Neotropical Juncaceae

Henrik Balslev

Aarhus University, Denmark. 

Description

Perennial and usually rhizomatous or rarely annual herbs, glabrous or with hairy leaf margins. Rhizome creeping, ascending or erect, unbranched or branched, naked or covered by scales or splitting leaf bases. Culms erect, ascending or rarely procumbent, terete (rarely flattened, J. ensifolius Wikstr.), smooth or longitudinally ridged, naked or leaf bearing. Leaves linear or filiform, with an open and sometimes auriculate or closed sheath, spirally arranged or rarely distichous, blades sometimes reduced. Inflorescence terminal, sometimes pseudolateral, compound, cymose or racemose, usually in heads or spike-like clusters, rarely reduced to a single terminal or lateral flower. Flowers generally small, tepals <8mm long, perfect or imperfect, actinomorphic. Perianth segments 6 in 2 whorls of 3, glumaceous, equal or almost equal, free. Stamens 6 in 2 whorls of 3, opposite the perianth segments, inner whorl sometimes reduced; filaments filiform or somewhat flattened and widened at base; anthers oblong to linear, basifixed, obtuse or mucronate, dehiscing by 2 longitudinal lateral slits. Carpels 3, connate; ovary superior, 1-locular, 3-septate or 3-locular; style 1, up to 10 mm long but usually shorter; stigmas 3, terete and of equal diameter throughout or sometimes tapering distally, twining with adaxial papillae; fruit an orbicular to oblong or ellipsoidal, 3-lobed, round or trigonous, loculicidal or circumscissile capsule. Seeds usully many per capsule, sometimes 3; outer seed coat hyaline, whitish or light brown, sometimes drawn out into tail-like appendages, often with distinct sculpturing; inner seed coat brown to castaneous or yellow.

Notes on delimitation

  • The complete trimerous, pentacyclic flowers and the capsular fruit delimits Juncaceae morphologically from other glumiflorous families such as Poaceae and Cyperaceae and the grass-like habit and internal anatomy separates it from the related Thurniaceae.
  • Molecular data suggest that Juncaceae and Cyperaceae are sister taxa and that together they are sister to Thurniaceae (Jones 2005).

Distribution in the Neotropics

  • Distichia Nees & Meyen in the Andes from Colombia to northernn Chile/Argentina.
  • Luzula DC. and Juncus L. mostly in the temperate zone and at high elevations in the tropical zone.
  • Oxychloe Phil. in the Andes from Patagonia to Peru.
  • Patosia Buchenau in the Andes from Bolivia to northern Chile/Argentina.
  • Rostkovia Desv. in southernmost South America and Ecuador.

Distinguishing characters (always present)

  • Trimerous (very rarely dimerous), pentacyclic (rarely tetracyclic) flowers.
  • Glumaceous tepals.
  • Tricarpelate (very rarely bicarpelate) ovary.
  • Capsule.
  • 3 (Luzula) or more seeds.

Other important characters

  • Grass-like.
  • Smooth herbs.
  • Without silica.

Key differences from similar families

Juncaceae are grass-like herbs with herbaceous culms, linear leaves, and usually many flowered inflorescences of small glumaceous flowers - just like Poaceae and Cyperaceae. Juncaceae differ from both these families in having:

  • Smooth leaves and culms without silica encrustations.
  • Complete whorls of 3+3 tepals.
  • Usually 3+3 stamens.
  • Gynoecium of 3 carpels.
  • Fruit with at least 3 seeds and usually many more.

Number of genera

Five:

  • Distichia
  • Juncus
  • Luzula
  • Oxychloe
  • Patosia
  • Rostkovia

Useful tips for generic identification

  • Distichia - cushion forming, leaves distichous, flowers unisexual.
  • Juncus - grass-like, smooth, many-flowered culms, many seeded capsules.
  • Luzula - grass-like, hairy leaf margins, many-flowered culms, 3-seeded capsules.
  • Oxychloe - cushion forming, leaves prickly, flowers with two bracteoles.
  • Patosia -  cushion forming, leaf margin finely serrate, flowers with one bracteole.
  • Rostkovia - grass-like, single-flowered culms, capsule orbicular and many seeded.

Status

  • Juncaceae are native in alpine meadows and in grasslands, especially in the highlands, where they may be weedy. Some are endemic to the Andes (all Distichia, Oxychloe, Patosia and species of Juncus and  Luzula), whereas other are widespread, often temperate species.

Important literature

Balslev, H. 1996. Juncaceae. Flora Neotropica Monograph 68: 1-168.

Jones, E. et al. 2005 The Juncaceae-Cyperaceae interface: a combined plastid gene analysis. In J. T. Columbus et al. (eds). Monocots: Comparative Biology and Evolution, 2 vols. Rancho Santa Ana Botanic Garden, Claremont, CA.

Kirschner, J. et al. 2002. Juncaceae 1: Roskovia to Luzula, Species Plantarum: Flora of the World Part 6: 1-237.

Kirschner, J. et al. 2002. Juncaceae 1: Juncus subg. Juncus, Species Plantarum: Flora of the World Part 7: 1-336.

Kirschner, J. et al. 2002. Juncaceae 1: Juncus subg. Agathryon, Species Plantarum: Flora of the World Part 8: 1-192.

How to cite

Balslev, H. (2009). Neotropical Juncaceae. 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/Juncaceae.htm.