Page:Collected Physical Papers.djvu/343

 XXIII

ON A VEGETABLE PHOTO-ELECTRIC CELL

When light is incident on the sensitive pulvinus of Mimosa pudica, the response is by contraction and the resulting fall of the leaf. A diminution of electric resistance of tissues of plants under stimulation has also been demonstrated in the previous paper. The electromotive response of galvanometric negativity, under the excitatory action of light, has previously been demonstrated in my work on Comparative Electro-physiology. I here describe a new and interesting method for this demonstration.

For obtaining the normal response, we take a vigorous leaf and pin it on a paraffined block of wood. Two pieces of thin muslin in connection with non-polarisable electrodes are spread over two areas of the leaf A and B; when these pieces of muslin are moistened with normal saline, they become practically transparent. When light from an arc lamp is thrown on A, that area becomes galvanometrically negative and the responsive current flows in {he direction GAB. Light thrown on B (A being shaded) causes a response in the opposite direction, (left illustration fig. 96).

The fact, that the electromotive response under light is the same as that under a different mode of stimulation such as mechanical, is demonstrated as follows. The moist piece of cloth on A is rubbed