7 o MOVEMENTS OF CURVATURE 



Mimosa were irresponsive to blows, whereas others found that they remained 

 sensitive 1 . 



A single stimulation of the pulvinus of Mimosa causes its irritability 

 to be transitorily suppressed during the return movement, and it is only 

 gradually restored after the return-movement has been completed. Hence 

 the same stimulus induces at first a feeble, and later a pronounced response 2 , 

 and if gentle blows are struck on the primary pulvinus at intervals of three 

 to five minutes, the irritability is sufficiently restored during the intervals 

 to enable each stimulus to produce a moderate response. During the 

 period of insensitivity following mechanical stimulation, the pulvinus of 

 Mimosa remains irritable to photonastic, heliotropic, and other stimuli, so 

 that the absence of a response to mechanical stimuli is due to the temporary 

 inhibition of the power of perceiving such stimuli, and not to the motor- 

 mechanism being temporarily ineffective. Nothing is, however, known as to 

 the way in which this special sensitivity is suppressed and restored. 



It is hardly to be expected that all sensitive plants should react 

 in this respect in a precisely similar fashion to Mimosa, but in general 

 any sudden explosive stimulatory reaction appears often to be followed 

 by a more or less transitory diminution of excitability. This applies to 

 the stamens of Cynareae, although here the excitability soon returns, and 

 is partly restored before the stamens have re-expanded 3 . A complete 

 suppression of excitability does not always follow as the result of stimu- 

 lation, for Pfeffer 4 has shown that the leaves of Oxalis remain excitable 

 during the return movement. In the same way the voluntary muscles 

 of animals can be kept permanently contracted in a condition of tetanus 

 by rapidly repeated stimuli. 



On the other hand, Cnscuta affords an instance in which stimulation- 

 induces a periodic inhibition of the contact-irritability. The tentacles of 

 Drosera, however, remain permanently irritable, although the sensitivity 

 is so far decreased by stimulation, that a weak continuous stimulus is 

 unable to produce a permanent curvature, the tentacles gradually straighten- 

 ing again 5 . It is highly probable that further specific peculiarities will 

 be discovered, and investigations in this direction are likely to throw light 

 upon the phenomena of irritability in general. 



Both the stimulatory and the return movements begin slowly, increase 

 to a maximum and then gradually cease, while not only in the case of 

 Mimosa, but also where the movement is slow, the response to stimulation 

 takes place more rapidly than the return movement. The occurrence of 



1 The literature is given by Pfeffer, 1. c., 1873, p. 56. 



3 Pfeffer, I.e., 1873, p. 60. 



3 Cohn, Abhdlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft I, p. 16. 



* Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1885, Bd. I, p. 521. 



5 Cf. Pfeffer, 1. c., 1885, p. 514. 



