178 PHYTOHORMONES 



greater than 15 minutes and therefore curvature in these 

 zones is dependent on transport of the redistributed auxin 

 from above. From the fifth to the tenth zone the initial curva- 

 ture therefore moves downwards at 12 mm. per hour, the 

 rate of auxin transport. After only 30 minutes of curving 

 the curvature begins to decrease very rapidly in the upper- 

 most zones, so that they are straight within 2 hours 

 ("autotropism"). This is due to the fact that the gravita- 

 tional auxin redistribution does not persist and the auto- 

 tropism is ascribed, as we have seen in V D, to the temporary 

 exhaustion of the food factor at the point where growth 

 has been rapid. The zone of maximum curvature is thus 

 quite narrow and moves rapidly down the plant. The final 

 state of the plant thus shows a very localized curvature in 

 the base, the upper part being almost entirely straight. In 

 decapitated plants this autotropic straightening is very much 

 less marked, and is delayed until regeneration begins, 

 150 minutes after decapitation. This affords clear proof 

 that autotropism is dependent upon a supply of auxin. 



Thus it may be seen that all the peculiarities of the 

 curvatures in Avena may be satisfactorily explained in terms 

 of what we know about auxin and its distribution. 



IV. OTHER TROPISTIC RESPONSES 

 J. Traumatotropism 



The curvature of plants towards or away from a wound — 

 traumatotropism — was first ascribed by Stark (1921) to the 

 influence of special traumatotropic hormones which were 

 produced in the damaged tissue. Beyer, however (1925), 

 made it clear that all organs which curve towards a wound 

 are those whose growth is retarded by removal of the tip. 

 Correspondingly, if the tip be removed, such organs lose 

 their ability to curve towards the wound. Hence he deduced, 

 in agreement with Paal, that the curvatures are due to 

 interference with the normally present growth-promoting 

 substance, rather than to the production of a growth- 

 inhibiting substance. He also found that even in decapitated 



