Neotropical Hydrocharitaceae

Anna Haigh

Royal Botanic Gardens, Kew, UK. 

Description

Habit: aquatic herbs, sometimes emergent, stems stoloniferous or erect, corms often present when stems stoloniferous. Leaves basal, alternate, opposite or whorled, simple, sessile or petiolate; basal sheath present, distinct from blade or petiole, tubular; blades linear or expanded; aerenchyma present in abaxial part of leaf (Hydrocharis L. and Limnobium Rich.). Inflorescences often complex, of 1-many flowers, sessile in leaf axils or terminal on long scapes, subtended by 1-2 bracts, the scapes occasionally with up to 10 longitudinal ridges or wings. Flowers actinomorphic, bisexual (often cleistogamous) or unisexual (plants dioecious), sessile or pedicellate (staminate flowers) or mostly sessile (pistillate flowers), the pedicels often long and filiform in staminate flowers (the elongated hypanthium often gives impression of pedicel in pistillate flowers); perianth often fused to form hypanthium; sepals (2)3; petals (absent or 2)3; androecium of 12-13 stamens (when present), the stamens in several whorls of mostly 3, the filaments distinct or united, the anthers basifixed or dorsifixed; gynoecium syncarpous, the ovary inferior (if present), the carpels 3-20+, the locules 1 or falsely 6-9, the styles 1-9, sometimes bifid, the stigmas linear, dry, papillose; placentation laminar. Fruits berry -like, dehiscing irregularly or breaking up at maturity. Seeds 3-numerous, ellipsoid to cylindric or fusiform.

Notes on delimitation

  • Placed in the Alismatales, they appear to be most closely related to the Alismataceae and Limnocharitaceae.
  • Here, following APG II, the previously separate Najadaceae has been included in the Hydrocharitaceae.

Distribution in the Neotropics

A cosmopolitan family of 18 genera and c.115 species. 10 genera comprising 21 species occur in the Neotropics.

  • Apalanthe Planchon:  monospecific (A. granatensis Kunth) Planchon in fresh waters of tropical South America.
  • Egeria Planchon:  two species in subtropical and temperate fresh waters in South America (E. densa Planchon - almost cosmopolitan due to its invasive nature).
  • Elodea Michx:  five species in the Americas, two of which are widespread in the fresh waters of the Andes and temperate South America.
  • Halophila Thouars:  ca. 10 species in the warm marine areas of the world, 3 species are native to the Neotropics.
  • Hydrilla Rich:  monospecific (Hydrilla verticillata (L.f.) Royle is native to the old world but an invasive and naturalized in the fresh waters of the Americas).
  • Limnobium Rich.: 2 species in tropical and temperate American fresh waters, 1 of these in the Neotropics.
  • Najas L: about 40 species in fresh waters worldwide, seven species are native to the Neotropics, and Najas graminea Delile - naturalized.
  • Ottelia Persson:  21 species of which one is native to the fresh waters of the Neotropics.
  • Thalassia Banks ex König:  two species, one of which is native to the salt waters of the Caribbean.
  • Vallisneria L: six species of which one is native to the Neotropical fresh waters.

Distinguishing characters (always present)

Other important characters

  • Fruits berry -like.
  • Sepals and petals usually 3.
  • Ovary inferior.
  • Inflorescences subtended by 1-2 bracts.

Number of genera

See above

Notable genera and distinguishing features

  • Najas is the largest genus in the Neotropics with 7 species, sometimes armed with prickles on internodes, leaf margins usually serrate to minutely serrulate.

Status

  • Native. Two species Hydrilla verticilata and Najas graminea are naturalised.

General notes

  • Hydrocharitaceae occupy streams, rivers, lakes, bays and oceans.
  • Two Neotropical genera are strictly marine (Halophila and Thalassia), the rest occupy freshwater habitats.
  • Hydrocharitaceae are of limited economic use, except as aquarium plants. Their invasive nature, however, is more important.

Important literature

Cook, C.D.K., 1998. Hydrocharitaceae. In: K. Kubitzki (ed.), The families and genera of vascular plants 4: 234-248. Springer Verlag, Berlin.

Haynes, R.R., 2004. Hydrocharitaceae. Pp. 444-446. In: Smith, N., Mori, S.A., Henderson, A., Stevenson, D.W. & Heald, S.V. (eds.). Flowering Plants of the Neotropics. New York Botanical Garden, Princeton University Press, Princeton.

How to cite

Haigh, A. (2009). Neotropical Hydrocharitaceae. 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/Hydrocharitaceae.htm.