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Herbs, subshrubs, or shrubs, rarely trees, very rarely climbing, stems often fibrous, sometimes succulent. sometimes armed with stinging hairs; epidermal cells of leaves, sometimes stems, perianths mostly with prominent cystoliths punctiform to linear; Leaves alternate or opposite, stipules present, rarely absent; leaf blade simple. Inflorescences cymose, paniculate, racemose, spicate, or cluster-capitate, usually formed from glomerules, sometimes crowded on common enlarged cuplike or discoid receptacle, rarely reduced into a single flower. Flowers unisexual (plants monoecious or dioecious), rarely bisexual in partial flowers; actinomorphic, very small, (1-)4-or 5-merous, rarely perianth absent in female flowers. Calyx absent. Perianth lobes imbricate or valvate. Male flowers: stamens as many as and opposite to perianth lobes, filaments inflexed in bud; anthers 2-locular, opening lengthwise, rudimentary ovary often present. Female flowers: perianth lobes free or connate, usually enlarged in fruit and persistent, occasionally absent; staminodes scarious, opposite to the perianth lobes, or absent. Ovary rudimentary in male flowers, sessile or shortly stipitate, free or adnate to the perianth; 1-locular, ovule solitary, erect from the base; style simple, or absent; stigma diverse, capitate, penicillate-capitate (brushlike), subulate, filiform, ligulate, or peltate. Fruit usually a dry achene, sometimes a fleshy drupe, often enclosed by the persistent perianth. Seed solitary, endosperm usually present; embryo straight; cotyledons ovate elliptical or orbicular.
Herbs or small shrubs [lianas, trees], herbs annual or rhizomatous perennial, usually pubescent, sometimes with stinging hairs, deciduous. Leaves opposite or alternate and spirally arranged, simple; stipules present or absent; petioles present. Leaf blades paired, equal in size (except in Pilea , which may have unequally paired leaves), dotted with linear or rounded marks formed by cystoliths (variously shaped calcium carbonate crystals inside epidermal cells). Inflorescences axillary or terminal, of paniculately or racemosely arranged cymes, or spikelike. Flowers bisexual or unisexual (staminate or pistillate), staminate and pistillate flowers on same or different plants; perianth hypogynous. Staminate flowers usually pedicellate; tepals 4-5, white or green; stamens 4-5, equaling tepals in number; filaments inflexed in bud, reflexing suddenly as flowers open; anthers basifixed, dehiscing by longitudinal slits; pollen ejected explosively; pistillode 1. Pistillate flowers usually sessile; tepals 2-4, hypogynous, greenish or reddish, distinct or connate; staminodes present or absent; pistil1, 1-locular; placentation basal; ovule1; style present or stigma sessile; stigma linear [capitate]; Bisexual flowers: tepals 4; stamens4; pistil1. Fruits achenes, free or loosely or tightly surrounded by persistent, accrescent perianth.
Herbs, sometimes epiphytic, shrubs, climbers or trees, occasionally with stinging hairs. Leaves simple, rarely radially lobed, petiolate, stipulate, mostly with cystoliths. Inflorescence axillary, monoecious or dioecious, cymose, spicate or capitate, rarely enveloped by a caducous spathe. Flowers small, unisexual, rarely bisexual; perianth 1-whorled; tepals usually 3–5, occasionally 1, 2 or absent, free or connate, imbricate or valvate, persistent. Male flowers: stamens usually 3–5, opposite tepals, or reduced to 2 or 1; filaments inflexed in bud, rarely straight; anthers bilobed; pistillode usually present. Female flowers: ovary superior, rarely immersed in sockets in the inflorescence, sometimes adnate to calyx, usually 1-locular; ovule 1, basal, orthotropous; style 1; stigma 1, simple; staminodes sometimes present. Fruit an achene, rarely a fleshy drupe. Seed with endosperm; embryo straight.
Female flowers with 3–5 tepals, which are free or united, often very unequal, often accrescent after pollination, rarely absent; staminodes, when present, scale-like, inflexed (sometimes actively ejecting the achene when ripe); ovary superior, syncarpous, erect, usually somewhat laterally compressed, symmetrical or asymmetrical, glabrous, 1-celled, with 1 erect basal ovule (placenta absent or very small); style usually absent or very short; stigma capitate, brush-like, or linear
Leaves alternate or opposite, petiolate or sessile, sometimes the opposite leaves are anisophyllous with each of a pair of leaves unequal in shape and/or size; stipules usually present, lateral or often intrapetiolar, often fused; lamina simple to deeply 3–5-lobed, rarely 7-lobed, margin entire, serrate or dentate, usually strongly 3-nerved from the base with much smaller lateral nerves from the midnerve, or sometimes evenly penninerved
Stinging hairs are large, unicellular and have a calcified wall, bulbous tip, and a soft-walled, sack-like lower part; the bulbous tip is easily detached, liberating an irritating fluid from the sack-like part; the sack-like base of the stinging hairs is embedded in, or occasionally mounted on, a small epidermal protuberance on stems, petioles or inflorescences, see Tab. 23
Annual or perennial herbs, shrubs, lianas, or small trees with soft wood; monoecious or dioecious (rarely polygamous); stinging hairs are present in some genera while stiff, non-stinging hairs (without bulbous tip, sack-like base and irritating fluid) are frequent
Male flowers with (1)2–5 tepals; stamens equal in number and opposite the tepals or solitary, inflexed in bud until reflexing suddenly to forcibly eject the pollen; anthers 2-thecous opening by longitudinal slits; rudimentary ovary usually present
Flowers minute, unisexual, actinomorphic or (especially the female flowers) zygomorphic, with one whorl of tepals or rarely the female flowers naked, sessile or pedicellate, pedicel often articulated just below the perianth
Inflorescences are extremely varied, mostly cymose, often of densely clustered flowers, and subtended by involucral bracts; occasionally the inflorescence axis is contracted into a flattened, disk-shaped, fleshy receptacle
Herbs, undershrubs, or rarely soft-wooded trees, rarely climbing, often armed with stinging hairs; epidermal cells mostly with prominent cystoliths; stems often fibrous
Cystoliths (incrustations of calcium or cellulosis most easily visible in dried material) are generally present in the epidermal cells, dot-like, elongated, or linear
Seed with a thin membranaceous testa, usually not fused to the endocarp, mostly with little or no endosperm, and the cotyledons correspondingly swollen and fleshy
Fruit an achene, consisting of the hardened ovary-wall, often enclosed by the persistent accrescent perianth
Flowers very small, unisexual, usually cymose, sometimes crowded on a common enlarged receptacle
Stamens as many as and opposite the calyx-lobes; filaments inflexed in bud; anthers 2-celled
Ovary free or adnate to the calyx, 1-celled; style simple; ovule solitary, erect
Leaves alternate or opposite, simple; stipules present or absent
Male flowers with a 4–5-lobed calyx, lobes imbricate or valvate
Female calyx like that of the male, often enlarged in fruit
Seed mostly with endosperm and a straight embryo
Fruit a dry achene or fleshy drupe
Rudimentary ovary usually present
Staminodes scale-like or absent
Petals absent
