Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
69
cells (Fig. 10F) usually contain several chloroplasts
(with poorly developed grana), mitochondria, lipid
droplets, free ribosomes and rough endoplasmic
reticulum, as well as plastids with electron-opaque
bodies resembling protein crystalloids. Branched
plasmodesmata with enlarged walls frequently form
connections between vascular parenchyma cells.
f. Phyllosphere
Seagrasses often bear epiphytes including bacteria, diatoms, algae, hydroids and sponges (see
Borowitzka et al., Chapter 19). Some filamentous
bacteria have been also associated with decaying
seagrass leaves by tunneling thick outer tangential
and radial walls in T. testudinum (Porter et al., 1989)
and in T. ciliatum (Barnabas, 1992). On the other
hand, symbiotic fungal hyphae have been reported
only from Z. muelleri, but only in its subtidal form.
These fungal hyphae have been observed throughout
the leaf intercellular spaces and might be involved
in enhancing solute transfer within the seagrass leaf
(Kuo, 1984; Kuo et al., 1990a).
Webster and Stone (1994b) concluded that
the wall composition of vegetative parts of
Heterozostera nigricaulis (as H. tasmanica) is quite
unlike that of terrestrial graminaceous monocotyledons and is more akin to the walls of dicotyledons.
Chemistry and biochemistry of wall compositions in
other seagrass species are also discussed in detail by
Mateo et al., Chapter 7.
IV. Reproductive Morphology and Anatomy
A. Flowers
1. Reproductive Strategies
Generative reproduction is carried out in seagrasses
by regular flowering. The majority of seagrass
species are perennial and only certain populations of
Zostera marina (Keddy and Patriquin, 1978; McMillan, 1983b; Van Lent and Verschuure, 1994; den
Hartog, unpublished), Halophila decipiens (McMillan and Soong, 1989; Kuo and Kirkman, 1995;
Kenworthy, 2000), and Halophila tricostata (Kuo
et al., 1993) have been identified as annuals. The
perennial populations may show both generative and
vegetative growth, while the annul populations depend completely on generative growth i.e. recruitment from seeds.
Nine of the 12 seagrass genera are dioecious (although some species of Halophila, e.g. H. decipiens,
H. capricorni and H. beccarii are monoecious), in
comparison with less than 10% of the entire number of angiosperm genera. Dioecy probably plays
an important role in the outcrossing mechanism in
seagrasses (McConchie and Knox, 1989a; see also
Waycott et al., Chapter 2 and Ackerman, Chapter 4).
The Posidoniaceae are exclusively monoecious; in
contrast, the Cymodoceaceae are entirely dioecious,
while the marine Hydrocharitaceae and the Zosteraceae contain both monoecious and dioecious genera. All genera of the eurysaline aquatic plants are
monoecious, with the exception of Lepilaena, which
has monoecious as well as dioecious taxa.
Most aquatic plants retain the floral systems of
their terrestrial ancestors, which were not originally
adapted to function in water. Some species have
acquired floral modifications that allow pollination
to function efficiently in aquatic habitats, a phenomenon known as hydrophily. For the reproductive
organs, seagrasses exhibit unique structural adaptations for marine submerged pollination. Many floral structures in seagrasses are simplified with a reduced perianth in comparison with terrestrial flowering plants, and these structures may vary with seagrass groupings. This fact indicates that the affinities between the seagrass families are not at all clear
(Tomlinson, 1982). Furthermore, it also reflects the
fact that seagrasses have evolved from more than
one common ancestor, through different evolutionary pathways, and geological periods (den Hartog,
1970; Larkum and den Hartog, 1989; Philbrick and
Les, 1996; Les et al., 1997). Most of their freshwater or terrestrial cousins no longer exist and identification of seagrass evolutionary pathways is quite
difficult, if not impossible (but see Chapters 1 & 2).
Floral development and morphology in seagrasses
have been reported in several species including Amphibolis (see McConchie and Knox, 1989a). In A.
antarctica, the flowers form on lateral branches
while in A. griffithii, they may also develop terminally on an upright branch from the rhizome. Female
flowers develop from a pair of primordia, which, in
A. griffithii develops three stigmas, while in
A. antarctica, may form secondary branches. The
ovary wall bears the initials of the grappling apparatus, comprising four comb initials in A. griffithii and
a further inner set of horns in A. antarctica.
