SENSITIVITY AND MOVEMENT 247 



duced 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 subpetiole, 

 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 propagated 

 through the tubular system as a wave of relaxation or 

 negative tension. The initial change of pressure caused 

 by such a mechanical injury is comparatively large ; 

 hence a much more violent disturbance is produced in 

 the adjacent pulvini than can possibly arise when the 

 primary stimulus is due to shock. A traumatic stimulus 

 can accordingly be transmitted over a relatively large 

 distance ; it will not only reach the main pulvinus, but 

 may also travel through the stem to other leaves." 



Haberlandt claims that the mode of transmission is 

 hydrodynamic, and states that when the transmitting 

 tubes are cut a drop of transparent liquid escapes which 

 is not derived from the xylem elements, and which is 

 chemically cell sap, not water. In a paper published 

 in 1916 Ricca, an Italian botanist, criticises the whole 

 of Haberlandt's work and contradicts his statement that 

 the fluid does not escape from the xylem. Ricca obtained 

 responses quite as rapid with decorticated petioles as 

 with corticated ones, and holds that the excitation is 

 transmitted by the xylem and not by Haberlandt's 

 leptome tubes. By means of ingeniously devised ex- 



