Page:Collected Physical Papers.djvu/358

 XXV

THE SELF-RECORDING RADIOGRAPH

A diurnal periodicity is generally exhibited in the various activities of the plant and in the movements of its different organs. This periodicity must be related to external variations, notably of temperature and light. It would be impossible to analyse the resulting phenomenon unless a continuous record of changes in the intensity of light is secured with the same exactitude as that of variation of temperature. As regards these two principal factors, the effect induced by the rise of temperature is often antagonistic to that of increasing intensity of light. A rise of temperature, for example, enhances the rate of growth up to an optimum; light, on the other hand, acts as a stimulus in retardation of growth.

For full analysis of diurnal periodicity in plants, it thus becomes necessary to devise means for continuous record of variation of temperature and the changing intensity of light. In regard to the variation of temperature, a simple and reliable type of thermograph has been devised, by which it is easy to obtain a continuous record of the variation of temperature throughout the day and night. No apparatus is, however, available for the continuous registration of variation of the intensity of light.

Selenium is well known for the characteristic diminution of its electric resistance under illumination.