68 MOVEMENTS OF CURVATURE 



stimulation growth curvatures. The closure of the leaf of Dionaea, however, 

 due to seismonic stimulation is partly produced by growth l . On the other 

 hand, the disturbances of growth in growing shoots produced by shaking 

 are to be regarded as the result of seismonic stimulation. Although at 

 present only nutation curvatures are known to result from contact-stimula- 

 tion, it is hardly to be expected that the potential powers of the plant 

 should find expression in this direction alone, and in fact we have in the 

 secondary thickening of the hooks and tendrils of many tropical climbers 

 induced by contact a special response which may or may not be accom- 

 panied by curvature. Furthermore, the movements produced in the cilia 

 of certain organisms by contact-stimuli are not due to growth, but are the 

 result of contractility, just as the movements of an animal produced by 

 a tickling sensation are due to muscular contraction. 



In regard to sensitivity, the duration of the latent period, and the 

 rapidity of the reaction, no definite line of demarcation' can be drawn 

 . between seismon c and contact-stimulatioa It is true that the latter never 

 produces so rapid a reaction as occurs in the leaf of Mimosa pudica^ in 

 which, under favourable conditions the latent period may be less than 

 a second, while the sinking of the primary petiole and the folding of 

 a pair of leaflets may be performed in two to five seconds. The stamens 

 of Centaurea jacea and the leaves of Dionaea muscipula move with about 

 the same rapidity. Burdon-Sanderson 2 found that at 20 C., when the 

 leaves of the latter plant are moderately responsive, the latent period after 

 mechanical stimulation was about one second, and the closure of the leaf- 

 lobes required five to six seconds. Sensitive tendrils may, however, begin 

 to curve five to twenty seconds after contact-stimulation, so that the 

 reaction is more rapid than the movements produced by seismonic stimu- 

 lation in less sensitive plants such as Robinia> Oxalis> and Acacia lophantha. 

 Since the sensitivity and power of reaction are largely dependent upon the 

 stage of development and upon the external conditions, their precise 

 determination is of subordinate interest and importance. It is however 

 worthy of note, that under special conditions Mimosa pudica may show 

 only a slow and feeble power of reaction, while when the plant has been 

 kept for some time at a low temperature, such as 5 to 10 C., it temporarily 

 or permanently loses the power of response to seismonic stimuli. 



In the case of the leaves of Mimosa pzidica and the stamens of Cynareae 

 and Berberis every successful stimulation excites the full amplitude of 

 movement. This is however not always the case, for even the strongest 

 mechanical stimulation only produces a partial folding or drooping of the 



1 How far the curvature of the pulvini of Mimosa pudica is a matter of growth is uncertain^ 

 but the latter does appear to take part in the performance of many sleep movements. 



a Burdon-Sanderson, Phil. Trans., 1882, Pt. I (p. 48 of reprint); Biol. Centralbl., 1882, Bd. II, 

 p. 497. 



