THE OAKS This is the great family of the cup-bearers, whose fruit, the acorn, is borne in a scaly cup that never breaks into quarters, as does the husk that holds a chestnut, beechnut, or hickory nut. All oak trees bear acorns as soon as they come to fruiting age. This is the sign by which they are known the world over. Seldom is a full-grown oak without its little insignia, for the cups cling after the nut falls, and one grand division of the family requires two seasons to mature its fruit. For this reason, half-grown acorns are seen on the twigs after the ripe ones fall.
We cannot say of oak trees that they all have sturdy trunks, rough bark, and gnarled limbs, for not all of them have these characteristics. But there is a certain likeness in oak leaves. They are simple, five-ranked, generally oval, and the margins are generally cut into lobes by deep or shallow bays. Most oak leaves have leathery texture, strong veins, and short petioles. They are leaves that out last the summer, and sometimes persist until spring growth unseats the stalks; sometimes, as in the "live oaks," they hang on three to five years.
The twigs of oak trees are more or less distinctly five angled, and the winter buds cluster at the ends. This in sures a group of young shoots, crowded with leaves, on the ends of branches, and a dense outer dome of foliage on the tree.
Nearly three hundred distinct species of oaks are recog nized by botanists, and the list is growing. New species are in the making. For instance, a white oak and a bur oak grow near enough for the wind to "cross-fertilize" their pistillate flowers. The acorns of such mixed parent age produce trees that differ from both parents, yet reveal characteristics of both. They are "hybrids," and may be called new varieties of either parent. Other species of oak are intercrossing by the same process—the interchange of pollen at the time of blossoming. This proves that the oak family is young, compared with many otheriamilies, whose members are too distantly related to intercross.
Though geologically young, the oak family is one of the most important, furnishing timber of superior strength and durability for bridge-building, ship-building, and other construction work. Tanning has depended largely upon oak bark. As fuel, all oak trees are valuable.
Fifty species of oak are native to North American forests. Twice as many grow east of the Rocky Moun tains as west of the Great Divide. No species naturally passes this barrier. The temperate zone species extend southward into tropical regions, by keeping to high alti tudes. Thus we find American oaks in the Andes and Colombia; Asiatic species occur in the Indian Archi pelago. No Old World species is native to America. Each continent has its own.
East of the Rocky Mountains the oaks hold a place of preeminence among broad-leaved trees. They are trees of large size, and they often attain great age. They are beautiful trees, and therefore highly valued for ornamental planting. This has led to the introduction of oaks from other countries. We have set European, Japanese, and Siberian oaks in our finest parks. Europe has borrowed from our woods the red oak and many others. All coun tries are richer by this horticultural exchange of trees.
Our native oaks fall into two groups: the annual-fruit ing and the biennial-fruiting species. The first group matures its acorns in a single season; the second requires two seasons. It happens that annuals have leaves with rounded lobes, while biennials have leaves with lobes that end in angles and bristly tips. The bark of the annual trees is generally pale; that of the biennials, dark. Hence the white oak group and the black oak group may be easily distinguished at a glance, by the bark, the leaf, and the acorn crop.