i86 



TROPIC MOVEMENTS 



zone of the root, away from the injured side or injurious agency. This 

 negatively traumatropic curvature is about as rapidly produced as a geo- 

 tropic one, and is shown as the result of comparatively trifling injuries, 

 while severe injury may cause the growing apex to perform a complete 

 coil (Fig. 40). 



Since we are here dealing with a tropic stimulus which is only per- 

 ceived at the root-apex, no reaction is shown when the tip of the root is 

 removed by a transverse cut, or when it is entirely killed by the injury. 

 The removal of an oblique slice from one side of the apex produces, 



however, a corresponding traumatropic curvature, 

 while, according to Spalding *, the incision must 

 pass through the meristem below the root-cap in 

 order to be effective. MacDougal 2 regards the 

 periblem as being the irritable and responsive 

 region, but without bringing forward any conclu- 

 sive proof. 



Naturally gentle rubbing has no effect, but 

 the energy of growth of the roots in soil is such 

 that when in contact with stones sufficient pressure 

 and friction might be exerted to produce a trau- 

 matropic curvature away from the hindrance. 

 The root-apex, except in the case of the roots of 

 Vanilla planifolia> does not appear to possess 

 any thigmotropic irritability, for the curvatures 

 observed by Darwin away from the side to which 

 pieces of paper, glass, or mica had been attached were apparently trau- 

 matropic in origin, and were due to the means of attachment employed. 

 Indeed the local application of alcohol or of a solution of shellac readily 

 produces a traumatropic curvature away from the point of application. 



The traumatropic stimulation is not the result of the generally occurring 

 transitory reaction, but is due to the cessation of the correlative influences 

 which normally radiate from the injured zone. This produces asymmetric 

 disturbances which induce an acceleration of growth on the side opposed to 

 the injury. Spalding found, in fact, that if the root was embedded in 

 plaster-of-paris immediately after being injured, a traumatropic curvature 

 was shown as soon as it was set free eight days afterwards 3 . During this 



FIG. 40. Seedlings of Vicia 

 Faba. The radicles have curved, 

 as shown, sixteen hours after the 

 application of silver nitrate at c. 

 In B the injury and resulting cur- 

 vature are greater than in A. 



1 Spalding, Annals of Botany, 1894, Vol. vm, p. 432. 



2 MacDougal, Botanical Gazette, 1897, Vol. xxm, p. 307. 



3 Cf. Spalding, 1. c., p. 426 ; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 373. The curvature 

 observed by Nemec (Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 87) as the result of attaching particles 

 of plaster-of-paris to one side of a root was possibly traumatropic in character. It is also possible, 

 however, that a local retardation of growth might act as a tropic stimulus to the primary meristem, 

 or that the curvature might be more or less mechanically induced. 



