The Betulaceae are monoecious trees and shrubs comprising 6 genera and 120 species. The leaves are alternate and simple; the stipules are often deciduous. The male inflorescences are pendulous catkins composed of aggregations of variously simplified 3-flowered cymes, each with a subtending bract and varying numbers of bractlets and 2-18 stamens. A 1-6-parted minute calyx is often present in each flower. The female inflorescences are conelike, erect or pendulous aggregations of 2-3-flowered cymes, each with a subtending bract and varying numbers of bractlets. Each female flower has a single compound 2-carpelled pistil with 2 deeply divided styles and an inferior or nude ovary. A placental septum divides the lower part of the ovary into 2 locules, each containing 2 ovules. The fruit is a 1-seeded indehiscent nut or samara.

Each "thumbnail" image below is linked to a larger photograph.

Alnus nepalensis, alder. This photo shows male inflorescences on the lower left, developing fruit above, and small female inflorescences in the lower center.
Alnus rhombifolia, alder. Note small pinkish cone-like, female inflorescences in upper part of photo and pendulous catkins of male flowers below (top left). This close view of a male inflorescence shows the cymose clustering of male flowers (top right). In the lower photo 3 female inflorescences can be seen. Note the deeply parted pink styles. Green bracts hide the ovaries of the female flowers.
Alnus sp., alder, Mt. Spokane, WA, 2002.
Betula sp., birch. These photos show an old brown male inflorescence and green fruiting female inflorescences. These eventually would shatter into hundreds of winged fruitlets.
Carpinus betulus, European hornbeam, Corvallis, OR, July 2003.
Corylus cornuta, filbert, hazelnut, vic. Corvallis, OR..

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