to arise from torpor of the nervous system, blistering the head and internal stimuli afford the most probable means of relief. The person should associate as much as possible with noisy, bustling people, and shun solitude and all such studies as have a tendency to produce abstraction.

CHAPTER XVIII.

SLEEP OF PLANTS.

DURING night, plants seem to exist in a state analogous to sleep. At this period they get relaxed, while their leaves droop and become folded together. Such is peculiarly the case with the tamarind tree, and the leguminous plants with pinnated leaves; but with almost all plants it takes place in a greater or lesser degree, although in some the change is much more striking than in others. The trefoil, the Oxalis, and other herbs with ternate leaves, sleep with their leaflets folded together in the erect posture. The cause of the different states in which plants exist during the day and night has never been correctly ascertained-some attributing it to the influence of light, some to the vicissitudes of temperature, and others to atmospherical humidity. Probably the whole of these influences are concerned. It is very evident that the presence of certain stimuli during the day puts the leaves in a state of activity, and excites their development; while the want of such stimuli in the night time throws them into repose, relaxes them, and occa

sions them to be weighed down, as if the sustaining principle which kept them in energy was suspended in the torpor of sleep. The principal of these stimuli is unquestionably light; indeed, Linnæus, from the observation of stove plants, seems to have demonstrated that it is the withdrawing of light, and not of heat, which produces the relaxation or Sleep of Plants, as it is commonly denominated. The effect of light upon the leaves of the Acacia is peculiarly striking. At sunrise they spread themselves out horizontally; as the heat increases they become elevated, and at noon shoot vertically upwards: but as soon as the sun declines they get languid and droop, and during night are quite pendent and relaxed. During day, the leaves of most plants are spread out and displayed, and at the same time, inclined towards the sun. Those of the Helianthus annuus, the Helianthemum annuum, and Croton tinctorium follow the course of the sun in their position; and most buds and flowers have a tendency to turn their heads in the direction of the great luminary of day. As an instance of this, let us look at the sun-flower, which confronts the source of light with its broad yellow expansion of aspect, and hangs its gorgeous head droopingly so soon as the object of its worship declines. The leaves of a great number of vegetables present changes in their position corresponding to the different hours of the day. "Who does not know," says Willdenow, "that the species of Lupinus, especially Lupinus luteus,

turn, in the open air, their leaves and stalks towards the sun, and follow its course in so steady a manner, as to enable us to specify the hour of the day from their direction." Such phenomena were not unknown to Pliny and Theophrastus.

The analogy between animal and vegetable life is still farther demonstrated by the well known fact, that while some creatures, such as the cat and owl, sleep during the day, and continue awake at night, certain plants do the same thing. Such is the case with the Tragopogon luteum, which becomes closed, or in other words, goes to sleep at nine in the morning, and opens at night. Every hour of the day, indeed, has some particular plant which then shuts itself up: hence the idea of the Flower Dial by means of which the hour of the day can be told with tolerable accuracy. Some plants, which shut themselves up in the day time, flower at night. The night-flowering Cereus, a species of cactus, is a beautiful instance of the kind; and there are other plants which exhibit the same interesting phenomenon. Nothing, indeed, can be more beautiful than the nocturnal flowering of certain members of the vegetable world. Linnæus used to go out at night with a lantern into his garden to have an opportunity of witnessing this remarkable peculiarity in the plants by which it is exhibited.

The analogy between the two kingdoms is rendered yet more striking, when it is recollected that, (with such exceptions as the above,) plants in

crease much more rapidly during night, which is their time of sleep, than in the day-time, which may be considered the period of their active or waking existence.

The state in which plants exist in the winter season, resembles the hybernation of animals: there is the same torpor and apparent extinction of vitality. Heat and light have the power of both reviving plants and putting an end to hybernation. Between plants and animals, however, there is this difference: that while most plants become torpid in winter, only a small number of animals get into that state; but even in such dissimilitude we can trace an analogy; for as there are animals upon which winter has no torpifying influence, so are there likewise plants. The Helleborus hyemalis or christmas rose, flowers at the end of December, and the Galanthus nivilis, or snow-drop, in the month of February.