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The families of flowering plants

L. Watson and M.J. Dallwitz

Urticaceae Juss.

Habit and leaf form. Shrubs, lianas, and herbs, or trees (a few); laticiferous, or non-laticiferous, without coloured juice. ‘Normal’ plants. Without conspicuous aggregations of leaves. Self supporting, or climbing. Mesophytic. Conspicuously heterophyllous (e.g. Urtica pilea), or not heterophyllous (mostly). Leaves small to large; alternate, or opposite; when alternate, spiral; petiolate (usually), or sessile; non-sheathing; simple. Lamina entire (usually), or dissected (rarely); conspicuously asymmetric (sometimes), or not conspicuously asymmetric; when dissected, palmatifid; pinnately veined, or palmately veined; cross-venulate. Leaves stipulate (usually), or exstipulate (e.g. Parietaria). Stipules interpetiolar, or intrapetiolar; free of one another, or concrescent. Lamina margins entire, or serrate, or dentate. Leaf development not ‘graminaceous’. Domatia occurring in the family (recorded in two genera); manifested as pockets.

General anatomy. Plants with laticifers (non-articulated and unbranched, e.g. in Urtica), or without laticifers (usually?).

Leaf anatomy. The leaf lamina dorsiventral. Leaves with ‘pearl glands’ (sometimes, some Urtica species), or without ‘pearl glands’. Stomata anomocytic, or anisocytic. Hairs present; eglandular and glandular (in addition to stinging hairs, q.v.; the eglandular hairs mostly unicellular, sometimes bracket-shaped or hooked, occasionally absent; the glandular ones mostly comprising few-celled heads on unicellular stalks). Urticating hairs present, or absent (notoriously common in the Urticeae, generally silicified and brittle and skin-piercing towards the apex and discharging the irritant contents when broken: see illustration), or absent. Lamina with secretory cavities (sometimes), or without secretory cavities. Secretory cavities when present, containing mucilage. Cystoliths usually present (these variously spherical, ellipsoidal, ovoid or fusiform). The mesophyll containing mucilage cells (commonly), or not containing mucilage cells; containing crystals (generally common), or without crystals. The crystals mostly druses. Minor leaf veins without phloem transfer cells (4 genera).

Axial (stem, wood) anatomy. Young stems often tetragonal. Secretory cavities present, or absent; when present, with mucilage, or with latex. Cork cambium present; initially deep-seated, or initially superficial (usually). Nodes tri-lacunar. Primary vascular tissues in a cylinder, without separate bundles; collateral (neary always), or bicollateral (in Myriocarpa). Internal phloem present (Myriocarpa), or absent. Secondary thickening developing from a conventional cambial ring (usually), or anomalous (in Myriocarpa). The anomalous secondary thickening from a single cambial ring. Primary medullary rays narrow.

The wood diffuse porous. The vessels small to large (but usually medium sized); solitary, radially paired, and in radial multiples (but often predominantly solitary). The vessel end-walls simple. The vessels without vestured pits; with spiral thickening, or without spiral thickening. The axial xylem without fibre tracheids; with libriform fibres (the fibre pitting typically simple, but occasionally with very small borders); including septate fibres (commonly), or without septate fibres. The fibres without spiral thickening. ‘Included’ phloem present (Myriocarpa), or absent. The wood storied, or partially storied.

Reproductive type, pollination. Unisexual flowers present. Plants monoecious, or dioecious, or polygamomonoecious. Female flowers with staminodes (these scalelike), or without staminodes. Gynoecium of male flowers vestigial (usually), or absent. Pollination anemophilous; mechanism conspicuously specialized (the filaments reflexing violently).

Inflorescence, floral, fruit and seed morphology. Flowers solitary (rarely), or aggregated in ‘inflorescences’. The ultimate inflorescence units cymose. Inflorescences axillary; in loose, glomerate, spiked, racemed, panicled or capitate cymes, the flowers often condensed into heads etc., or sometimes crowded on a common receptacle which may be concave or convex; with involucral bracts, or without involucral bracts. Flowers bracteate, or ebracteate; minute, or small; regular; (3–)4–5(–6) merous. Hypogynous disk absent.

Perianth sepaline, or vestigial to absent (sometimes, in female flowers); (2–)4–5(–6); free, or joined; 1 whorled; persistent; accrescent (often), or non-accrescent. Calyx (the perianth being thus interpreted) (2–)4–5(–6) (in males), or 3–5 (when present, in females); 1 whorled; polysepalous, or gamosepalous; regular; persistent; accrescent, or non-accrescent; imbricate, or valvate.

Androecium (2–)4–5(–6). Androecial members free of the perianth; free of one another; 1 whorled. Androecium exclusively of fertile stamens. Stamens (3–)4–5(–6); isomerous with the perianth; oppositisepalous; inflexed in bud (uncoiling elastically); filantherous (the filaments usually wrinkled). Anthers dorsifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or decussate. Pollen grains aperturate; 2 aperturate, or 3 aperturate, or 7–15 aperturate; porate, or foraminate; 2-celled (in Parietaria, Pellionia and Urtica).

