Nearly three hundred distinct species of oaks are recognized 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 parentage 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 other families, 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 Mountains 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 altitudes. Thus we find American oaks in the Andes and Colombia; Asiatic species occur in the Indian Archipelago. No Old World species is native to America. Each continent has its own.
East of the Rocky Mountains the oaks hold a place of preëminence 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 countries are richer by this horticultural exchange of trees.
Our native oaks fall into two groups: the annual-fruiting 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.
THE WHITE OAK GROUP
The White Oak
Quercus alba, Linn.
The white oak has no rival for first place in the esteem of tree-lover and lumberman. Its broad, rounded dome, sturdy trunk, and strong arms (see illustration, [38]), and its wide-ranging roots enable a solitary tree to resist storms that destroy or maim other kinds. Strength and tenacity in the fibre of root and branch make it possible for individuals to live to a great age, far beyond the two centuries required to bring it to maturity. Such trees stir within us a feeling of reverence and patriotism. They are patriarchs whose struggles typify the pioneer's indomitable resistance to forces that destroyed all but the strong.
White oak trees in the forest grow tall, lose their lower branches early, and lift but a small head to the sun. The logs, quarter-sawed, reveal the broad, gleaming "mirrors" that make a white oak table beautiful. The botanist calls these the medullary rays—thin, irregular plates of tissue-building cells, that extend out from the central pith, sometimes quite to the sap-wood, crowding between the wood fibres, which in the heart-wood are no longer alive. A slab will show only an edge of these mirrors. But any section from bark to pith will reveal them.