214 GROWTH HORMONES IN PLANTS 



Only the most important types of traumatic response will be 

 mentioned, following the general presentation of Stark (1917). 



Amputations. — If one cotyledon is removed from seedlings of 

 HeUanthus, Sinapis, and many other plants, a positive traumatic 

 curvature appears in the hypocotyl. Positive curvatures occur 

 also, following the removal of part of the leaf lamina (e.g., in 

 Ficaria). After the unilateral removal of leaflets from a com- 

 pound leaf [e.g., in members of the rose family), curvatures 

 commonly appear in the petiole. When entire leaves are removed 

 {e.g., in Sonchus palustris), the axis of the shoot frequently shows 

 a positive, traumatic curvature. 



Fig. 64. — Course of a wound curvature produced by a transverse incision near 

 the base of an Avena coleoptile. An initial positive curvature is followed by a 

 negative and a second positive curvature. Ordinate: d value; abscissa: time in 

 hours. {After Purdy, 1921.) 



Wounding by Incisions. — Transverse incisions bring about 

 positive traumatic bending in coleoptiles. For example, when 

 an incision is made near the base of an Avena coleoptile, a curva- 

 ture appears above the wound. Positive wound curvatures can 

 be observed also in hypocotyls, (e.g., of Phaseolus) . Purdy (1921) 

 studied the course of curvature in the Avena coleoptile and 

 reported that the initial positive curvature is followed by a 

 negative and a second positive curvature (Fig. 64). 



Longitudinal grooves bring about small, positive, traumatic 

 curvatures in Helianthus, Lupinus, Phaseolus, and various 

 grasses. This type of curvature was observed by Boysen Jensen 

 and Nielsen (1925). Later, Weimann (1929) found that a 

 secondary negative curvature appears in both normal and 

 decapitated Avena coleoptiles, whether rotated on a cHnostat 

 or not. 



