CHAPTER XX 



ON THE VELOCITY OF TRANSMISSION OF EXCITATORY 

 WAVES IN PLANTS 



Difficulties in accurate determination of velocity of transmission, due to unknown 

 variations of excitability arising from injury, and variations of conductivity 

 through fatigue — A perfect method of obtaining accurate and consistent results — 

 Relative advantages of studying conduction in plants as compared with animals 

 — Determinations of velocity of transmission in centripetal and centrifugal direc- 

 tions — Preferential conductivity in centrifugal direction— Diminution of con- 

 ductivity and excitability by fatigue — Within a certain critical interval, organ 

 ' refractory ' to further stimulus — Increased velocity of transmission with in- 

 creasing stimulus — Measurement of diminution of conductivity by cold — Fibro- 

 vascular elements the best conducting channels — Conductivity lengthwise greater 

 than crosswise - Electric mode of determination of velocity of transmission — 

 Indifferent parenchymatous tissues do not transmit stimulation — Comparative 

 tables showing velocity of transmission in various plant and animal tissues. 



IN the last two chapters the effects of various agencies on the 

 power of conduction were demonstrated qualitatively. It is 

 important, however, to obtain, if possible, the quantitative 

 values of this conductivity and its variations. ' The absolute 

 value of conduction can be obtained from the determination 

 of the velocity of transmission of excitation through the 

 tissue. This determination of velocity may be made roughly, 

 by observing the time taken for the application of a stimu- 

 lus, say by cut or hot wire contact at a given point, to produce 

 motile effects on a leaflet at a known distance. 



A result thus obtained, however, would, for reasons to be 

 given presently, prove very indefinite, and no two such results 

 in succession could be trusted to agree. In order to ascer- 

 tain the exact quantitative effects of various agencies on 

 conductivity, we must first be completely assured that our 

 determinations of velocity under normal conditions are trust- 

 worthy. 



