Trunks are not,however, the only parts of plants which direct their course towards the air and the light of the sun. There are flowers which, quitting their perpen dicular direction, present their surface directly to that luminous body, and fol low its situation in its diurnal course. This%airt of motion has been called, by some writers, natation; and the plants which are subject to it, have been term ed heliotrope ; that is, turning with the sun. Of this kind are bastard-rocket, dyer's-weed, sunflower, turnaole, and the greatest part of the compound flow. ere with plain tongue-shaped petals. In these flowers, the disk or surface looks towards the east in the morning, the south at noon, and the west at night. The spikes or ears of corn, which hang down by their weight, are observed, in like roamer, to incline themselves to.
wards the sun, never to the north. The stems of drabs, trientalis, and a species of bastard fever-few, with egg-shaped and notched leaves, incline or hang down wards during the night.
The observations of Hales and Bon net establish, that these motions are oc casioned, not by any twisting in the stem, but by the dryness of the fibres, which, by being exposed to the heat of the sun. contract, and thus determine the notation of the flowers and young stems. It is in this manner that moisture and dryness alternately dilate and contract the plant improperly called the rose of Jericho ; an appearance which is likely observed in the beards of oats, and in those of the capsules of crane's bill.
The direction of the leaves of several plants suffer considerable changes during the night This is so certain, that if a botanist who is accustomed to the part or habit of plants were to examine, in a summer night, the plants which cover any particular meadow, he would find several which be could not recognize by that character. The same changes hap pen when the moisture of the day cor responds to that of the night.
The change of direction just mention ed is particularly sensible in compound leaves. During the heat of the sun in the day-time, the pinnated or winged leaves of several plants, particularly those of the pea-bloom or leguminous tribe, rise vertically upwards, and form a right angle with the common foot-stalk ; the lobes, or lesser leaves, which stand op posite, being applied closely together by their upper surface. Several simple
leaves, particularly those of sigesbecka, and Indian mallow, (urena) when their upper surface is exposed to an ardent sun, become, in like manner, concave; which demonstrates their analogy with the winged leaves just mentioned.
In that state of the atmosphere which generally precedes a storm, and in a close, moist, and cloudy air, the winged leaves extend themselves along the com mon foot-stalk. The same appearance is observed in the leaves of the sensi tive plant, when it has been kept for se veral days in a cellar below ground. Af ter sun-set, and during the fall of the dew, they incline still lower, hang vertically downwards, and are applied closely toge ther, like the leaves of a book, by the lower surface, under the stalk with which they stand at right angles. The odd lobe. if there is one at the extremity of the leaf, folds itself up till it has reached the Sat pair of lobes or smaller leaves, in its neighbourhood. This motion, which Lin flans calls the sleep of plants, and can be produced by an artificial as well as natu ral dew, has been observed, not only in compound leaves, such as those of the pea-bloom plants, but likewise in some simple leaves, particularly those of bal sam, and bastard fever-few. The small leaves of false acacia and liquorice hang downwards during the night, but are not united by the under surface, like the greater part of leguminous plants. Those of the sensitive plant, mimosa pudica, extend themselves longitudinally along the common foot-stalk, and infold one another mutually. The small lobes of se veral species of trefoil, lucerne, and lo tus, arc united only by their summits, and form a cavity, which contains the young flowers, and shelters them from cold and other injuries to which they are liable in the night-time. In some simple leaves, a similar appearance is observed. Thus the upper leaves of garden orach app. proach during the night, unite perpendi cularly, embrace the young shoot, and do not relinquish that posture till the sun has dissipated the humidity of the air. See Milne's excellent Botanical Dictiona ry, to which we have been indebted for these observations.