A popular garden tree in residential areas, noted for its grace and elegance, immediately identifiable at a distance by its white or silvery bark with characteristic pattern of black fractures. Sizable specimens are on the lawn above the south end of Lomita Drive and at 626 Alvarado Row. A variety with deeply lacerated leaves may also be seen, for example at the southeast corner of Salvatierra Street and Coronado Avenue. Two cutleaf weeping birches ('Dalecarlica') planted in 1918 are east of the front lawn of The Knoll (illustration right).
By July or August the female catkins, about 1/4 inch in diameter, are ripening into seed cones. While the cone is ripening there is a sharp-moving boundary between ripe (brown) and unripe (bright strawberry red). If taken to pieces the cones will be found to be composed of scales shaped like a fleur-de-lys. Under each scale are three seeds in the form of tiny nuts with two transparent wings, some plumper than others. The cones exhibit very fine spirals conforming to the same Fibonacci numbers as do pine cones. At the same time similar objects about 1/8 inch in diameter are also present on the tree. These are the male catkins that will live through the winter to produce pollen in the spring. Canoe birch (B. papyrifera) and other American birches are available, but not in the range of horticultural varieties developed in Europe.
For dioecious trees (male and female flowers on separate plants) that make use of the wind to carry pollen from the male to the female organs, cross-pollination occurs freely, provided there is a female tree nearby to receive the pollen. Birches are monoecious (separate male and female flowers on the same plant). It seems strange that monoecious trees that evolved later should have their pollen producing anthers placed so close to the pollen-receiving stigma, given that the idea is to receive pollen from another tree, if the evolutionary consequences described by Darwin are to result. Various temporal, spatial, and other less obvious devices have evolved to work against self-pollination and can be studied in the spring; for example, you can watch a grevillea flower day by day to see how it protects itself.
As a deciduous tree, the silver birch is hard to beat for airiness and beauty. From the Himalayas comes B. jacquemontii, whose pure white trunk is free of dark markings. There are six in the courtyard at the south end of the School of Education, another nearby, and a row at the west facade of the Graduate School of Business.
Illus. right: cutleaf weeping birch 'Dalecarlica'
Illustrations (links open new windows): leaf and catkins |Additions/Revisions:
Name derivation, genus | species Latin name for birch | hanging downRelated material: Canopy Trees for Palo Alto Tree Library