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(Last Modified On 5/9/2013)
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(Last Modified On 5/9/2013)
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Family
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HYDROCHARITACEAE
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Contributor
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C. DEN HARTOG
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Description
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Annual or perennial marine or fresh-water herbs, having either a creeping monopodial rhizome with roots at the nodes and distichously arranged leaves, or an erect main axis with roots at the base and spirally arranged or verticillate leaves. Leaves submerged, sometimes floating or partly emerged, linear, lanceolate, elliptic, ovate, or orbicular, either sessile and then sometimes sheathing basally, or differentiated into blade and petiole; main veins more or less parallel, straight or curved, connected by perpendicular or ascending cross-veins; ligule absent; stipules sometimes present; squamulae intravaginales present. Flowers regular, or rarely slightly irregular (Vallisneria), unisexual, sometimes with rudiments of the other sex present, or bisexual, sessile or pedicellate, solitary or in cymose inflorescences, enclosed by a pedunculate or sessile spathe of 2 free or partly to completely connate spathal leaves; perianth of 1 or 2 whorls of 3 free segments; stamens (2-)3-several, arranged in 1 or more whorls, the anthers basifixed, 2- or 4-thecate, dehiscent lengthwise, the filaments more or less slender, sometimes absent; gynoecium paracarpous, (2-)3-15-carpelled, the ovary inferior, linear, ellipsoid, or ovoid, 1-loculed, the placentas parietal, either protruding nearly to center of the ovary or obsolete, the ovules several, orthotropous to anatropous, erect or pendulous, the styles (2-)3-15, often more or less split into 2 stigmatic branches, a long filiform hypanthium often occurring between the ovary and perianth. Fruits indehiscent, opening by decay of the pericarp, or rarely stellately dehiscent, the pericarp fleshy or membranous; seeds several, fusiform, ellipsoid, ovoid, or globose, the embryo straight, either with the hypocotyl and cotyledon not distinctly separated and with an inconspicuous plumule at the base of a lateral groove, or with a well differentiated hypocotyl and cotyledon and a large well developed plumule, endosperm absent.
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Habit
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herbs
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Distribution
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The family consists of 14 genera with about 100 species and is widely dis- tributed in the tropical and temperate areas of the world. Three genera, Hydrilla, Thalassia, and Halophila, occur in Panama.
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Reference
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Hartog, C. den. The Sea-grasses of the World. Pp. 213-268. 1970. Hutchinson, J. The Families of Flowering Plants. Ed. 2. Vol. 2. Pp. 538- 541. 1959.
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Key
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a. Caulescent fresh-water plants; upper and middle leaves opposite or in whorls of 3-8, not differentiated into petiole and blade, 1-nerved, at most 4 cm long; tannin cells present ....................... 1. Hydrilla aa. Marine plants with creeping rhizomes. b. Leaves distichous, linear, sheathing basally, 9-17-nerved, to 60 cm long; tannin cells numerous ............... 2. Thalassia bb. Leaves arranged in pairs or in a pseudowhorl, differentiated into a short petiole and an elliptic or ovate blade, to 2.5 cm long, 3-nerved, connected by ascending cross- veins; tannin cells absent ............. 3. Halophila
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