CHAPTER X 



THE SIGNIFICANCE OF GROWTH SUBSTANCES FOR 

 TRAUMATIC AND THIGMATIC CURVATURES 



The curvatures that plants exhibit in response to wounds and 

 other mechanical irritations are well-known, but the causal 

 relationship between the initial stimulus and the final response 

 is not easy to explain. Phototropic and geotropic phenomena 

 bear certain distinct similarities to the traumatic and thigmatic 

 curvatures, for all are the result of differential growth. It seems 

 appropriate, therefore, to examine these womid and touch 

 responses in the light of the growth-substance explanation. 



General Survey of the Phenomena. — Traumatotropism of roots 

 was observed first by Darwin (1881) and investigated later by 

 many others. It was found that if a root tip was treated on one 

 side with silver nitrate or wounded otherwise, the resulting 

 traumatic curvature was in a direction away from the wound. 

 Following such unilateral wounding of the tip, transmission of the 

 stimulus takes place, and bending occurs in the growing zone at 

 some httle distance away. The growing zone itself is also sensi- 

 tive to womiding, though to a lesser degree than the tip. 



Haberlandt (1921. 1922) postulated the presence of wound 

 hormones to account for the stimulation of cell division at or near 

 the regions of injury in plants (see also Wehnelt, 1927). The 

 observed facts which prompted this hypothesis probably belong 

 to a different categoiy from that which includes traumatic 

 curvature phenomena. Traumatic sensitivity, as we know from 

 the experiments of Stark (1917, 1919), is widely distributed in 

 aerial organs, in the stems of seedlings, and in older stems and 

 leaves. 



The thigmatic stimulus first studied in tendrils is rather widely 

 distributed in etiolated seedhngs, older stems, influorescences, 

 petioles, etc. (Stark, 1916). Curvatures can be obtained in these 

 plant parts by stroking one surface with a cork rod, whereupon 

 the stimulated side becomes concave in due course of time, and a 

 positive curvature takes place. 



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