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Page:Popular Science Monthly Volume 18.djvu/525

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shown at A and B (Fig. 12), where the leaves are seen at night crowded together, as if for mutual protection. Not less striking is the contrast between the night and day aspect of Cassia corymbosa, as seen in Fig. 13. Here the horizontally extended leaflets sink down vertically at night, and at the same time rotate so that the lower surface faces outward.

PSM V18 D525 Cassia corymbosa in diurnal and nocturnal state.jpg
Fig. 13.—Cassia corymbosa: A, plant during day; B, same plant at night. Both figures copied from photographs.

As a conclusion from all his experiments, observations, and reflections upon this subject, Mr. Darwin states:

"The great sweeps made by the stems of twining plants, and by the tendrils of other climbers, result from a mere increase in the amplitude of the ordinary movement of circumnutation. The position which young leaves ultimately assume is acquired by the circumnutating movement being increased in some one direction. The leaf-blades of various plants assume a vertical position through modified circumnutation, in order to protect their upper surfaces from being chilled