The Oak is most abundant on clay soils, but is at its best when growing in deep sandy loam, where there is also plenty of humus. Its roots in such soil strike down to a depth of five feet, and therefore it thrives in association with Beech, whose roots are much nearer the surface, and whose fallen leaves supply it with humus.
The Oak is more persistently attacked by insects than any other tree. One authority (Leunis) has tabulated the species that get their living mainly or entirely from their attacks on the foliage, timber, or bark, and they number about five hundred. With some species this warfare is waged on so extensive a scale, that in some years by early summer the Oaks are almost divested of their foliage, and a new crop of leaves becomes a necessity. But the reserve forces of the Oak are quite equal to this drain, and the tree does not appear to suffer, though a much less thorough attack would be serious to a Conifer. One of the worst of these Oak-spoilers—though it by no means restricts its energies to attacks on this tree—is the Mottled Umber Moth (Hibernia defoliaria), whose pretty caterpillars may be seen hanging by silken threads from the leafless twigs.
A striking Oak insect is the Stag Beetle (Lucanus cervus), which, in warm evenings in the south of England, may be seen flying round the Oaks, the size and antler-like jaws of the male arousing feelings of respect in the minds of those who are not acquainted with its habits. The formidable
looking "horns" are usually harmless. The beetle spends its larval stage in the wood of unhealthy Oaks, and, when mature, seeks his hornless mate among its foliage.
Perhaps the most interesting of the Oak's pensioners to the woodland rambler will be the varied forms of gall on different parts of the tree. There is the so-called Oak-apple, of uneven surface and spongy to the touch, which certain people still wear on May 29th, in honour of Charles II.; the well-rounded hard Bullet-gall of Cynips kollari, the Artichoke-gall of Cynips gemmæ, the Spangle-galls of Neuroterus lenticularis, so plentiful on the back of the leaf, and the Root-gall of Biorhiza aptera. All these galls are abnormal growths, due to the irritation set up by the Gall-wasps named, when they pierced the young tissues in order to lay their eggs in them. Where any of these galls are perforated it may be known that the Gall-wasp whose grub fed within has flown, but where there is no such perforation the grub is still within, feeding upon the flesh of the gall, or in the chrysalis stage, awaiting translation to the winged condition.
Several Oaks of foreign origin are also grown in our parks and open spaces; among them the Holm Oak (Quercus ilex) whose evergreen leathery leaves have toothed or plain edges, and occasionally the lower ones develop marginal spines, whence its name of Holm or Holly Oak. It is notable for retaining its lower branches, so that its appearance, as Loudon remarks, "even when fully grown, is that of an immense bush, rather than that of a timber tree." It is a native of Southern Europe and North Africa, and appears to have been introduced about the middle of the sixteenth century. It usually attains a height of from twenty to thirty feet, but occasionally specimens are seen up to sixty feet. It has a much thinner, more even bark than that of our native Oak, and of a black colour. The long brown acorns do not ripen until the second year.