CHAPTER XXX 



DETERMINATION OF VELOCITY OF TRANSMISSION OF 

 EXCITATION IN PLANT TISSUES 



Transmission of excitation in plants not due to hydromechanical disturbance, but 

 instance of transmission of protoplasmic changes Difficulties in accurate 

 determination of velocity of transmission A perfect method Diminution of 

 conductivity by fatigue Increased velocity of transmission with increasing 

 stimulus Effect of cold in diminishing conductivity Effect of rise of tem- 

 perature in enhancing conductivity Excitatory concomitant of mechanical 

 and electrical response Electrical methods of determining velocity of trans- 

 mission Method of comparison of longitudinal and transverse conductivities 

 Tables of comparative velocities in animal and plant Existence of two 

 distinct nervous impulses, positive and negative. 



WHEN a point in the tissue is stimulated the state of excita- 

 tion is often found to be transmitted to a distance. This is 

 well seen in the case of sensitive plants, where the excitation 

 applied at one point is found to give rise to motile responses 

 of the distant leaf or leaflets. 



In this transmission of excitatory impulses to a distance 

 in the plant we have a phenomenon which would seem to be 

 analogous to nervous transmission in the animal. For certain 

 reasons to be given presently, however, it has usually been 

 supposed that there is actually nothing in common between 

 the two. ' The nervous system belongs exclusively to the 

 animal organisation, and, indeed, to the more highly de- 

 veloped Metazoa only. Plants, unicellular animals, and the 

 lower Metazoa have no nerves, and if in exceptional cases 

 (as in the excitatory movements of many plants) there are 

 forms of activity which resemble the vital manifestations of 

 the animal organisation, as effected by nerves, it is easy to 

 prove that the resemblance is merely superficial.' l 



1 Biedermann, Electro-Physiology, English translation, 1898, Vol. ii. p. 32. 



