TRANSMISSION IN MIMOSA ^ * 645 



*\ 

 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 accord- 

 ingly be transmitted over a relatively large distance ; it will not only 

 reach the main pulviuus, but may also travel through the stem to other 

 leaves. 



The rise of pressure produced by shock causes the cell-sap of the 

 transmitting elements to travel in the same direction as the wave of 

 compression. Traumatic stimulation, on the other hand, leads to a fall 

 of pressure, and the cell-sap accordingly moves towards the region of 

 reduced pressure, that is, in the direction opposite to that of the wave 

 of relaxation. 



It is impossible here to consider in detail, how the shock of the 

 waves of compression and relaxation is transferred at various points to 

 the sensitive parenchyma in the pulvini. It may, however, be noted 

 that variations of pressure must result in alterations in the volume and 

 form of the transmitting tissues, and that these latter changes in their 

 turn affect the two or three layers of collenchyma which surround the 

 transmitting tissue in the pulvini, and so indirectly bring about 

 sudden deformation of the neighbouring sensitive parenchymatous 

 elements. When the initial stimulus is feeble, the ultimate deformation 

 will presumably be restricted to the immediate neighbourhood of pits, 

 since the exceedingly thin and elastic limiting membranes will respond 

 to the slightest variation of pressure by bulging towards the side of 

 reduced pressure, the sudden protrusion constituting a fresh mechanical 

 stimulus. 



The velocity of transmission in Mimosa is naturally affected by a 

 variety of circumstances, but depends, above all, upon the amount of 

 friction between the liquid contents and the walls of the tubes, upon the 

 resistance that has to be overcome during the filtration of the liquid 

 through the 'transverse septa and upon the turgor-pressure in the 

 transmitting system. According to Dutrochet and Bert, the rate of 

 transmission through leaf and stem of the adult plant varies between 

 2 and 15 mm. per sec. Experiments performed by the author have 

 shown that the stem usually transmits stimuli rather more slowly than 

 the leaf, though the two organs do not always exhibit a marked 

 difference in this respect. In a typical case, the velocity of trans- 

 mission was found to be 8*5 mm. per sec. in the petiole, and 6'5 mm. 

 per sec. in the stem. K. Linsbauer, 34 ' working with a kymograph, has 

 estimated the average velocity of transmission of a traumatic stimulus, 

 produced by making an incision in the primary petiole, at 31 "2 mm. 

 per sec. ; when the main petiole is severed altogether, the stimulus 



