Urticaceae

Non-woody (herbaceous) perennials with rhizomes or annuals, usually hairy, sometimes with stinging hairs, deciduous. Stinging hairs have a swollen, bulb-like or cylindric base and a stiff, nearly transparent tip that breaks off upon contact, acting like a tiny hypodermic needle injecting histamine and other substances that cause pain, a burning sensation and itching. Non-stinging hairs are soft and flexible and are not swollen at the base.

Leaves opposite or alternate and spirally arranged, simple, with stalks; leaf edges with teeth; stipules present or absent. Leaf blades dotted with linear or rounded marks formed by calcium carbonate crystals inside cells lining the leaf surface.

Flowers small, inconspicuous, in leaf axils or at the top of the stem, arranged in cymes resembling panicles, racemes or spikes. Each flower producing either pollen or seeds (rarely both) on the same plant or on different plants. Petals none. Flowers producing only pollen are usually each on a stalk, and have 4-5 white or green sepals; 4-5 stamens (same number as sepals); and a small non-functional pistil that cannot produce seeds. Stalks of stamens are curved inward in bud and straighten as the pollen is ejected explosively. Flowers producing only seeds usually have no stalk; and have 2-4 greenish or reddish sepals that are separate or fused; 1 pistil that has one chamber; ovary located above point of attachment of other flower parts (superior); style 1 or none; may sometimes have small remnants of stamens that do not produce pollen. Flowers producing both pollen and seeds have 4 sepals, 4 stamens, and 1 pistil. Fruits achenes, free or loosely or tightly surrounded sepals that persist and greatly enlarge after flowering.