Urticaceae

Urticaceae species can be shrubs (e.g. Pilea), lianas, herbs (e.g. Urtica, Parietaria), or, rarely, trees (Dendrocnide, Cecropia). Their leaves are usually entire and bear stipules. Urticating (stinging) hairs are often present. They have usually unisexual flowers and can be both monoecious or dioecious. They are wind-pollinated. Most disperse their pollen when the stamens are mature and their filaments straighten explosively, a peculiar and conspicuously specialised mechanism.

While the stings delivered by Urticaceae species are often unpleasant, they seldom pose any direct threat to health, and deaths directly attributed to stinging are exceedingly rare; species known to cause human fatalities include Dendrocnide cordata^[4][5] and Urtica ferox.^[6]

The APG II system puts the Urticaceae in the order Rosales, while older systems consider them part of the Urticales, along with Ulmaceae, Moraceae, and Cannabaceae. APG still considers "old" Urticales a monophyletic group, but does not recognise it as an order on its own.

The fossil record of Urticaceae is scattered and mostly based on dispersed fruits. Twelve species based on fossil achenes are known from the Late Cretaceous of Central Europe. Most were assigned to the extant genera Boehmeria (three species), Debregeasia (one species) and Pouzolzia (three species), while three species were assigned to the extinct genus Urticoidea.^[7] A Colombian fossil flora of the Maastrichtian stage has yielded leaves that resemble leaves of the tribe Ceropieae.^[8] In the Cenozoic fossil leaves from the Ypresian Allenby Formation preserve distinct trichomes, and have been attributed to the Tribe Urticeae in the fossil record. The leaves had originally been identified as Rubus by earlier workers on the Eocene Okanagan Highlands, but Devore et al (2020) interpreted the preserved hairs along the stem and major veins as stinging trichomes, rather than simple hairs or thorns.^[9]

Modern molecular phylogenetics suggest the following relationships^[10][11] (see also ^{[12][13][14][15][16][17][18][19][20][21]}):

The Urticaceae are subject to many bacterial, viral, fungal, and nematode parasitic diseases. Among them are:^[23]

• Bacterial leaf spot, caused by Xanthomonas campestris which affects Pellionia, Pilea, and other genera
• Anthracnose, a fungal disease caused by Colletotrichum capsici which affects Pilea
• Myrothecium leaf spot, a fungal disease caused by Myrothecium roridum which affects plants throughout the Urticaceae, as well as other angiosperms^[24][25]
• Phytophthora blight, a water mold disease caused by Phytophthora nicotianae which affects Pilea
• Southern blight, a fungal disease caused by Athelia rolfsii which affects both Pellionia and Pilea

• 
  Pilea cadierei
• 
  Pilea pumila
• 
  Dendrocnide sp.
• 
  Elatostema umbellatum
• 
  Urtica dioica
• 
  Boehmeria nivea
• 
  Parietaria judaica flowers
• 
  Urtica dioica stinging hairs
• 
  Leaves of Dendrocnide meyeniana
• 
  The dotted bumps on the leaves of Urtica thunbergiana

• Pignatti, Sandro (1982). Flora d'Italia (in Italian). Bologna: Edagricole. ISBN 978-88-506-2449-2.
• Friis, Ib (1989). Urticaceae. Flora of tropical East Africa. Rotterdam: A.A. Balkema and the Royal Botanic Gardens, Kew. ISBN 978-90-6191-352-8.

Wikimedia Commons has media related to Urticaceae.

• Stevens, P. F. "Angiosperm Phylogeny Website, Version 13". Missouri Botanical Garden. continuously updated.