IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 69 



leaves of Robinia or of Oxalis. A strong blow also acts as a submaximal 

 excitation upon the leaves of Mimosa pudica when their irritability has been 

 diminished by keeping the plant at a low temperature *. 



It is obviously advantageous that the response should be more marked 

 when the stimulus is more intense or prolonged r or is increased by repetition 

 and summation. This applies more especially to organs endowed with 

 contact-irritability, for in this way they are enabled to a certain extent to 

 so adapt their response as to perform their special function in the best 

 possible manner. A few touches usually suffice to produce a distinct 

 reaction, although in very sensitive tendrils a single contact, if sufficiently 

 intense, will produce a response, while three or four touches are required to 

 produce a curvature in the highly-sensitive tendrils of Drosera*. Even 

 a single contact, however, may represent a series of local variations of 

 pressure, and it cannot be denied that a sudden maximal explosive 

 movement is better attained by the release of strains previously prepared, 

 than by changes in the rate of growth. 



As in other cases the result of a transitory mechanical stimulation may 

 gradually disappear, whereas when the stimulus is continuous the new 

 position of equilibrium assumed will depend upon the intensity of the 

 stimulus, upon the awakened counter-actions, and upon the accommodation 

 of the plant to the stimulus, which is mainly due to its depressed excitability. 

 So long as the plant maintains the position induced by stimulation and 

 reacts to a rise in the intensity of the same stimulus, no accommodation 

 other than that involved in a certain depression of the excitability can take 

 place. This latter appears to be of general occurrence ; and in many cases, 

 as, for example, in the leaves of Mimosa pudica> it goes so far that the 

 stimulated organ in spite of the continued application of mechanical or of 

 weak induction shocks returns to its original position and is no longer 

 responsive to mechanical excitation 3 . If the return to the original position 

 has taken place during the continued application of gentle shaking, the 

 sensibility is only weakened and an increase in the intensity of the 

 mechanical shocks brings about the usual movement. It is owing to these 

 facts that some authors have found that continually-shaken -plants of 



1 Pfeffer, Physiol. Unters., 1873, p. 69 ; Unters. a. d. bot. lust, zu Tubingen, 1885, Bd. I, p. 520 ; 

 Macfarlane, Biological kctures, 1894, P- I 9- According to G. Haberlandt (Ann. du Jard. bot. de 

 Buitenzorg, 1898, Suppl. n, p. 35) gentle rubbing excites a sub-maximal movement in the leaves of 

 Biophytum sensitivum. In such circumstances the movement may be produced by repeated stimu- 

 lation as in the case of tendrils, although single stimuli may be ineffective. According to Burdon- 

 Sanderson (Proceedings of the Royal Society, 1877, Vol. xxv, p. 411) the sudden maximal move- 

 ment of the leaves of Dionaea imiscipula may be excited by the summation of the action of repeated 

 gentle blows. Cf. also Darwin, Insectivorous Plants. Macfarlane's statement (1. c., p. 187) that at 

 least two blows are required to produce a response in Dionaea miiscipula appears only to apply 

 under special conditions. 



3 Darwin, Insectivorous Plants, 1875, P- J 9- 



8 Pfeffer, Physiol. Unters., 1873, p. 56 ; Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. I, p. 521. 



