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There are some curious facts regarding climbing plants: their stems generally turn from left to right round the pole used for support; others follow a contrary direction; while to some it seems to be a matter of indifference. Mr. Darwin has concluded that light is an influential cause. If plants of this class are placed in a room near a window, the stem requires more time to perform the half revolution during which it is turned away from the light than for that which is toward the window. In one case the whole circle was completed in five hours and twenty minutes; of this the half in full light only required an hour, while the other could not traverse its part in less than four hours and twenty minutes—a very striking variation. Some Chinese ignamas (Diascorea batatas) in full growth were placed in a completely darkened cave, and others in a garden; in every case those were in darkness lost the power of climbing round their supports; those exposed to the sun were twisting, but as soon as they were put in the cellar they grew with straight stems.

The sleep of plants, which certainly has a connection with light, is another curiosity in nature. Flowers and leaves of some growths seem to fade at particular hours, the corolla being closed, which after a state of lethargy blows out afresh; others the flower falls and dies without having closed. In the case of the convolvulus the flower is drawn up at noon, Linnaeus noted the hours in which certain plants blow and fade, and thus a floral dial; but science has not yet been able to explain these curious relations to light.