The white oak attains the elevation of seventy or eighty feet, with a diameter of six or seven feet; but its proportions vary with the soil and climate. Soon after their unfolding, the leaves are reddish above and white and downy beneath; when fully grown, they are smooth and of a light green on the upper surface. In autumn, they change to a bright violet color, and form an agreeable contrast with the surrounding foliage which has not yet suffered by the frost. This is the only oak on which a few of the dried leaves remain till the circulation is renewed in the spring. By this peculiarity and by the whiteness of the bark, from which it derives its name, it is easily distinguishable in the winter. This tree puts forth flowers in May, which are succeeded by acorns of an oval form, large, very sweet, contained in rough, shallow, grayish cups, and borne singly or in pairs, by peduncles eight or ten lines in length, attached, as in all species of annual fructification, to the shoots of the season. The fruit of the white oak is rarely abundant, and frequently, for several years in succession, a few handfuls of acorns could hardly be collected in a large forest where the tree is multiplied. Some stocks produce acorns of a deep blue color. Of all the American oaks, this is the best and the most generally used, being strong, durable, and of large dimensions. It is less employed than formerly in building, only because it is scarcer and more costly. Among the uses of this wood, the most important is in ship-building. In all the dock yards of the northern and middle states, except Maine, it is almost exclusively employed for the keel, and always for the lower part of the frame and the sides: it is preferred for the knees, when sticks of a proper form can be found. In the smaller ports south of New York, the upper part of the frame is also made of white oak; but such vessels are less esteemed than those constructed of more durable wood. The medicinal properties of oak bark depend on its astringency, and that again on its tannin. The inner bark of the small branches is the strongest, the middle bark next, and the outer bark is almost useless.