Page:Researches on Irritability of Plants.djvu/131

 

CHAPTER IX

DETERMINATION OF THE LATENT PERIOD

Difficulties of accurate determination of Latent Period—Advantages of Resonant Recorder—Simultaneous tracings of tuning fork exciter and Resonant Recorder—Automatic stimulation at a definite moment—Identical value of latent period in successive determinations—Accurate measurement of time-interval shorter than .005 second—Latent period little affected by inertia of recorder—Tabular statement of value of different specimens of Mimosa—Effect of season on latent period.

When the motile pulvinus of Mimosa is subjected to an exciting shock, a short time elapses between the incidence of this shock and the initiation of the responsive movement. This short interval is known as the Latent Period. In a responding muscle, similarly, contraction does not occur instantaneously on the application of stimulus. The latent period in this case is determined from the record of the muscle-twitch. When after the application of stimulus the muscle has not yet begun to contract, the record appears as a straight line. Then on the commencement of contraction, the recording-lever is jerked up and the curve likewise bends upwards. The length of the straight portion of the record, between a mark that represents the incidence of the shock and the flexure at the initiation of response, gives us the duration of the latent period. We have, however, to determine the time-value of this length. This is done by means of a sinuous curve drawn below the record by a tuning-fork, vibrating 100 or 200 times in a second. Experimenting in this manner, the latent period for frog's muscle has been determined at about .01 second.

There are still several difficulties to be encountered in making this determination with any great exactness.

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