TRAUMATROPISM 



385 



ing appeared as a small, dark spot on one side of the radicle^ 

 The seedlings were then suspended in a moist chamber, over 

 water, and at a temperature of 14.4 C. After 24 hours almost 

 all of the radicles showed a marked curvature from the 

 wounded side. Other means can be used to produce this 

 result : a thin slice cut off obliquely from the tip of the radicle, 

 a drop of shellac, of copper sulphate, of potassium hydrate, or 

 steam at about 95 C. (SPAULDING) in general, any agent 

 which irritates strongly without killing. 



Concerning the place on the root where the injury must be 

 made in order that a response should appear, SPAULDING finds 

 that if the branding is made further from 

 the apex than 1.5 mm. no traumatic curva- 

 ture will occur ; or if an oblique cut at the 

 apex should not involve the growing root tip, 

 as at 6, Fig. 104 S it is without effect. So 

 it seems that the pro- 

 liferating root tip is 

 the sensitive part. 



The point of maxi- 

 mum curvature is, how- 

 ever, not at the root tip, 

 but lies in the region 

 of most rapid growth. 

 The length of time 

 elapsing between injury and response 

 varies with the species and with the tem- 

 perature. Thus, at a temperature of 18 C. 

 the curvature begins in from 45 to 55 

 minutes, and reaches a maximum within 

 24 hours (WIESNER). 



The long, latent period and the slowness 

 of complete response give an insight into 

 the reason for this separation of the per- 

 ceiving and responding zones. During the 

 latent period the irritated tissue is be- 

 coming stretching tissue through the im- 

 bibition of water, while the root tip is 

 becoming generated in advance, leaving 



FIG. 104. Diagram 

 showing the rela- 

 tions of the root 

 tip, r.t, and the 

 root cap, r.c. ; a, b, 

 line of a cut which 

 involves only the 

 root cap. 



FIG. 105. Median longi- 

 tudinal section through 

 a gypsum cast, b, sur- 

 rounding and repress- 

 ing a root of Vicia faba. 

 Natural size. (From 

 PFEFFEB, '93.) 



2c 



