The Oak is subject to a considerable amount of variation, probably due to differences of situation, soil, etc., and some authors have sought to elevate certain of the varieties into species by giving them distinctive names. It does not appear to be certain, however, that these forms are at all constant, and they are connected by intermediate forms that make the identification of many individuals a matter of difficulty. In one of these forms (_sessiliflora_) the stalk of the acorns connecting them with the branch is very short, but the leaves have a distinct footstalk, from half an inch to an inch long. This form is more plentiful in the north and west, and is conspicuous in the Forest of Dean. A second form, known as _pedunculata_, has the leaf-stalk short or absent, the base of the leaf broad and somewhat heart-shaped, and the stalk upon which the acorns are borne very long. A third form (_intermedia_), commonly known as Durmast, has short leaf-stalks, short stalks to the acorns, and the under side of the leaf downy. _Pedunculata_ is found more on the lower hills and the sides of valleys, whilst _sessiliflora_ prefers higher ground, with a southern or western aspect. [Illustration: _Pl. 4._ Bole of Oak.] [Illustration: _Pl. 5._ Flowers of Oak.] 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.