Page:Researches on Irritability of Plants.djvu/137

114 conducting-strip, so long there will be a deflection in the galvanometer. By means of its screw-adjustment, the rod is gradually raised till the line of junction between platinum and ebonite is exactly reached; the deflection in the galvanometer will now cease suddenly. In this way the point of interruption or 'break' is determined with precision. By pulling the thread in connection with one arm of the recording-lever, we then trace a slightly curved line on the smoked plate. This indicates the exact position in succeeding records of the moment of application of stimulus. This mark of stimulation is shown in the printed records as a vertical line. After making this mark on the plate, the peg is removed. It is easy to see that in successive experiments stimulation will occur at that definite moment which corresponds to this marked line of stimulation.

K′ represents a key-device by which the make-shock is prevented from exciting the plant. One end of a lever carries a bent metal-rod of U-shape, which is partly immersed in cups of mercury by means of a spring. During the depressed position of this key, the secondary coil is short-circuited. When the handle K is slightly pressed, there is a 'make' of the primary current. But the make-shock is short-circuited as K′ is still in the depressed position. Further pressure of the handle K lifts K′ up, removing the short-circuit of the secondary coil. When the break-shock is given by the contact-breaker of the falling plate, there is no short-circuit to divert the shock, which now passes through the plant and excites it.

The sequence of these events, then, is as follows:—

By turning the disc D the recording-plate is lifted and held arrested in the up-position. The pressure of the handle K releases the plate-carrier, which then begins to fall. At the same time, the primary circuit is completed and a make-shock is induced in the secondary. But this make-shock is diverted by the short-circuit key K′, which is still in the depressed position. Further pressure of the handle K removes the short-circuit by lifting the ends of K′.