Page:Collected Physical Papers.djvu/375

Rh At first sight this would appear to be very unlikely, for the most effective rays are in the ultra-violet region with wave-length as short as 20 × 10−6 cm.; but electric waves used in wireless signalling are 50,000,000 times as long. The perceptive power of our retina is confined within the very narrow range of a single octave, the wave-lengths of which lie between 70 × 10−6 cm. and 35 × 10−6 cm. It is difficult to imagine that plants could perceive radiations so widely separated from each other as the visible light and the invisible electric waves.

The results obtained prove, however, that electric waves are effective in modifying the rate of growth. The experiment was carried out with the help of a portable electric Radiator, the intensity of radiation being capable of variation. The Radiator was placed at a distance of 200 metres from the growing plant which was suitably mounted on the Balanced Crescograph.

Effect of feeble Stimulation.—The response was found to be an acceleration of growth as seen in fig. 113 (a). This is analogous to the stimulating action of light of sub-minimal intensity.

Effect of strong Stimulation.—Even more striking is the effect of stronger stimulation; the balance wasimmediately upset, indicating a retardation of the rateof growth (fig. 113 b). The latent period, i.e., the interval between the incident wave and the response, was only a few seconds. The record given in the figure was obtained with the moderate magnification of 2,000 times.

Under an intensity of stimulus slightly above the sub-minimal, the response exhibited retardation of