Page:Researches on Irritability of Plants.djvu/148

Rh which 100 would be due to make and the alternating 100 to break.

In order to subject the plant to brief alternating shocks applied at definite moments, and having definite duration, the sliding interrupter and its connections already described were appropriately modified. The vertical sliding contact-plate now consists of a conducting platinum sheet, except for a narrow non-conducting interruption made of a piece of inlaid ebonite. The sliding-plate and the contact rod now short-circuit the secondary, except during the brief interval when the narrow strip of ebonite removes the short-circuit. It is during this definite interval that the plant is subjected to the alternating shock. The breadth of the strip is so chosen that the duration of the shock is about .05 second. During this time the plant will receive 10 alternating shocks, 5 of make and 5 of break. If desired, a simple device may be introduced by which the duration of the shock can be modified. This modification consists in inlaying a right-angled triangle instead of a linear piece of ebonite on the metallic plate. The base of the right-angled triangle is kept horizontal. It follows that, by adjusting the rod from right to left, the interval of the removal of short-circuit can undergo continuous reduction of duration.

The mark which indicates the beginning of stimulation is made in the usual manner, and in successive experiments the stimulation is initiated at this precise moment. The duration of stimulus is also constant. In the following experiments, it is to be remembered, we are to determine the effect of changing one factor whilst maintaining others constant. Thus we have in each case to take one record under standard and one or more under modified conditions.

The series of experiments to be described below have in each case been repeated at least twelve times, with