Neotropical Urticaceae

Description

Herbs, shrubs, trees or lianas, monoecious or dioecious. Stems with or without stinging hairs, rarely with spines, occasionally releasing watery latex when cut. Leaves alternate or opposite, blades simple, frequently toothed or entire, rarely lobed; 3-veined rarely pinnately veined; upper and or lower surface of leaves with cystoliths embedded below epidermis, visible only once leaf has been dried; stipules one or two at each node or absent, borne in the axils of leaves or adjacent to petiole, free or fused, where fused simple or forked. Inflorescences axillary, rarely terminal, paniculate, cymose, racemose or spicate, frequently composed of glomerules, bracteate, bracts discrete or completely enclosing the flowers.  Flowers unisexual or bisexual, where bisexual unisexual flowers also present, with or without a perianth, where present perianth a single whorl of tepals. Staminate flowers 2-5-merous, flowers actinomorphic, tepals free, valvate or imbricate in bud, frequently with a subapical appendage, frequently opening explosively; stamen number equal to tepal number; filaments frequently inflexed in bud; anthers 2-locular, opening lengthwise; pollen mono- or polycolporate; a single pistillode present; pistillate flowers 1-4-merous, zygomorphic or actinomorphic; tepals, where present, free or fused and connate; staminodes present or absent, opposite tepals, where present frequently inflexed; ovary 1-locular, ovule solitary; style simple, or absent; stigma capitate, penicillate, subulate or filiform. Fruit usually a dry achene or a fleshy drupe, perianth frequently persistent and enlarged in fruit. Seed with endosperm; embryo straight; cotyledons ovate elliptical or orbicular. 

Notes on delimitation

• Urticalean Rosid, sister to Moraceae (Sytsma et al. 2002). Where Poikilospermum Zipp. ex Miq. is ascribed to the Cecropiaceae there is strong evidence for the inclusion of Cecropiaceae within a monophyletic Urticaceae (Sytsma et al., 2002; Datwyler & Weiblen, 2004; Monro, 2006). Where Poikilospermum is excluded and ascribed to the Urticaceae, however, the relationship between the two groups is ambiguous (pers. obs.).

Distribution in the Neotropics

• The family is cosmopolitan with c. 49 genera, 2,000 spp. in total; and 16 genera, c. 450 spp. in the Neotropics.

Distinguishing characters (always present)

• Leaves with cystoliths, stipules, inflorescences bracteate, perianth reduced to a single whorl of tepals.

Other important characters

• Stigma almost always solitary.

Key differences from similar families

• Ulmaceae: gynoecium 2-3 carpelled in flower (vs. 1), ovule apical (vs. basal);  anthers not dehiscing explosively (vs. dehiscing explosively).
• Moraceae: gynoecium 2-3 carpelled in flower (rarely fruit) (vs. 1), ovule apical (vs. basal); anthers not dehiscing explosively (vs. dehiscing explosively).

Number of genera

16 genera in the Neotropics

• Boehmeria Jacq.
• Cecropia Loefl.
• Coussapoa Aubl.
• Discocnide Chew.
• Hemistylus Benth.
• Gyrotaenia Griseb.
• Laportea Gaudich.
• Myriocarpa Benth.
• Parietaria L.
• Pilea Lindl. (incl. Sarcopilea Urb.)
• Phenax Wedd.
• Pourouma Aubl.
• Pouzolzia Gaudich.
• Rousselia Gaudich.
• Urera Gauduch.
• Urtica L.

Useful tips for generic identification

Key to genera of Neotropical Urticaceae and Cecropiaceae.

1. Leaves opposite, occasionally strongly unequal at each node and appearing alternate … 2
1. Leaves alternate … 4

2. Stems and leaves with stinging hairs; stipules forked; stigma capitate … Urtica
2. Stems and leaves without stinging hairs; stipules not forked; stigma capitate or threadlike … 3

3. Herb, shrub, small tree or vine; leaves never succulent, with punctiform cystoliths; stipules narrowly ovate with a length to width ratio > 1:4; stipules free, lateral; stigma thread-like … Boehmeria
3. Herbs, epiphytes or shrublets; leaves always succulent, with fusiform, 'V' or 'X' shaped cystoliths; stipules fused, intrapetiolar, deltate, cordiform, auriculate, ovate, obovate or oblong with a length to width ratio < 1:4; stigma capitate … Pilea

4. Stipule scars prominent and completely encircling the stem; trees or shrubs, rarely a vine; leaves > 50 mm, entire, 3-lobed, palmate or compound pinnatifid, or palmatifid; secondary and third order venation consistently parallel, tertiary veins spaced < 1 mm apart; where leaves weakly incised, a tree … 5
4. Stipule scars not encircling the stem, prominent or not; herbs, and or epiphytes, shrubs and or vines or trees; leaves 2-400 mm, entire, rarely lobed, never palmate, compound pinnatifid or palmatifid; the secondary and third order venation parallel or not, where parallel not consistently so, where leaves > 50 mm, tertiary veins spaced > 1 mm apart … 8

5. Leaves palmately lobed or compound pinnatifid, or palmatifid … 6
5. Leaves entire or lobed, where lobed, entire leaves also frequently found on the same branch … 7

6. Flowers borne in clusters on a cymose inflorescence; seeds > 5 mm diameter … Pourouma
6. Flowers borne in compact racemes borne on a cymose inflorescence; seeds < 2 mm diameter … Cecropia

7. Leaves lobed, although entire leaves also found on the same plant; flowers borne in clusters on a cymose inflorescence … Pourouma
7. Leaves entire, never lobed; flowers borne in capitate globose heads borne on a cymose inflorescence … Coussapoa

