IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 63 



(Fig. 23), of Martynia and of Bignonia show no external structural sign 

 of their power of rapid movement on stimulation. Furthermore the 

 sudden closure of the leaf of Dionaea muscipula (Fig. 24) and of Aldro- 

 vanda is produced by the influence of contact acting on the midrib and 

 lamina. 



In all these cases the responding organ is also the percipient one, 

 but in Masdevallia muscosa, according to Oliver, the movement of the 

 labellum is produced by touching the neighbouring part of the flower 

 and not by touching the motile zone T . 



The above-named plants respond to any sufficiently intense mechanical 

 shock or disturbance, whether produced by wind, rain, contact with solid 

 bodies, or vibrations propagated through the soil. They may hence be 



FIG. 22. A flower of Herberts vttlgaris after the re- 

 moval of the anterior petals and stamens (magnified'. 

 The stamen (a) is unstimulated, but contact has caused the 

 stamen (6) to curve over and apply itself to the stigma (,). 



FlG. 23. Longitudinal sections of the flower of 

 Mimtilns luteus. In A the stigmas () are unstimu- 

 lated, in Ba. touch has caused them to close together. 



said to possess a seismonic irritability as distinguished from the sense of 

 touch (contact or thigmotropic irritability) shown by tendrils, by certain 

 algae and fungi, as well as by the tentacles of Drosera. In these cases 

 a response is produced only by contact with a solid body, whereas the 

 strongest bending or shaking caused by wind, water, or the impact of 

 a thread of mercury, as well as rubbing with a wet rod covered with 

 10 or 15 per cent, gelatine, fail to awaken any irritable response. At the 

 same time sensitive tendrils respond to the lightest contact with a solid 

 body, such as fails to awaken any response in the highly irritable leaflets 

 of Mimosa. The tentacles of Drosera are almost as sensitive as tendrils, 

 the head of the tentacle perceiving the stimulus to which the stalk responds 

 by bending 2 . 



1 Oliver, Annals of Botany, 1888, Vol. I, p. 244. 



2 For details see Pfeffer, Unters. a. d. Lot. Inst. zu Tubingen, 1885, Bd. i, p. 483. 



