Page:Researches on Irritability of Plants.djvu/93

70 be induced in the moist thread by drying. The horizontal rod holding the cork of the contact-maker is soldered to a tube which can move up and down a vertical rod. These adjustments enable the point of electrolytic contact to be brought to a level with the point of connection on the specimen. The rod is insulated on ebonite.

All the practical difficulties having thus been eliminated, I shall now proceed to show the various records obtained under this mode of periodic stimulation of uniform intensity. In fig. 32 is given a photograph of the apparatus with its accessories. The recorder is of duplex type, for taking two sets of records at the same time.

Before entering upon the detailed consideration of the results of these experiments, I may say that at the beginning of this investigation my attention was roused by the apparently capricious variations in the responses obtained under conditions which were rigidly uniform.

A long-continued investigation through the different seasons of the year has given me the clue to what at first appeared to be so anomalous. The outward response, it is obvious, is dependent on two factors—the intensity of the impinging stimulus and the capacity for reply possessed by the plant itself. It is easy to see that the second of these factors must be dependent on the vigour of the plant, or in other words, on its tonic condition, which in its turn is modified by the environmental condition. Thus in unfavourable circumstances the plant may fall into an atonic or sluggish condition. The absorption of energy from without, by Whatever form of stimulation, will improve the tonic condition of the plant, with consequent enhancement of excitability.

Taking a plant in a subtonic condition, then, we may expect that any application of stimulus will increase its excitability, a fact which will find expression in a growing amplitude of response. This enhancement of excitability will reach a limit at which the plant will be in an optimum condition. After reaching this climax there may be a