Gynoecium ostensibly 1 carpelled (i.e. with no obvious evidence of more than one carpel). Carpels reduced in number relative to the perianth. The pistil 1 celled. Gynoecium monomerous (ostensibly), or syncarpous (supposedly, theoretically); of one carpel (at least, usually with no evidence of syncarpy), or synstylovarious to eu-syncarpous (theoretically); superior (usually), or partly inferior (Pipturus, etc.). Carpel (if treated as monomeric) shortly stylate, or non-stylate; apically stigmatic; 1 ovuled. Placentation basal. Ovary if recognised as syncarpous, 1 locular. Gynoecium stylate, or non-stylate to stylate. Styles 1. Stigmas 1 (usually), or 2 (e.g. Phenax); dry type; papillate; Group II type. Placentation if recognised as syncarpous, basal. Ovules in the single cavity 1; funicled, or sessile; ascending; non-arillate; orthotropous to hemianatropous; bitegmic; crassinucellate. Outer integument contributing to the micropyle, or not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; proliferating, or not proliferating. Synergids pear-shaped. Hypostase present (in Urtica). Endosperm formation nuclear. Endosperm haustoria present; chalazal (short, in Urtica). Embryogeny asterad.

Fruit fleshy, or non-fleshy. The fruiting carpel (treated as monomeric) indehiscent; an achene, or nucular, or drupaceous. Fruit if recognised as syncarpous, indehiscent; achene-like, or a nut, or a drupe (rarely). The drupes with one stone. Fruit 1 seeded. Seeds scantily endospermic, or non-endospermic. Endosperm oily, or not oily. Cotyledons 2. Embryo achlorophyllous (2/2); straight.

Physiology, phytochemistry. C3. C3 physiology recorded directly in Parietaria, Pilea. Not cyanogenic. Alkaloids present, or absent. Anthraquinones detected (Boehmeria); polyacetate derived. Arbutin absent. Iridoids not detected. Proanthocyanidins present, or absent; when present, cyanidin. Flavonols present, or absent; when present, kaempferol and quercetin, or quercetin. Ellagic acid absent (8 species, 5 genera). Aluminium accumulation not found. Sieve-tube plastids S-type (without starch).

Geography, cytology. Temperate to tropical. Cosmopolitan except frigid zones. X = 6–14.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Malviflorae; Urticales. Cronquist’s Subclass Hamamelidae; Urticales. APG III core angiosperms; core eudicot; Superorder Rosanae; fabid. APG IV Order Rosales.

Species 550. Genera 49; Aboriella, Achudemia, Archiboehmeria, Astrothalamus, Australina, Boehmeria, Chamabainia, Cypholophus, Debregeasia, Dendrocnide, Didymodoxa, Discocnide, Droguetia, Elatostema, Forsskaolea, Gesnouinia, Gibbsia, Girardinia, Laportea, Lecanthus, Leucosyke, Maoutia, Meniscogyne, Myriocarpa, Nanocnide, Neodistemon, Neraudia, Nothocnide, Obetia, Oreocnide, Parietaria, Pellionia, Petelotiella, Phenax, Pilea, Pipturus, Poulzolzia, Procris, Rousselia, Sarcochlamys, Sarcopilea, Soleirolia, Touchardia, Urera, Urtica.

General remarks. The details of anomalous stem anatomy in Myriocarpa need pursuing.

Economic uses, etc. Commercial cordage fibre (ramie) is obtained from Boehmeria nivea, a few (e.g. Pilea spp.) are grown as novelties, and nettles (Urtica spp.) constitute edible and quite palatable greens.


Why, look you, I am whipped and scourged with rods,
Nettled, and stung with pismires, when I hear
Of this vile politician, Bolingbroke
(‘1st Henry the Fourth’, i., 3 — ‘pismires’ = ants)

Nay, mark, how Lewis stamps as he were nettled;
I hope all’s for the best
(‘3rd Henry the Sixth’, iii., 3)

Illustrations. • Laportea aestuans: Thonner. • Le Maout and Decaisne: Pilea, Parietaria. • Le Maout and Decaisne: Urtica, Boehmeria. • Dendrocnide longifolia, as Laportea: Hook. Ic. Pl. 26 (1898). • Gonostegia pentandra, as Urtica: R. Wight, Ic. Pl. Indiae Orientalis 2 (1843). • Parietaria judaica (as P. diffusa): Eng. Bot. 1278, 1868. • Parietaria officinalis (B. Ent.). • Procris pedunculata, as P. laevigata: Hook. Ic. Pl. 13 (1877–79). • Urtica dioica, U. urens and U. pilulifera: Eng. Bot. 1279, 1282 and 1280 (1868). • Urtica dioica (B. Ent.). • Urtica pilulifera (B. Ent.). • Urticastrum decumana, as Urtica: R. Wight, Ic. Pl. Indiae Orientalis 2 (1843). • Helxine (= Parietaria). • Obetia tenax, as Urera: Hook. Ic. Pl. 18 (1888). • Stinging leaf hair of Urtica dioica, and epidermis of Pilea serpyllifolia: Solereder, 1908.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 15th April 2018.’.