Page:Researches on Irritability of Plants.djvu/90

Rh period of such contact would be impracticably long—more than 15 seconds.

So continuous a tetanisation would undoubtedly fatigue or even injure the tissue. The contact made by a seconds-hand, again, though sufficiently brief, would have the serious defect that its movements were jerky and would therefore make the duration of contact unequal.

I succeeded in overcoming these difficulties by using a released revolving disc, which could be made to complete an electrical circuit for any definite short period that was required. For this I employed a phonograph motor, an axis of which, carrying a disc, could be adjusted to revolve once in a second.

This disc is usually held stationary by a lever-clutch, and can be started only by the pressure on it of the revolving rod of the Periodic Starter. It is re-arrested after one complete revolution and is not again released till the next rod comes into position, after an interval of 10, 15, or 20 minutes, as the case may be. There is also attached to the disc a sector whose arc is one-tenth the circumference of the circle. This sector, during the revolution, will press against a closing-key, the period of closure being then ·1 second. By increasing or diminishing this arc the time of closure, and with it the duration of the tetanising shock, can be correspondingly changed. It is necessary that the sector should, at the moment of the release, be at the greatest possible distance from the closing-key. By the time it reaches this key it will have acquired a constant and definite velocity. Thus the periods of closure, and consequent duration of the exciting-shock, will be identical in successive experiments.

There is another possible source of variation which must be guarded against. The electrodes of the secondary coil are connected with the plant by means of moistened threads. These threads, in long-continued experiments, may become more or less dried up, the electrical resistance being thus increased in an unknown manner. The intensity Rh