386 EFFECT OF MOLAR AGENTS UPON GROWTH [Cn. XIV 



the irritated protoplasm behind. This can be demonstrated if 

 (following PFEFFER, '93) the freshly irritated radicle is con- 

 rined in a plaster cast (Fig. 105). The growth of the tip is 

 slackened so that it does not leave the irritated tissue far 

 behind ; at the same time, the zone of stretching encroaches 

 upon the root tip, so that, if the radicle is released after several 

 days, the curvature, although taking place, occurs abnormally 

 near the tip. If the response were transmitted a fixed dis- 

 tance backward from the tip, we should expect that in the 

 confined plant either the turning would take place at the normal 

 position or would altogether fail to occur. Since neither of 

 these effects follows, the conclusion is strengthened that the 

 identical plasm which is irritated responds, producing the 

 traumatic curvature. 



Temperature, controlling as it does the rate of growth, con- 

 trols the length of the latent period. According to the obser- 

 vations of WIESNER, this is at 



Thus, 25 C. seems to be about the optimum temperature for 

 the traumatropic response ; and this is a temperature 10 to 

 12 below the optimum for protoplasmic movements (Chap. 

 VIII, 2). 



The true explanation of traumatropism remains, after pro- 

 longed discussion, uncertain. DARWIN regarded it as a 

 response to stimulation, but DETLEFSEN and WIESNER sought 

 to show that it is the immediate mechanical result of the 

 injury. Thus, DETLEFSEN assumed tensions in the root cap, 

 which caused the root when cut on one side to shorten on the 

 other ; but this conclusion was disproved by SPAULDING, who 

 found that the usual results do not follow when the root cap is 

 injured without injuring the root tip. WEISNER'S explanation 

 was based on the observation that decapitated roots grow more 



