VELOCITY OF TRANSMISSION OF EXCITATION 455 



nerve of fern. This, for the relatively sluggish vegetable 

 tissue, is undoubtedly very high. 



In connection with this question of velocity of trans- 

 mission, a fact not hitherto taken into account is, that there 

 are two distinct kinds of nervous impulses, travelling with 

 different velocities namely, the hydro-positive and the true 

 excitatory negative. Of these the velocity of the former is 

 greater. In the nerves of higher animals, where the velocity 

 of transmission of true excitation is also great, it is not 

 generally easy to distinguish one from the other, so rapid 

 is their succession. But their occurrence as distinct waves, 

 even in animal tissues, I shall be able to demonstrate in 

 a subsequent chapter. In plants, however, where the velocity 

 of transmission of true excitation is not very high, it generally 

 lags perceptibly behind the positive wave (p. 59). Burdon 

 Sanderson, in his determination of the velocity of trans- 

 mission of excitation in Dioncea^ arrived at the exceptionally 

 high result of 200 mm. per second. I have shown, however, 

 that the wave whose velocity he measured was not of true 

 excitation, but of hydro-positive disturbance (p. 231). 



In the present chapter it has been my object to demon- 

 strate the reality of true excitatory propagation in plants 

 similar to that in the animal. The examples given will be 

 found more fully described on referring to my book on * Plant 

 Response.' I shall, however, in the course of the present 

 work describe new and extremely delicate means by which 

 the modifications of conductivity may be studied in plants 

 uncle varying physiological conditions. 



