12
C. den Hartog and J. Kuo
that indicate intimate affinities to the Limnobiumgroup. 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 Hydrocharitaceae 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.
Key to the Marine Genera
1a. Very coarse plants with a thick rhizome and
strap-shaped leaves; leaf margins with very
coarse nerves, after decay remaining as persistent strands. Flowers with three petals and three
sepals. Male spathe with numerous flower-buds
which become detached just before flowering,
the flower then floating freely at the water surface. Pollen spherical, free. Female flower on
a long peduncle, which spirally contracts after
pollination. Tannin cells present . . . . . Enhalus
1b. Moderately coarse or even very delicate plants
with more slender rhizomes. Male spathe containing only one flower, shed after anthesis. Pollen spherical, arranged into moniliform
strings. Tannin cells present or absent.
2a. Leaf-bearing branches arising from the rhizome
at distances of several internodes; each internode covered by a scale. Leaves distichous, linear; nerves parallel. Spathal leaves partly connate. Styles 6–8, each divided into two stigmata.
Parietal placentas protruding far into the centre
of the ovary. Fruit stellately dehiscent. Tannin
cells present . . . . . . . . . . . . . . . . . . . . . . Thalassia
2b. Leaf-bearing branches arising from the thin
rhizome at each node. Leaves petiolate, in
pairs, in pseudo-whorls or distichously arranged; with a pinnate nervation. Spathal leaves
free. Styles 3–6, not divided. Parietal placentas protruding only slightly into the ovary. Fruit
dehiscent by decay of pericarp. Tannin cells
absent . . . . . . . . . . . . . . . . . . . . . . . . . . . Halophila
Enhalus L.C. Richard, M´ em. Inst. Paris 12, 2 (1812)
64,71,74.
Type species: Enhalus koenigi Rich. (= E. acoroides
(L. f .) Royle)
Enhalus is a monotypic genus, widely distributed
along the coasts of the Indian Ocean and the tropical
part of the western Pacific.
Thalassia Banks ex K¨ onig in K¨ onig and Sims, Ann
Bot. 2 (1805) 96
Type species: Thalassia testudinum Banks ex K¨ onig
This genus contains two well-defined species. T.
hemprichii is widely distributed in the coastal waters of the Indian Ocean and the western Pacific. T.
testudinum is restricted to the Gulf of Mexico and the
Caribbean, and it has been recorded from Bermuda.
Fossil leaf material of T. testudinum has been
found in the Avon Park deposits from the Middle
Eocene of Florida (Lumbert et al., 1984).
Halophila Du Petit Thouars, Gen. Nov. Madag. 2
(1806) 2
Type species: Halophila madagascariensis Steudel,
validated by Doty & Stone, 1967.
The typification of Halophila has been a weary
affair, because the original author of the genus
C. den Hartog and J. Kuo
that indicate intimate affinities to the Limnobiumgroup. 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 Hydrocharitaceae 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.
Key to the Marine Genera
1a. Very coarse plants with a thick rhizome and
strap-shaped leaves; leaf margins with very
coarse nerves, after decay remaining as persistent strands. Flowers with three petals and three
sepals. Male spathe with numerous flower-buds
which become detached just before flowering,
the flower then floating freely at the water surface. Pollen spherical, free. Female flower on
a long peduncle, which spirally contracts after
pollination. Tannin cells present . . . . . Enhalus
1b. Moderately coarse or even very delicate plants
with more slender rhizomes. Male spathe containing only one flower, shed after anthesis. Pollen spherical, arranged into moniliform
strings. Tannin cells present or absent.
2a. Leaf-bearing branches arising from the rhizome
at distances of several internodes; each internode covered by a scale. Leaves distichous, linear; nerves parallel. Spathal leaves partly connate. Styles 6–8, each divided into two stigmata.
Parietal placentas protruding far into the centre
of the ovary. Fruit stellately dehiscent. Tannin
cells present . . . . . . . . . . . . . . . . . . . . . . Thalassia
2b. Leaf-bearing branches arising from the thin
rhizome at each node. Leaves petiolate, in
pairs, in pseudo-whorls or distichously arranged; with a pinnate nervation. Spathal leaves
free. Styles 3–6, not divided. Parietal placentas protruding only slightly into the ovary. Fruit
dehiscent by decay of pericarp. Tannin cells
absent . . . . . . . . . . . . . . . . . . . . . . . . . . . Halophila
Enhalus L.C. Richard, M´ em. Inst. Paris 12, 2 (1812)
64,71,74.
Type species: Enhalus koenigi Rich. (= E. acoroides
(L. f .) Royle)
Enhalus is a monotypic genus, widely distributed
along the coasts of the Indian Ocean and the tropical
part of the western Pacific.
Thalassia Banks ex K¨ onig in K¨ onig and Sims, Ann
Bot. 2 (1805) 96
Type species: Thalassia testudinum Banks ex K¨ onig
This genus contains two well-defined species. T.
hemprichii is widely distributed in the coastal waters of the Indian Ocean and the western Pacific. T.
testudinum is restricted to the Gulf of Mexico and the
Caribbean, and it has been recorded from Bermuda.
Fossil leaf material of T. testudinum has been
found in the Avon Park deposits from the Middle
Eocene of Florida (Lumbert et al., 1984).
Halophila Du Petit Thouars, Gen. Nov. Madag. 2
(1806) 2
Type species: Halophila madagascariensis Steudel,
validated by Doty & Stone, 1967.
The typification of Halophila has been a weary
affair, because the original author of the genus