Page:Researches on Irritability of Plants.djvu/71

48 shown, be increased by sliding the secondary nearer and nearer to the primary coil. At a great distance the intensity of the shock is very feeble, whereas in the nearest position it is most intense. If a scale be placed to mark the relative position of the secondary to the primary, we may be assured of obtaining an identical intensity of shock whenever we place the secondary at the same point on the scale; or we can obtain an increasing intensity of stimulus by progressive movement along the scale towards the primary. There is, however, no simple relation between the distance and the intensity—that is to say, equal decrement of distance does not mean equal increment of intensity. All that we are sure of, is that the sliding in of the secondary coil secures an increasing intensity of stimulation. In order to be certain of obtaining quantitative values of intensity, the scale has to be specially calibrated.

In subjecting the plant to the secondary shock, if we begin with feeble intensity of stimulus, by placing the secondary at a great distance, and gradually increase the intensity by sliding the secondary nearer and nearer, we shall obtain that scale-reading at which the stimulus begins to be effective. This particular intensity, the feeblest that is effective, we designate the minimal stimulus. As We now proceed to increase the stimulation by pushing the secondary nearer to the primary, we find the amplitude of the response is progressively enhanced, and ultimately we reach an intensity beyond which there is no further increment of amplitude. This intensity we designate the maximal stimulus. When the plant is in an exceedingly vigorous condition, the minimal intensity is low and the range between maximal and minimal is narrow. But if the plant be in a less favourable tonic condition, then the minimal stimulus is relatively high and the range between minimal and maximal is wider.

As different induction-coils have different constants, the