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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