Page:Researches on Irritability of Plants.djvu/134

Rh acquiring increasing velocity. After this the velocity becomes uniform. If this uniformity should be required throughout the record, the tracing of response may be taken during this later period only.

The mode of procedure, therefore, is first to make the recording-writer vibrate at its own definite frequency of, say, 100 times per second. The recording-plate is then released and later, when its motion has become uniform, we pass through the pulvinus an electrical stimulus of an instantaneous break-shock. There should be a mark made on the recording-plate corresponding exactly to the moment of stimulation. The horizontal record, consisting of a series of dotted points representing one-hundredth of a second, is suddenly deflected upwards on the initiation of the responsive fall of the leaf. The number of dots intervening between the mark of stimulation and this point gives us the value of the latent period for the specimen.

Stimulation cannot be effected by hand at any exact predetermined point on the record. This must be done automatically by the moving plate itself. We cannot again give an instantaneous break-shock without previously completing the primary of the Ruhmkorff coil, which causes a disturbing make-shock.

In order to avoid this the secondary electrodes, during make, should be short-circuited by means of a thick conducting-wire; the secondary shock is thus practically diverted from the plant through the path of least resistance, which is the conducting wire. All these requirements are provided for in practice by the special mechanical devices of the apparatus.

The essential parts of the automatic arrangement by which a break-shock is given, at a predetermined point on the recording-plate, are shown in fig. 66.

The recording-plate is allowed to drop by pressing the