Full-grown oaks vary in height from sixty to one hundred and thirty feet, the difference depending upon situation; the tallest, of course, being those that have been drawn up in forests, at the expense of their branches. Trees growing freely in the open are of less height, and are made to appear comparative dwarfs by the huge proportions of the bole. In the forest this may be no more than ten feet in girth, but in isolated specimens may be as much as fifty-four feet (Cowthorpe Oak), with a much broader base. The thick rough bark is deeply furrowed in a large network pattern, which affords temporary hiding-places for insects. The branches are much given to turn and zig-zag from side to side--a character that makes them very useful in boat-building, as "knees" of various angles may be cut from them without having recourse to bending. The best knees are to be obtained from Oaks grown in the hedgerow. [Illustration: Oak. A, female flower; B, male flowers.] The Oak flowers in April or May, and the blossoms are of two distinct forms--male and female. The males are in little clusters, which are borne at intervals along a hanging stalk, two or three inches in length. They are green, and therefore inconspicuous; but examined separately, they will be found to have a definite calyx, whose margin is cut into an uncertain number (4-7) of lobes. There are no petals, but attached to the sides of the calyx there are ten stamens. The female flowers are fewer, and will be found on short erect stalks above the male catkins. Each female flower consists of a calyx, invested by a number of overlapping scales, and enclosing an ovary with three styles. The ovary is divided into three cells, each containing two seed-eggs. An acorn should therefore contain six kernels, but, as a rule, only one of the seed-eggs develops, though occasionally an acorn contains two kernels. The overlapping scales at the base of the female flower become the rough cup that holds the acorn.