Page:Collected Physical Papers.djvu/407

Rh

A very sensitive method of record of the action of light by the induced electromotive variation of galvanometric negativity is afforded by the photo-electric cell in which two similar leaves, immersed in an electrolyte, form the voltaic elements. There is no current in the cell when it is kept in the dark, but exposure of one of the two leaves gives rise to a responsive current. The amplitude of response increases with the intensity and duration of exposure to light. An abnormal positive response occurs when the physiological vigour of the specimen is feeble and below the normal.

In the green leaves two opposite reactions are simultaneously induced under the action of light; of these the A-reaction is associated with the building up process and storage of energy by assimilation; the D-reaction of break-down and expenditure of energy occurs under excitatory action of stimulus. The A-effect is often masked by the predominant D-reaction; the existence of A is demonstrated by the positive electric response exhibited by actively assimilating plants like Hydrilla; it is also shown as an after-effect on the cessation of stimulus (p. 330).

The living plant is in a state of unceasing activity, absorbing and storing energy from without, setting free and dissipating it from within. The expenditure of energy may be manifested in movement, or it may not be externally perceptible, being employed in working the internal mechanism of the body—such for instance, as the distribution of water, which as I have shown