lower side is, in 

 the one case, partly inactivated by the wound enzjrnaes, in 

 the other case not. The auxin inactivation may also play 

 an important part in positive curvatures. 



In conclusion, it is evident that the whole phenomenon 

 of traumatotropism needs to be reconsidered in the light of 

 our present knowledge of auxins. 



K. Electrotropism 



Movements of shoots and roots towards electrodes have 

 been known for a very long time; they are of interest to us 



