until a severe frost followed by a thaw brings them down. The buds, leaves, and flowers are all much attacked by gall-forming insects, many different kinds being found on one and the same tree.
It is not until the oak is from sixty to a hundred years old that good seeds are obtained from it. Oaks will bear acorns earlier than this, but they are apt to be barren. A curious fact is the tendency to produce large numbers of acorns in a given favorable autumn, and then to bear none, or very few, for three or four years or even longer. The twisted, "gnarled" character of old oaks is well known, and the remarkably crooked branches are very conspicuous in advanced age and in winter (Plate II). The bark is also very rugged in the case of ancient trees, the natural inequalities due to fissures, etc., being often supplemented by the formation of "burrs."
A not inconsiderable tendency to variation is shown by the oak, and foresters distinguish two sub-species and several varieties of what we regard (adopting the opinion of English systematic botanists) as the single species Quercus robur.
Besides forms with less spreading crowns, the species is frequently broken up into two—Q. pedunculata, with the female flowers in rather more lax spikes, and the acorns on short stalks, the leaves sessile or nearly so, and not hairy when young; and Q. sessiliflora, with more crowded sessile female flowers, and leaves on short petioles and apt to be hairy. Other minute characters