Page:Researches on Irritability of Plants.djvu/155

CHAPTER XI

VELOCITY OF TRANSMITTED IMPULSE IN PLANTS

Detection of transmitted excitation by means of electromotive variation—Specific tissue for conduction of excitation—Hydro-mechanical theory of transmission of stimulus—Propagation of excitatory protoplasmic change—Physiological test—Automatic record of transmission-period—Conditions for obtaining constant velocity—Determination of velocity of transmission in Mimosa—Differential method of determining velocity—Constancy of results—Tabular statement of different determinations of velocity—Effect of intensity of stimulus on velocity of transmission—Effects on sub-tonic tissue and on tissue in optimum condition—After-effect of stimulus in enhancing conductivity—Effect of optimum condition—Disturbing action of leakage of exciting current—Effect of fatigue—Effect of temperature—Velocity of transmission in Biophytum and Averrhoa—Direction of preferential conduction

We have hitherto dealt with the reaction of tissues which exhibit the excitatory condition by motile response, as in pulvinus in the case of the plant and muscle in the case of animal. In the animal, again, we meet with certain conducting-tissues in which the excitatory protoplasmic change is transmitted to a distance, and, should one of these nerves happen to lead to a contractile muscle, the transmission of the excitatory change is conspicuously exhibited by the contraction of the terminal organ.

We now come to the question whether there is a transmission of a true excitatory change in the plant, and if so whether there is in it any specific conducting-tissue, corresponding to the nerve of the animal, for the conveyance of excitation? Since the transmission of excitation depends on the propagation of a protoplasmic change, it follows that a conducting-tissue must be characterised by a more or less protoplasmic continuity. Should the plant possess

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