producing the greenish aspect of the foliage, ; during the latter period, on the contrary, it is the lower or whitish surface that is presented to the observer.Thus there is a diurnal and a nocturnal position.As the day declines, the leaves, which at noon were horizontal, are seen gradually to erect themselves upon the branch, often becoming nearly perpendicular to it, and this move- ment of erection is accompanied by a movement of torsion in the basal part of the leaf, which often traverses au arc of 90°.
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J. CHESTER LYMAN (1876) studied this question.