GROWTH SUBSTANCES FOR GEOTROPISM 



205 



tion of growth substance in the root tips (Fig. 62). It may be 

 considered, therefore, that displacement of growth substance is 

 responsible for the geotropic curvature in roots. 



Similarity of Growth Substance in the Root and Coleoptile. — 



The experimental evidence shows that the growth substance of 

 the coleoptile has a retarding effect upon the growth of the root ; 



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A 



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 B 



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Fig. 62.— Effect of temperature on the geotropism of seedling roots of Vicia 

 Faba. The upper member of each pair of diagrams shows the set-up, while the 

 lower one shows the response after 24 hours. The plants were grown at 20° C, 

 but those without cross-hatching were kept at 5° for 24 hours preceding the 

 experiment. The tips were excised and replaced on the stumps in different 

 combinations as follows: A, 20° tip on 5° stump; B, 5° tip on 5° stump; C, 5° tip 

 on 20° stump; D, 20° tip on 20° stump. The greatest curvatures occurred in 

 those roots bearing tips which had grown at 20° C, A and D, due probably to 

 the greater growth-hormone content of the tips. {After Hawker, 1933.) 



therefore, placement of a coleoptile tip upon a horizontal, decapi- 

 tated root will produce in it a positively geotropic curvature, since 

 the growth substance accumulates on the under side of the tip. 

 These observations alone are not sufficient for a complete explana- 

 tion of positively geotropic curvature in the root. It is necessary 

 to show, in addition, that growth substances that have a pro- 

 moting effect on the growth of the Avena coleoptile are actually 

 formed by the root tip. It must be shown, also, that the increased 

 growth which takes place following removal of the root tip is not 

 produced by wound substances of an entirely different nature 

 (Biinning, 1928). 



