The stinging hairs that bring some members of this family forcibly to our attention each consist of a long slender cell containing the irritating substance in a sac-like base. The bulbous tip of the hair is broken off upon contact, leaving a sharp point through which the irritant is injected when slight pressure on the hair compresses the sac at the base.

1. Leaves alternate.

2. Plant without stinging hairs (but not necessarily glabrous); leaves entire; tepals of pistillate flowers equal, fused at the base; style essentially absent, remaining ± central and terminal as the fruit matures, the stigma a deciduous tuft of hairs; achene very shiny and glossy, terete.

Parietaria

2. Plant with stinging hairs; leaves prominently toothed; tepals of pistillate flowers separate, becoming unequal in fruit (2 larger, 2 smaller); style and elongate persistent stigma linear-subulate, becoming lateral as the fruit matures; achene ± dull, usually speckled until fully ripe, strongly compressed.

Laportea

1. Leaves opposite.

3. Plants usually 1 m or more tall, with stinging hairs; tepals of pistillate flowers 4, separate, becoming unequal (2 larger, 2 smaller), ciliate to densely hispid with straight to curved bristles; stigma a deciduous tuft of hairs.

Urtica

3. Plants less than 1 m tall, without stinging hairs (pubescence, if any, minute and of hooked or ± appressed hairs); tepals of pistillate flowers 3, glabrous, and separate or completely united and bearing some hooked bristles; stigma linear.

4. Flowers in dense (± interrupted) elongate unbranched cylindrical spike-like inflorescences; achene (somewhat fleshy) completely enclosed by the perigynium-like perianth, which bears mixed straight and hooked bristles.

Boehmeria

4. Flowers in loose spreading branched clusters; achene evident, not hidden by the separate glabrous tepals.

Pilea