Neotropical Dioscoreaceae

Description

Rhizomatous perennials, rhizome often short and usually subtending one to several annually to perennially replaced tubers. Stems herbaceous or woody at base, twining (tendrils lacking), rarely erect or absent in acaulescent perennial herbs with a basal rosette of leaves. Leaves opposite to alternate, entire to compound, base often cordate, venation campylodromous (or leaves compound), secondary venation reticulate; petiole usually with a basal and apical pulvinus except in acaulescent species. Inflorescences usually axillary, sometimes borne on a leafless, herbaceous scape, flowers solitary or in panicles, cymes, spikes, or racemes. Flowers trimerous, epigynous, almost all actinomorphic, rarely hermaphrodite, usually unisexual and plants dioecious. Tepals 6. Stamens usually 6. Ovary inferior, 3-locular, rarely 1-locular. Fruit usually capsular, rarely leathery and indehiscent or samaroid; dehiscence loculicidal, irregular or indehiscent. Seeds smooth, flattened and winged, or wingless and either globose and smooth, or with 16-18 longitudinal ridges.

Notes on delimitation

• Dioscoreales. See Angiosperm Phylogeny Website. Older treatments/textbooks suggest Dioscoreaceae are "primitive" monocots among the closest to Dicots. This is not so.
• Most treatments pre-2000 separate Taccaceae, but these are now considered together.

Distribution in the Neotropics

• Dioscorea L. - throughout the Neotropics.
• Tacca parkeri Seem. - Northern South America only.

Distinguishing characters (always present)

• Campylodromous leaf venation (or homologous compound leaves).
• No tendrils.
• Inferior ovary.

Other important characters

• Perennial or annual.
• Usually underground tuber.
• Twining stem (a few erect dwarf Dioscoreas and Tacca parkeri are rosette -forming herbs).
• Basal and apical petiolar pulvini.
• Reticulate secondary leaf  venation.
• Dioecious.
• 3-winged capsule with 3 locules (Dioscorea) or 6-ribbed leathery indehiscent fruit with 1 locule (T. parkeri).

Key differences from similar families

From Smilacaceae:

• No tendrils (tendrils from petiole in Smilacaceae).
• Basal and apical pulvini (none in Smilicaceae). 
• Inflorescence a spike/raceme or derived form (not an umbel).
• Dioecious
• Ovary inferior (not superior).
• Fruit a capsule or leathery/indehiscent (not a berry).

From Menispermaceae:

• Campylodromous (not pinnate) venation.
• Parts in 3s/6s, (Menispermaceae not so).

Closest relative Burmanniaceae is morphologically very reduced and usually heteromycotrophic.

Number of genera

• 2, Dioscorea and Tacca Forst. & Forst. f. (1 rather rare species in Northern South America, T. parkeri).

Useful tips for generic identification

See below

Notable genera and distinguishing features

Dioscorea:

• Possesses a stem.
• Inflorescence with flowers solitary or in panicles, cymes, spikes, or racemes but not a pseudo-umbel.
• Bracts usually small and never dimorphic as in Tacca.
• Flowers usually unisexual.
• Ovary 3-locular.

Tacca:

• Lacks a stem.
• Inflorescence a cymose pseodo-umbel.
• Large foliaceous and pendent filiform bracts.
• Flowers perfect.
• Ovary 1-locular.

Status

• Both genera native.
• Cultivated native (rarely) and non-native Dioscorea also encountered.

Important literature

Barroso, G.M., D. Sucre, E.F. Guimares, L.F de Carvalho, M. Conceicao Valente, J. Dames e Silva, J.B. da Silva, F.R. Timno Rosenthal Cinesio Maximo Barbosa, A.N. Roseira and O.M.B Alda F. Barbosa. 1974. Flora de Guanabara Dioscoreaceae. Sellowia 25: 9-256.

Drenth, E. 1972. A revision of the family Taccaceae. Blumea 20: 367-406.

Kirizawa, M. Flora Fanerogamica da Reserva do Parque Estadual das Fontes do Ipiranga (Sao Paulo, Brasil). 196 - Dioscoreaceae. Hoehnea 9:131-133.

Knuth, R. 1924. Dioscoreaceae. In: Engler H. G. A., ed. Das Pflanzenreich, 87 (IV. 43), Leipzig, 1-387.

Pedralli, G.G. 2002. Levantamento floristico das Dioscoreaceae (R. Br.) Lindley da cadeia do Espinhaco, Minas Gerais e Bahia, Brasil. Bol. Bot. Univ. Sao Paolo 20: 63-119.

Prieto C, A., J.L. Fernandez-Alonso and R.L. Liesner. 2000. Nota sobre la familia Taccaceae  (Liliidae) y su presencia en Colombia. Caldasia 22: 265-270.

Sosa, V., B., G. Schubert, and A. Gomez-Pompa. 1987. Flora de Veracruz: Dioscoreaceae. Xalapa: Instituto Nacional de Investigaciones Sobre Recursos Bioticos.

Tellez V., O. and B. G. Schubert. 1994. Dioscoreaceae. Pp.54-65 in Flora Mesoamericana Volumen 6, Alismataceae a Cyperaceae, eds. G. Davidse, M. Sousa S. and A. O. Chater. Mexico D.F.: Universidad Nacional Autonoma de Mexico.

[Knuth's treatment is still the only regional monograph. The quality of the others is variable; The Flora Mesoamericana treatment is much better than those from Brazil].

How to cite

Wilkin, P. (2009). Neotropical Dioscoreaceae. In: Milliken, W., Klitgård, 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/Dioscoreaceae.htm.