DEFOLIATION (from de, and folium, a leaf), the fall of the leaves. A term opposed to frondescentia, the annual renovation of the leaves, produced by the unfolding of the buds in spring.

Most plants in cold and temperate climates shed their leaves every year. This happens in autumn, and is generally announced by the flowering of the common meadow saffron. The term is only applied to trees and shrubs; for herbs perish down to the roots every year, losing stem, leaves, and all.

All plants do not drop their leaves at the same time. Amongst large trees, the ash and walnut, although the latest in unfolding, are soonest divested of them. The latter seldom carries its leaves above five months. On the oak and hornbeam the leaves die and wither as soon as the colds commence; but they remain attached to the branches until they are pushed off by the new ones, which unfold themselves the following spring. These trees are doubtless a kind of evergreens. The leaves are probably destroyed only by cold; and perhaps would continue longer on the plant, but for the force of the spring sap joined to the moisture.

In mild and dry seasons the liiach, privet, yellow jessamine of the woods, and maple of Crete, preserve their leaves green until spring, and do not drop them till the new leaves are beginning to appear. The fig-tree, and many other trees that grow between the tropics, are of this particular class of evergreens. The trees in Egypt, says Dr Hasselquist, cast their leaves in the latter end of December and beginning of January, having young leaves ready before all the old ones have fallen off; and, to forward this operation of nature, few of the trees have buds; the sycamore and willow, indeed, have some, but with few and quite loose stipula or scales. Nature did not conceive buds to be so necessary in the southern as in the northern countries; and this occasions a great difference between them.

Lastly, some trees and shrubs preserve their leaves constantly throughout the whole year, and are not in the least influenced by the clemency or inclemency of seasons. Such are the firs, juniper, yew, cedar, cypress, and many other trees, hence denominated evergreens. These preserve their old leaves a long time after the formation of the new, and do not drop them at any determinate time. In general the leaves of evergreens are harder and less succulent than those which are renewed annually. The trees are generally natives of warm climates; as the alaternuses of France and Italy, and the evergreen oak of Portugal and Suabia.

Some herbaceous perennials, as the house-leeks and navel-worts, enjoy the same privilege with the evergreen trees, and resist the severities of winter; some even can dispense with the earth for a time, being replete with juices, which the leaves imbibe from the humidity of the atmosphere, and which in such plants are of themselves sufficient for effecting the purposes of vegetation. It is for this reason that, unless in excessive hot weather, gardeners seldom water fat succulent plants, as the aloes, which rot when they are moistened, if the sun does not quickly dry them up.

The leaves of all the evergreen shrubs and trees have a thin compact skin or cover over their surface, as is easily discovered by macerating them in water, in order to separate the parenchyma or pulp from the vessels of the leaves, which cannot be effected in any of these evergreens till a thin parchment-like colour is taken off. These trees and shrubs are found by experiment to perspire but little, when compared with others which shed their leaves; and it is perhaps principally owing to this close covering, as also to the small proportion of moisture contained in their vessels, that they retain their verdure, and continue through the winter on the trees. The nutritive juices of these plants always abound more or less with an oily quality, which secures them from being injured by severe frosts; so that many of these evergreen trees are adapted to grow in the coldest parts of the habitable world.

With respect to deciduous trees, the falling off of the leaves seems principally to depend on the temperature of the atmosphere, which likewise serves to hasten or retard the appearance in question. An ardent sun contributes to hasten the dropping of the leaves; and hence in hot and dry summers the leaves of the lime-tree andhorse-chesnut turn yellow about the first of September, whilst in other years the yellowness does not appear till the beginning of October. Nothing, however, contributes more to hasten the fall of the leaves than immoderate cold or moist weather in autumn; but, on the other hand, moderate droughts serve to retard it. As a proof of this position, Mr Adanson relates, that in the year 1759, the leaves of the elm-tree, which generally fall off about the twenty-fifth of November, continued in verdure and vigour at Paris, where the autumn was remarkably dry, till the tenth of the following month.

It deserves to be remarked, that an evergreen grafted upon a deciduous tree, determines the latter to retain its leaves. This observation is confirmed by repeated experiments, particularly by grafting the laurel, or cherry-bay, an evergreen, on the common cherry; and the ilex, or evergreen oak, on the oak.