A family of trees with simple, alternate leaves, often asymmetrical at the base, and with small, usually hermaphrodite flowers in dense clusters on the twigs. The flowers have a 4–8-lobed perianth with imbricate lobes, 4–8 stamens opposite the perianth lobes, and an ovary of 2 fused carpels. They are wind-pollinated. The tiny, dry or fleshy fruits are often winged. Modern classifications recognize some 16 genera, with about 140 species, mostly in the northern temperate zone.
Subjects: Plant Sciences and Forestry.