Page:Collected Physical Papers.djvu/389

Rh 20 mm. per second. In thinner specimens the velocity is often found to be as high as 400 mm. per second. The velocity of impulse in Mimosa is slower than the nervous impulse in higher but quicker than that in lower animals.

The transmission of excitation is correspondingly modified by all conditions which modify the transmission of excitation in the animal nerve. The polar action of a constant electric current is identical in the two cases. In both Mimosa and animal nerve the velocity of transmission is increased within limits by a rise of temperature (fig. 120). In both, transmission

can be arrested temporarily or permanently by various physiological blocks. The conducting power is temporarily arrested by a block produced by the passage of an electric current in a portion of the conducting tissue through which the impulse is being transmitted; this electrotonic block is removed on the stoppage of the current. Finally, poisonous solutions abolish the conducting power of both animal and plant. These results offer conclusive proof that the conduction in the plant is a phenomenon of propagation of protoplasmic excitation as in the nerve of the animal.