Movements of Plants

Fritz Müller, in a letter from St. Catharina, Brazil, dated January 9, has given me some remarkable facts about the movements of plants.² He has observed striking instances of allied plants, which place their leaves vertically at night, by widely different movements; and this is of interest as supporting the conclusion at which my son Francis and I arrived, namely, that leaves go to sleep in order to escape the full effect of radiation.³ In the great family of the Gramineæ the species in one genus alone, namely Strephium, are known to sleep, and this they do by the leaves moving vertically upwards; but Fritz Müller finds in a species of Olyra, a genus which in Enlicher’s “Genera Plantarum” immediately precedes Strephium, that the leaves bend vertically down at night.⁴

Two species of Phyllanthus (Euphorbiaceæ) grow as weeds near Fritz Müller’s house; in one of them with erect branches the leaves bend so as to stand vertically up at night. In the other species with horizontal branches, the leaves move vertically down at night, rotating on their axes, in the same manner as do those of the Leguminous genus Cassia.⁵ Owing to this rotation, combined with the sinking movement, the upper surfaces of the opposite leaflets are brought into contact in a dependent position beneath the main petiole; and they are thus excellently protected from radiation, in the manner described by us. On the following morning the leaflets rotate in an opposite direction, whilst rising so as to resume the diurnal horizontal position with their upper surface exposed to the light. Now in some rare cases Fritz Müller has observed the extraordinary fact that three or four, or even almost all the leaflets on one side of a leaf of this Phyllanthus rise in the morning from their nocturnal vertically dependent position into a horizontal one, without rotating, and on the wrong side of the main petiole. These leaflets thus project horizontally with their upper surfaces directed towards the sky, but partly shaded by the leaflets proper to this side. I have never before heard of a plant appearing to make a mistake in its movements; and the mistake in this instance is a great one, for the leaflets move 90^o in a direction opposite to the proper one. Fritz Müller adds that the tips of the horizontal branches of this Phyllanthus curl downwards at night, and thus the youngest leaves are still better protected from radiation.

The leaves of some plants, when brightly illuminated, direct their edges towards the light; and this remarkable movement I have called paraheliotropism.⁶ Fritz Müller informs me that the leaflets of the Phyllanthus just referred to, as well as those of some Brazilian Cassiæ, “take an almost perfectly vertical position, when at noon, on a summer day, the sun is nearly in the zenith. To-day the leaflets, though continuing to be fully exposed to the sun, now at 3 p.m. have already returned to a nearly horizontal position.”⁷ F. Müller doubts whether so strongly marked a case of paraheliotropism would ever be observed under the duller skies of England; and this doubt is probably correct, for the leaflets of Cassia neglecta, on the plants raised from seed formerly sent me by him, moved in this manner, but so slightly that I thought it prudent not to give the case.⁸ With several species of Hedychium, a widely-different paraheliotropic movement occurs, which may be compared with that of the leaflets of Oxalis and Averrhoa; for “the lateral halves of the leaves, when exposed to bright sunshine, bend downwards, so that they meet beneath the leaf.”⁹

Charles Darwin

Down, Beckenham, February 22