Page:Researches on Irritability of Plants.djvu/54

Rh allowing successive stimuli to be maintained constant, or to be increased in a known manner.

The induction coil consists of a primary made of a few turns of thick wire enclosing a bundle of soft iron wire—thus forming an electro-magnet—and of a secondary consisting of a larger number of turns of thin wire. The secondary coil can be made to approach or recede from the primary, by means of a slide. When a current is suddenly started in the primary coil, by pressing a key an instantaneous make-induction-current is induced in the secondary. When, by releasing the key, this current is broken, an instantaneous break-induction-current, whose direction is opposite to that of make, is induced in the secondary. Owing to the greater suddenness with which the break is effected, the intensity of the break-shock is greater and of shorter duration than that of the make-shock. The intensity of either make- or break-shock may be increased by bringing the secondary nearer the primary. We can obtain successive uniform shocks, of either make or break, by maintaining the distance between the two coils constant.

We may subject the tissue to shock of either make or break at will by employing an additional short-circuiting key. When this key is down, the shock from the secondary coil is practically diverted across the better conducting-path provided by the key, so that for practical purposes none passes through the plant tissue. If it is desired to cause successive excitations by make-shock only, then the short-circuit key is raised when the primary circuit is made, and pressed down when this is broken. For exciting by break-shock, the short-circuit key is pressed when the primary is made and raised when the primary is broken (fig. 11).

In the response of animal tissue it is well known that while single induction-shocks are effective in the case of quickly reacting skeletal muscles, they induce hardly any