218 GROWTH HORMONES IN PLANTS 



The Growth-substance Explanation. — From the premises of the 

 growth-substance explanation, traumatic curvatures can be 

 explained in different ways. The supply of grow^th substance to 

 growing regions might be checked unilaterally, either by destruc- 

 tion of the tissue where it is formed or by removal of the growth 

 substance forming organs or parts of organs. Disturbance m 

 correlated growth activity and curvatures could be the result of 

 inhibition of stimulus transport on one side of the organ. There 

 is the possibility, also, that wound substances are formed 

 (Biinning, 1927) as a result of stimulation and that these sub- 

 stances influence growth. In addition, it may be necessary to 

 consider the possibility that unilateral disturbances in the food 

 supply to growing plant parts can produce curvatures. In view 

 of these different circumstances, an entirely unified explanation of 

 traumatic curvatures cannot be given. The discussion will be 

 confined to the description of a few characteristic traumatic 

 responses, relating them to the possible role of growth substances. 



Amputations. — Van Overbeek (1933) showed that the growth 

 substance necessary for growth of the hypocotyl in Raphanus is 

 supplied by the cotyledons. The early report of Heidmann 

 (1913) that the removal of one cotyledon produces a positive 

 traumatic curvature may now be interpreted as being due to the 

 removal of the growth-substance supply on one side. 



Wounding by Incisions — When a root or coleoptile is traumati- 

 cally stimulated on one side near the tip, the resulting curvature 

 of the root is negative, while that of the coleoptile is positive. 

 It is probable that these curvatures arise because of injury to the 

 tip; the distribution of growth substance from the tip is disturbed 

 and is lessened on one side. Lateral incisions in the Avena 

 coleoptile can modify curvatures by interfering with the transport 

 of growth substance. It is more difficult to explain the occur- 

 rences in the root. Here a wound below a transverse incision 

 produces a strong negative curvature. This phenomenon might 

 be the result of changes in the concentration of growth substance 

 or the formation of it on the side opposite the wound. 



The traumatic curvatures appearing as a result of transverse 

 incisions in the Avena coleoptile have been discussed. The 

 primary positive curvature is explainable as a disturbance of 

 correlated activity, which is brought about by checking the 

 transport of growth substance on one side. The traumatic 



