644 



STIMULUS-TRANSMITTING SYSTEM 



cell-sap ejected from the stimulus-transmitting elements. The large 

 size of the drop shows that this cell-sap can easily pass through the 

 pores in the wide pit-membranes of the transverse septa. The rows 

 of tubular cells therefore represent a continuous system, through which 

 variations of hydrostatic pressure can be transmitted over considerable 

 distances. 



In the author's opinion, transmission of stimuli in this system of 

 tubes takes place somewhat as follows : When the pulvinus of a 

 pinnule moves upwards in response to a shock, pressure is exerted 



Pig. 267. 



T.S. through the main petiole of Mimosa pudica ; b, stereome; h, hadrome. 

 The small circles indicate the position of the stimulus-transmitting elements. 



upon the highly turgescent transmitting cells, partly owing to changes 

 in the form and volume of the relaxed half of the pulvinus, and partly 

 owing to the mechanical effects of the curvature ; the local rise of 

 pressure produced in this way will be propagated along the system 

 of tubes, owing to the elastic tension of their walls, in the form of a 

 pressure-wave, like the pulsations which travel through the arterial 

 system of an animal. This wave of compression or positive tension 

 acts like a shock-stimulus upon the nearest pulvinus, and so leads to 

 an indirectly induced responsive movement. The initial rise of pressure 

 which starts the wave is not large ; hence the stimulus, while extending 

 from one pair of pinnules to the rest, does not convey a perceptible 

 shock to the comparatively insensitive pulvinus of the sub-petiole, and 

 never penetrates as far as the main pulvinus. 



Traumatic stimulation such as may be caused by the severance 

 of a pinnule instantly destroys the turgor of the injured transmitting 

 cells ; as a consequence a large local fall of turgor results, which is pro- 

 pagated through the tubular system as a wave of relaxation or negative 



