RATE AND MODE OE TRANSMISSION 633 



for the conduction of stimuli, it is necessary to take into account the 

 velocity of transmission. The rate of propagation of stimuli is in 

 general much lower among plants than it is in the case of animals. 

 According to Czapek, Kothert and others, heliotropic and geotropic 

 stimuli require five minutes in order to traverse a distance of one 

 or two millimetres. 3 "'' Traumatic stimuli travel much more rapidly ; 

 thus, Kretschmar 330 states that the traumatic stimulus produced by 

 cutting across a leaf or stem in certain water-plants ( Vallisneria, Elodea, 

 Hydrocharis), is propagated in the basipetal direction, under favourable 

 conditions, at a rate of 1-2 cm. per min., judging by the starting of 

 protoplasmic rotation, which constitutes the primary response. Fitting 331 

 has shown that an even more rapid transmission (1-2 cm. per sec. 

 in Passiflora coerulea) takes place in the case of the traumatic stimulus 

 which causes certain tendrils to curl up at the tip when they are 

 cut off. In the case of Mimosa pudica, finally, Dutrochet, Bert 

 and the author all agree in estimating the maximum velocity of 

 transmission at 1*5 cm. per sec. as regards the primary petiole, 

 the corresponding value for the stem being usually somewhat smaller ; 

 Linsbauer, however, arrives at much higher figures for Mimosa viz., 

 30-100 mm. per sec. at least. Even the greatest of these velocities 

 falls far behind the average rate of transmission of stimuli through the 

 nervous system of animals, which amounts to at least 30,000 mm. per 

 sec. As a matter of fact, lower velocities are known to occur in 

 the animal kingdom also ; thus, Engelmann states that stimuli are 

 transmitted through the heart-muscles, without the aid of nerve-fibres, 

 at rates varying between 6'4 and 177 mm. per sec. In any case, it 

 may be confidently asserted that the actual velocity of transmission in 

 any given case entirely accords with the nature of the requirements 

 which the resulting response is intended to satisfy. 



In approaching the question as to the existence of special arrange- 

 ments for the transmission of stimuli in plants, it is further necessary 

 to consider the various ways in which such transmission may be 

 effected. 



In the first place, stimuli may be transferred from cell to cell quite 

 independently of any special transmitting arrangements ; this state- 

 ment applies more particularly to shock-stimuli, which can be propagated 

 by purely mechanical means. In Mimosa pudica the contraction of 

 a single stimulated cell, or group of cells, in the irritable lower half of 

 the pulvinus leads to a deformation of the neighbouring cells, whereby 

 they are stimulated in their turn. Pfeffer has already pointed out 

 that a locally applied stimulus may spread in this way through the 

 entire sensitive tissue of the pulvinus. 



Where a shock- or wound-stimulus produces differences of hydro- 



