Power to Adjust Parts to Surroundings 



243 



a grip which holds the vine firmly and permits a still farther 

 ascent. Now it is easy to prove by experiment that it is really 

 the contact with the support which constitutes the stimulus 

 producing the bending, for anyone, by rubbing one side of a 

 tendril with a pencil, can call out 

 the turning, and watch all of the 

 steps in its progress. Even a mo- 

 mentary contact is followed by 

 a turning within a few minutes, 

 though the tendril will straighten 

 again in case the contact is not 

 maintained; but if the contact be 

 continuous the tendril will wind 

 completely around the pencil. The 

 advantage, the motor-mechanism 

 (which is growth), and the mode of 

 reception of the stimulus, in this 

 form of thigmotropism, are all suf- 

 ficiently obvious. 



Most persons who have knowl- 

 edge of plants would doubtless put 

 forward a different case as a type 

 of thigmotropism, viz., the well- 

 knowTi Sensitive Plant, which droops 

 promptly and completely at a touch 

 (figure 86). But I think this move- fig. 85. 

 ment is only accidentally thigmo- 

 tropic. Nobody has yet found, 

 even after study of the plant in 

 its native home, any satisfactory reason why the plant should 

 droop for a touch, while, on the other hand, it responds in 

 the same manner to other kinds of stimuli, — a scorch of flame, 

 a strongly-focussed light, a trifle of acid — -to which there can 

 be no question of adjustment. The leaves have, however, 



nr^ 



-Four successive stuges in 

 the thigmotropic curling of a tendril 

 around a support. (Copied, sim- 

 plified, from a wall-chart by Lau- 

 rent and Errera.) 



