This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Deciduous trees or shrubs with simple alternate stipulate leaves and monoecious flowers in catkins. Perianth none or bract-like. Flowers 2 or 3 together at the base of the bracts of the catkin. Stamens 2 to 5, with forked filaments and divergent anther-cells, or simple with connate anther-cells. Ovary 2-celled; cells 1-ovulate. Fruit a dry compressed lenticular often winged indehiscent nut, by abortion 1-celled and 1-seeded; seed pendulous. This order is limited to the two following genera, whose species are estimated at about thirty-five. They are scattered over the north temperate zone and the mountains of South America.