Page:Collected Physical Papers.djvu/415

Rh actively contractile from other inactive cells. In Mimosa the active substance is present in great abundance; in Neptunia the particular substance is quantitatively less and scattered. In the inactive Phaseolus the active substance is altogether absent. The pulvinus of Mimosa may thus be regarded as functionally equivalent to an active animal muscle.

Nervous tissue in plants.—Some of the crucial tests for discrimination of nervous impulse are (1) that excitation is initiated for transmission by the characteristic polar action of a constant electric current; (2) that the velocity of transmission is increased within limits, by a rise of temperature; (3) that the transmission can be arrested temporarily or permanently by various physiological blocks. The conducting power is temporarily arrested during the passage of an electric current in a portion of the conducting tissue through which the impulse is being transmitted, the block being removed on the stoppage of the current; (4) that the conducting power is permanently abolished by poisonous solutions. The results of application of these crucial tests offer conclusive proof that the conduction in the plant is a phenomenon of protoplasmic excitation as in the nerve of the animal.

The velocity of transmission in plants is accurately determined by the Resonant Recorder by which time intervals as short as $1⁄200$ sec. can be recorded. The velocity of transmission is sometimes as high as 400 mm. per second. It is slower than the nervous impulse in higher, but quicker than in lower animals (p. 369).

Rhythmic tissue.—I have demonstrated elsewhere the remarkable similarity of rhythmic mechanism in