Hydrocharitaceae Juss., Gen. Pl. (1789) 67; nom. cons.: pro parte (excluding the dicotyledons Nymphaea, Nelumbio, Trapa, Proserpinaca, and the monocotyledon Pistia which were included in the original description of the family). Typus: Hydrocharis L.

Monoecious or dioecious, annual or perennial aquatic plants, having either a creeping monopodial rhizome with unbranched roots at the nodes, and dis-tichously, rarely tristichously, arranged leaves, or an erect main axis (which may be highly contracted) with roots at the base, and spirally arranged or verti-cillate leaves. Leaves submerged, sometimes floating or partly emerged, linear, lanceolate, elliptic, ovate or orbicular, either sessile and then sometimes sheathing at the base, or differentiated into a leaf-blade and a petiole, always without a ligula; nerves more or less parallel, straight or curved, connected by perpendicular or ascending cross-veins. Stipulae sometimes present. Squamulae intravaginales present. Flowers actinomorphous or, rarely, slightly zygomorphous (Vallisneria), with a true, trimerous perianth, unisexual, and then sometimes with rudiments of the other sex, or bisexual, sessile or pedicellate, solitary or arranged in a cymose inflorescence, enclosed by a spathe. Spathe consisting of two free or partly to completely connate spathal leaves (bracts), pedunculate or sessile. Perianth consisting of 1 or 2 whorls of 3 segments. Stamens (2-) 3-several, arranged in one or more whorls; anthers basifixed, bi- or quadrilocular, longitudinally dehiscent; filaments more or less slender, sometimes absent. Pollen globose, sometimes released in moniliform chains (Halophila, Thalas-sia). Gynaeciumparacarpous. Ovary inferior, linear, ellipsoid or ovoid, consisting of (2-) 3-15 carpels, unilocular; between ovary and perianth often a long, filiform hypanthium. Placentas parietal either pro truding nearly to the centre of the ovary, or obsolete. Styles (2-) 3-15, often more or less split into two stigmatic branches. Ovules several, orthotropous to anatropous, erect or pendulous, with two integuments. Fruits indehiscent, opening by decay of the fleshy or membranous pericarp; or, rarely stellately dehiscent (Thalassia). Seeds several, fusiform, ellipsoid, ovoid or globose. Embryo straight, either with the hypocotyl and the cotyledon not distinctly separated and with a very inconspicuous plumula at the base of a lateral groove; or with a well differentiated hypocotyl and cotyledon and a large well developed plumula. No endosperm.

The family contains 17 genera, of which Thalassia, Halophila, and Enhalus are fully marine.

Thalassia as well as Halophila have been regarded to be sufficiently different from a morphological point of view to erect special subfamilies for them; some authors considered them even as separate families. Nakai (1943), for example, erected the family Thalassiaceae for the genus Thalassia, because of its 'confervoid' pollen (in fact strings of spherical pollen), its distichous linear leaves, its quadrilocular, laterally dehiscent anthers, and its superior ovary (an incorrect observation as the ovary is inferior). Nakai (1943) erected also the family Halophilaceae to contain the genus Halophila, because of its 'confervoid' pollen (strings of pollen as in Thalassia), its opposite, stipulate, petiolate, pinnately nerved leaves, its bilocular extrorse anthers, and its inferior ovary. In our opinion the family status is not really warranted for these genera; the subfamily status within the Hydrocharitaceae expresses in fact sufficiently the special position as well as the relationship of these taxa.

The status of a possible arrangement of the remaining 15 genera within subfamilies is still open to debate. Ascherson and Giirke (1889) and Eckardt (1964) distinguished two, Dandy in Hutchinson (1934) only one, and Dahlgren et al. (1985) three subfamilies, while Tomlinson (1982) refrains from giving an opinion on this subject. Cook (1998) does not arrive at a formal classification, but distinguishes three groups, (1) the Limnobium-group, (2) the Val-lisneria-group, and (3) the Elodea-group and the more or less alone standing genus Blyxa, that according to him could be considered to represent the archetype of the family. Cook places the seagrass Enhalus in the Vallisneria-group, but according to him, in spite of the reductions in many morphological and anatomical characteristics, it shows features that indicate intimate affinities to the Limnobium-group. Nakai (1943) regarded Enhalus as representing a family on its own, the Enhalaceae, mainly because it is a 'planta maritima'; this family is untenable, but unfortunately validly published in accordance with the rules of botanical nomenclature. On morphological grounds the seagrass Enhalus seems to be clearly related to the fresh-water genus Vallisneria, and has been classified by den Hartog (1970) within the subfamily Vallisnerioideae. Les et al. (1997) suggest another arrangement of the Hy-drocharitaceae based on the rbcL gene sequence. Independent molecular research by Tanaka et al. (1997) using the rbcL and mat K gene sequences indicates that Najas, generally classified as a family of its own (Najadaceae), is an in-group of the Hydrocharitaceae, and thus would lose its special status. Further, they demonstrated that the three marine genera, Enhalus, Halophila, and Thalassia form a monophyletic grouping, but the recognition of all marine Hydrocharitaceae as a separate monophyletic family is not strongly supported by the rbcL data. Therefore, Les et al. (1997) concluded that these genera must be retained as a single taxon, e.g. as a subfamily, within the Hydrocharitaceae rather than as a distinct marine family. In our opinion the three marine genera have in common that they fit the morphological basic plan of the Hydrocharitaceae and possess a set of physiological properties to deal with life in the marine environment. Apart from these basic characters the three marine genera show hardly any similarities. The molecular data probably indicate that the adaptation to the marine conditions in the three genera has followed a similar pattern and that probably the same physiological mechanisms are involved. For this reason we keep to the view that the two marine subfamilies Thalassioideae and Halophiloideae should be maintained and that Enhalus belongs to the Vallisnerioideae.

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