The Sleep of Flowers

Abstract

IN your “Notes” (vol. xii. p. 484) you mention a recent paper by M. Royer on this little-understood class of phenomena. We are acquainted with the objects of most of the spontaneous and periodical movements of plants, but of the physiological means by which these same movements are effected we know little or nothing. But it is important to remember that phenomena like in effect may be diverse in cause. The folding up of petals may have nothing physiologically in common with that of foliage-leaves. In fact, these phenomena may be divided into several classes. Thus movements due to irritation or concussion must be considered apart from those due to spontaneity, and the movements which form part of the series of processes of growth, such as the first unfolding of leaves and flowers, from those which occur in mature organs, though movements belonging to any two of these classes may be exhibited by the same plant, as in Oxalis and Mimosa. Cereus grandiflorus opens between 7 and 8 P.M., Mirabilis jalapa between 5 and 7 P.M. There is every probability that these times are those at which the insects which fertilise these two species also come forth, and that the same object exists in the case of other species which open and close their flowers more than once, “waking” and “sleeping;” but in the case of Cereus and Mirabilis the movement is one of growth only, though, no doubt, affected by external influences, such as the variation of heat and light. We have, however, cases of true “sleep” in Ipomæa nil and Calystegia sepium) which open between 3 and 4 A.M.; Tragopogan, the ligulate florets of which behave like petals, and which, opening at the same time, closes again before noon; Anagallis arvensis, opening at 8 A.M. and closing when the sky is overcast; the Mesembryanthaceæ, which open generally about 12—Mesembryanthemum noctiflorum, which opens between 7 and 8 P.M., being an exception; and Victoria regia, which opens for the first time about 6 P.M., closes in a few hours, opens again at 6 A.M., and closes finally and sinks in the afternoon; and in many other cases. Besides the causes mentioned in your note, the movements have been attributed to actinism. That they are not hygrometric is clear from the fact stated by Sachs, on the authority of unpublished experiments by Pfeffer (“Text-book of Botany,” p. 798), that they take place under water. These same experiments show them to be due to variations in the temperature, and when the temperature is constant, to variations in the intensity of light, and also to be accompanied, at least in some cases, with an increase of the length of the inner side of the phyllæ of the perianth when opening. Light certainly seems to have more to do with the movements of the “poor man's weather-glass” than heat, though perhaps atmospheric pressure might equally well be argued to be their cause. We must remember that as osmotic action is constantly going on at the root-hairs and in the growing parts of living plants, so a constant molecular diffusion of gases is going on through cell-walls, besides the passage of gases through stomata. “The movements of diffusion,” as Sachs says (p. 614), “tend to bring about conditions of equilibrium which depend on the co-efficients of absorption of the gas by a particular cell-fluid, on the molecular condition of the cell-wall, &c., on temperature, and on the pressure of the air. But these conditions are continually varying, and the equilibrium continually disturbed.” That a turgescence such as M. Royer describes occurs in many cases is well known, Space does not allow a detailed description of the physiological mechanism, but nearly all we yet know may be found in Sachs, who, however, attributes the phenomena directly solely to the passage of water and the elasticity of the cell-walls. Indirectly the cause may very possibly be heat acting as M. Royer supposes. It would be interesting to learn the effect of pollination on these plants, especially whether after it had taken place Victoria regia would re-open.