IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 65 



is awakened in the epidermis of man and of tendrils by weak induction 

 shocks, and furthermore, rubbing against a rough body acts in both cases 

 as a stronger excitation than rubbing with similar pressure against a smooth 

 one. In general the intensity of the excitation depends upon the amount 

 of surface in contact, upon the magnitude of the local variations of pressure, 

 and upon the rapidity with which they alter. The determination of the 

 numerical relations between these factors and the strength of the excitation 

 affords, however, no explanation of the actual nature of the sensation of 

 contact. 



In any case the deformations produced by varying local pressure in 

 the outer cell-walls of the epidermis create the conditions for an excitation 

 of the irritable protoplasm, which does not come into contact with the 

 object exercising pressure any more than in the case of the touch-corpuscles 

 in the skin of animals. The structure of the cell and cell-wall may therefore 

 aid considerably in the perception of the stimulus, although an excitation 

 is only possible when the protoplasm is endowed with this special form 

 of irritability. The pits which occur in the outer walls of the epidermis 

 in the tendrils of Cucurbitaceae and a few other plants undoubtedly act 

 in this way. Since, however, similar pits are present in the non-sensitive 

 portion of the tendril of Bryonia, it is evident that their presence does 

 not confer this special form of irritability upon the protoplasm of all cells 

 possessing them. Furthermore no pits are present in the epidermal walls 

 of the very sensitive tendrils of Passiflora and Cobaea \ and in some motile 

 organisms only a portion of the cilia are sensitive to contact, although 

 here the sensitive protoplasmic organs come into direct contact with foreign 

 bodies. 



It is, however, uncertain whether differences of pressure in the proto- 

 plasm act as the exciting stimuli, and also whether the entire protoplasm 

 or only the peripheral membrane, or only portions of the latter are able 

 to perceive contact stimuli. Even in the latter case, however, it is hardly 

 to be expected that so high a differentiation should be reached as in 

 the Pacinian or touch- corpuscles of vertebrate animals. A knowledge 

 of the nature and position .of the percipient organs does not, however, 

 reveal the mode of perception of the stimulus. 



An organ having seismonic irritability responds to every variation 

 of pressure if sufficiently intense, quite independently of its origin. Certain 

 highly sensitive plants even respond to sudden variations in the atmospheric 

 pressure, or to sudden changes of temperature, or to rapid alterati'ons 

 of transpiration and to the resulting water-currents 2 . The stimulus may 



1 Pfeffer, 1. c., p. 524. Haberlandt, Physiol. Anat., 2. Aufl., 1896, p. 478; Haberlandt, Sinnes- 

 organe im Pflanzenreich, 1901, p. 126 ; Strasburger, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 515. 



3 Long known in the case of Mimosa pudica. Munk (Die elektrischen- und Bewegungserschein- 

 ungen am Blatte von Dionaea, 1876, p. 105) observed that a sudden increase of transpiration acted 



PFEFFER. lit T? 