8. Inflorescences strongly congested; flowers obscured by numerous or prominent bracteoles … 9
8. Inflorescences not strongly congested, although flowers can be borne in compact glomerules or on a few-branched spike-like panicle; flowers not obscured by prominent bracteoles even though frequently very small … 15

9. Pistillate inflorescences or portions of inflorescence with 2-4 prominent green bracts and bracteoles …10
9. Pistillate inflorescences with >10 brown, grey-brown, brown or green-brown bracts and bracteoles … 12

10. Shrubs; leaf margins dentate or serrate … Hemistylus
10. Herbs; leaf margins entire … 11

11. Inflorescences enveloped by two prominent cordiform bracts, which completely obscure the flowers … Rousselia
11. Inflorescences subtended by 3-4 linear bracts, which do not completely obscure the flowers … Parietaria

12. Leaf margins entire … Pouzolzia
12. Leaf margins dentate, serrate or crenate for some part of its length … 13

13. Leaves not strongly unequal-sized at consecutive nodes; pistillate perianth tube absent, bracteoles conspicuous, glossy, much longer than the pistillate flowers … Phenax
13. Leaves of unequal sized at consecutive nodes or not; pistillate perianth tube present, bracteoles inconspicuous, papery less than half length of pistillate flowers … 14

14. Leaves of almost equal size at consecutive nodes or, where of unequal size, not consistently so; achene easily released from the perianth in fruit, shiny … Pouzolzia (P. parasitica)
14. Leaves of strongly unequal size at consecutive nodes; achene not easily released from the perianth in fruit, not shiny … 15

15. Upper leaf surface with punctiform cystoliths … 16
15. Upper leaf surface with fusiform cystoliths … 17

16. Tree > 3 m; young stems and leaves with stinging hairs, leaves clustered at the stem apices; inflorescences unisexual, without foliar bracts; the achene/ fruit a papery disc, 3.5-4.0 mm … Discocnide
16. Shrub < 3 m; young stems and leaves without stinging hairs; leaves not clustered at leaf apices; inflorescences bisexual or unisexual, where unisexual foliar bracts present or absent; achene/ fruit subcompressed ovoid, 1.5-3.0 mm, never papery … Hemistylus

17. Young stems and leaves with stinging hairs; stipules forked for 1/2 or more of length … Laportea
17. Young stems and leaves with or without stinging hairs; stipules forked or not forked, where forked, divided for 1/3 or less of length … 18

18. Stipules forked … Urera
18. Stipules not forked … 19

19. Pistillate inflorescences pendulous, thread-like; stigma foot-shaped; achene in fruit dry, not surrounded by a fleshy tepal or panicle branch; pistillate inflorescences with flowers borne singly along peduncle; plants without stinging hairs; stems without spines … Myriocarpa
19. Pistillate inflorescences erect, not thread-like; stigma penicillate; staminate inflorescence with flowers borne in clusters of 3-10 … 20

20. Plants without stinging hairs; pistillate infructescence branches becoming fleshy in fruit giving the appearance of a spadix, tepals reduced covering < Ό the achene length … Gyrotaenia
20. Plants with or without stinging hairs; pistillate infructescence not becoming fleshy, tepals becoming fleshy, covering > ½ achene length giving appearance of a berry … Urera

Notable genera and distinguishing features

• Cecropia: trees, 'pioneer species' characteristic of disturbed forest, trunks frequently with stilt or buttress roots, stems hollow, hosting extensive ant colonies and bearing nodes encircled by prominent stipule scars, leaves palmately lobed or compound.
• Pilea: understory herbs, epiphytes or shrublets, opposite succulent leaves with intrapetiolar stipules.
• Urera: shrubs, small trees and lianas of riparian and disturbed vegetation, occasionally with stinging hairs on the leaves and inflorescence, fruit fleshy and brightly coloured or white.
• Myriocarpa: small trees of riparian and disturbed vegetation, inflorescences few-branched spikes, often long and pendent.

Status

• Native, endemic, cultivated.

General notes

• A poorly studied family where little is known about generic relationships and where the monophyly of most genera remains to be tested. Understanding the floral anatomy, especially with regards to the hypanthium, also needs much work. In addition Andean Pilea is in great need of revision where a large number of new species (c. 50-100) await description.
• Many of the characters used in the key to genera are very small and it may be necessary to make measurements +/- 0.1 mm. A dissecting microscope (to X200) is therefore recommended.  The terminology used for the description of leaf shape is based on that in Stearn's Botanical Latin (1992).  Because of their small size and large number, it is usually fairly easy to find well-preserved flowers in all genera of Urticaceae.

Important literature

Friis, I. 1989. The Urticaceae: a systematic review. In: P.R. Crane & S. Blackmore (eds), Evolution, systematics, and fossil history of the Hamamelidae, vol. 2, Systematics Association Special Volume 40B. pp.285 - 308.  Oxford Science Publications, Oxford.

Sytsma, K.J., J. Morawetz, J.C. Pires, M. Nepokroeff, E. Conti, M. Zjhra, J.C. Hall, & M.W. Chase. 2002. Urticalean rosids: circumscription, rosid ancestry, and phylogenetics based on rbcL, trnL-F, and ndhF sequences. American Journal of Botany 89: 1531-1546.

Weddell, H.A. 1869. Urticaceae. Pp. 32-23564 in De Candolle, A., Prodromus, systematis naturalis regni vegetabilis. 16(1). Victoris Masson et Filii. Paris.

How to cite

Monro, A. (2009). Neotropical Urticaceae. In: Milliken, W., Klitgεrd, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Urticaceae.htm.