PROPAGATION OF MECHANICAL AND CHEMICAL STIMULI 93 



It is, however, probable that the conduction of stimuli in the stigmas 

 of Martynia and Mimulus, in the stamens of Sparmannia, and possibly in 

 the labellum of Masdevallia takes place in some other way. Furthermore, 

 the transference of stimuli in organs sensitive to contact-stimuli cannot 

 possibly be due to movements of water, since these organs do not respond 

 to repeated bending with its attendant movements of water. In such cases 

 we may assume that we are dealing either with a vital transmission of 

 stimuli which can only take place through intervening protoplasm, or with 

 a transference of stimulating materials, or of an electrical excitation from 

 cell to cell, for which the existence of living interprotoplasmic connexions 

 is not essential. It is in fact not inconceivable that dissimilar modes of 

 conduction may be excited at the same time. A simple instance of this is 

 afforded when the disturbance due to the response of a single stimulated 

 cell serves for the propagation of the stimulus through the whole of the 

 irritable organs, but not through the intervening non-motile tissue to 

 neighbouring motile organs. In addition, a mechanical disturbance can be 

 transferred so as to excite the rapid closure of the leaf of Dionaca, but not 

 the slow movements resulting from chemical stimulation. 



According to Oliver 1 , the transference of stimuli in the labellum of 

 Masdevallia is restricted to the vascular bundles, although it does not 

 appear to be due to a movement of water as in Mimosa. Even here, 

 however, a slow vital transmission of stimuli may also be possible, while 

 a transference of stimuli across active parenchyma tissue occurs both in the 

 pulvini of Mimosa and in irritable stamens. In addition, stimuli are 

 transferred mainly or entirely through parenchyma cells in the case of the 

 stigmas of Mimtdus and Martynia, according to Oliver 2 , when these respond 

 to seismonic stimulation, for the stimulation of one stigma may excite the 

 other after the intervening vascular bundles have been severed. Both 

 mechanical and chemical stimuli appear to be conducted through the 

 parenchyma cells of the tentacles of Drosera, but the rate of propagation 

 appears to be more rapid along the vascular bundles. 



The transmission of stimuli is in most plants extremely slow, but in 

 Mimosa pudica a rate of propagation of 15 mm. per second has been 

 observed 3 , and in the pulvinus itself, as well as in the stamens of Centaurea, 

 stimuli may travel still more rapidly. On the other hand, the impulse 

 radiating from the chemically or mechanically excited head of a tentacle 

 of Drosera does not appear to travel at a much greater rate than 10 mm. 



1 Oliver, Annals of Botany, 1888, Vol. I, p. 249. 



2 Oliver, Ber. d. hot. Ges., 1887, p. 168. 



3 Dutrochet, Recherch. anat. et physiol., 1824, p. 80; Bert, Mem. de 1'Acad. de Bordeaux, 1870, 

 T. via, p. 47 ; Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 325 ; G. Haberlandt, Das reizleitende 

 Gewebesystem der Sinnpflanze, 1893, p. 69. On the slow rate of propagation of stimuli in 

 Biophytum sensitivum, cf. Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl., p. 35. 



