Light is the most universal and important exciting cause in the vegetable world. The mouths of the stomata are opened by the influence of light. The leaves, young shoots, and tendrils turn to the light; it regulates the sleep of plants, as well as the diurnal motions of the daisy and sunflower. The opening of blossoms and of folded leaves which had been closed in sleep during the night, shows the susceptibility of their tissues to the influence of light, an influence beautifully exhibited by the orange-coloured Eschscholtzia, which shuts its golden blossoms under every passing cloud.
All M. Cohn’s experiments prove that in the Mimosa pudica, which is highly sensible to the action of light, heat, electricity, and touch, ‘the propagation of the external excitement, proceeds in the same mode as in animals, and there is little doubt that the vascular tissue (which contain spiral vessels) constitute the special bundles adapted for the purpose, and that the phenomena of contractibility depend upon a muscular tissue.’[[82]]
From Professor Franklin’s experiments it appears that ‘the motions resulting from external causes are owing to vital contractibility, and that they are governed by the same laws which regulate similar action in the animal kingdom. Their energies vary with the vigour of the plant; they are exhausted by over exercise, and require rest; and like animals they are lulled and put to sleep by chloroform and narcotics.’
END OF THE FIRST VOLUME.
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[1]. Professor Faraday.
[2]. Professor Helmholtz.
[3]. The address of the president, Sir William Armstrong, C.B., to the British Association at Newcastle-on-Tyne, 26th August, 1863.