GROWTH SUBSTANCES FOR TRAUMATIC CURVATURES 219 



curvatures caused by transverse incisions in roots are negative. 

 These curvatures may result from the unilateral inhibition of 

 growth-substance transport, which leads to a negative reaction, 

 since it checks growth in roots. In his discussion of wounding 

 and growth, Cholodny (1931c, /) showed how wounding can 

 modify phototropic and geotropic responses through effects upon 

 the distribution and activity of growth substance. 



Positive and negative traumatic curvatures resulting from 

 wounding a root have been explained in terms of the gradient of 

 growth substance between opposite sides of this organ (Fig. 61) 

 (Keeble and Nelson, 1935). It has been shown that the sap 

 which exudes following the removal of a root tip tends to inhibit 

 growth. Keeble and Nelson found that growth in length is 

 promoted when this exudate is washed away. In the light of 

 other work, it is probable that this root-inhibiting substance in 

 the sap is identical with the growth-promoting substance in the 

 tip of the Avena coleoptile (Cholodny, 1933?)). 



The positive traumatic curvatures which appear as the result 

 of longitudinal incisions can be explained neither by a change in 

 the production of growth substance nor its distribution. A 

 median longitudinal incision in stem organs causes a strong 

 decrease in -the rate of growth, but how this occurs has not been 

 determined. Many investigators have conjectured that wound 

 substances are produced as a result of stimulation and that these 

 substances have an effect upon the rate of growth. The fact that 

 Weimann (1929) observed secondary negative curvatures in 

 Avena coleoptiles with longitudinal incisions led him to conclude 

 that growth-retarding wound substances do not exist. Results 

 found in Avena obviously cannot be applied to all plants. Up to 

 the present, it has not been proved that w^ound substances exist. 

 The retardation of growth by longitudinal incisions may be due 

 to the destruction, as a result of the wounding, of the growth sub- 

 stances normally present. In the instances where growth is 

 promoted by wounding, the effects might be explained on the 

 basis of the freeing of growth substances already present in an 

 inactive form. 



SUMMARY 



The traumatic and thigmatic curvatures which occur following 

 wounding and mechanical stimulation are brought about by 



