Chapter 1 Taxonomy of Seagrasses
11
Recently, Campey et al. (2000) re-evaluated the
species boundaries of the members of the P. ostenfeldii species complex in one locality, and found that
there is a continuous variation of the character traits
within the complex, suggesting the existence of a
morphological continuum between the species; they
also did not find any allozyme variation. According
to them P. coriacea and P. robertsoniae are not separate species, and have to be regarded as synonyms.
Hydrocharitaceae
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 distichously, rarely tristichously, arranged leaves, or an
erect main axis (which may be highly contracted)
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 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, Thalassia). Gynaecium paracarpous. 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 protruding 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 G¨ urke (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 Vallisneria-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
11
Recently, Campey et al. (2000) re-evaluated the
species boundaries of the members of the P. ostenfeldii species complex in one locality, and found that
there is a continuous variation of the character traits
within the complex, suggesting the existence of a
morphological continuum between the species; they
also did not find any allozyme variation. According
to them P. coriacea and P. robertsoniae are not separate species, and have to be regarded as synonyms.
Hydrocharitaceae
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 distichously, rarely tristichously, arranged leaves, or an
erect main axis (which may be highly contracted)
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 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, Thalassia). Gynaecium paracarpous. 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 protruding 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 G¨ urke (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 Vallisneria-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