69
cells (Fig. 10F) usually contain several chloroplasts
(with poorly developed grana), mitochondria, lipid
droplets, free ribosomes and rough endoplasmic
reticulum, as well as plastids with electron-opaque
bodies resembling protein crystalloids. Branched
plasmodesmata with enlarged walls frequently form
connections between vascular parenchyma cells.
f. Phyllosphere
Seagrasses often bear epiphytes including bacteria, diatoms, algae, hydroids and sponges (see
Borowitzka et al., Chapter 19). Some filamentous
bacteria have been also associated with decaying
seagrass leaves by tunneling thick outer tangential
and radial walls in T. testudinum (Porter et al., 1989)
and in T. ciliatum (Barnabas, 1992). On the other
hand, symbiotic fungal hyphae have been reported
only from Z. muelleri, but only in its subtidal form.
These fungal hyphae have been observed throughout
the leaf intercellular spaces and might be involved
in enhancing solute transfer within the seagrass leaf
(Kuo, 1984; Kuo et al., 1990a).
Webster and Stone (1994b) concluded that
the wall composition of vegetative parts of
Heterozostera nigricaulis (as H. tasmanica) is quite
unlike that of terrestrial graminaceous monocotyledons and is more akin to the walls of dicotyledons.
Chemistry and biochemistry of wall compositions in
other seagrass species are also discussed in detail by
Mateo et al., Chapter 7.
IV. Reproductive Morphology and Anatomy
A. Flowers
1. Reproductive Strategies
Generative reproduction is carried out in seagrasses
by regular flowering. The majority of seagrass
species are perennial and only certain populations of
Zostera marina (Keddy and Patriquin, 1978; McMillan, 1983b; Van Lent and Verschuure, 1994; den
Hartog, unpublished), Halophila decipiens (McMillan and Soong, 1989; Kuo and Kirkman, 1995;
Kenworthy, 2000), and Halophila tricostata (Kuo
et al., 1993) have been identified as annuals. The
perennial populations may show both generative and
vegetative growth, while the annul populations depend completely on generative growth i.e. recruitment from seeds.
Nine of the 12 seagrass genera are dioecious (although some species of Halophila, e.g. H. decipiens,
H. capricorni and H. beccarii are monoecious), in
comparison with less than 10% of the entire number of angiosperm genera. Dioecy probably plays
an important role in the outcrossing mechanism in
seagrasses (McConchie and Knox, 1989a; see also
Waycott et al., Chapter 2 and Ackerman, Chapter 4).
The Posidoniaceae are exclusively monoecious; in
contrast, the Cymodoceaceae are entirely dioecious,
while the marine Hydrocharitaceae and the Zosteraceae contain both monoecious and dioecious genera. All genera of the eurysaline aquatic plants are
monoecious, with the exception of Lepilaena, which
has monoecious as well as dioecious taxa.
Most aquatic plants retain the floral systems of
their terrestrial ancestors, which were not originally
adapted to function in water. Some species have
acquired floral modifications that allow pollination
to function efficiently in aquatic habitats, a phenomenon known as hydrophily. For the reproductive
organs, seagrasses exhibit unique structural adaptations for marine submerged pollination. Many floral structures in seagrasses are simplified with a reduced perianth in comparison with terrestrial flowering plants, and these structures may vary with seagrass groupings. This fact indicates that the affinities between the seagrass families are not at all clear
(Tomlinson, 1982). Furthermore, it also reflects the
fact that seagrasses have evolved from more than
one common ancestor, through different evolutionary pathways, and geological periods (den Hartog,
1970; Larkum and den Hartog, 1989; Philbrick and
Les, 1996; Les et al., 1997). Most of their freshwater or terrestrial cousins no longer exist and identification of seagrass evolutionary pathways is quite
difficult, if not impossible (but see Chapters 1 & 2).
Floral development and morphology in seagrasses
have been reported in several species including Amphibolis (see McConchie and Knox, 1989a). In A.
antarctica, the flowers form on lateral branches
while in A. griffithii, they may also develop terminally on an upright branch from the rhizome. Female
flowers develop from a pair of primordia, which, in
A. griffithii develops three stigmas, while in
A. antarctica, may form secondary branches. The
ovary wall bears the initials of the grappling apparatus, comprising four comb initials in A. griffithii and
a further inner set of horns in A. antarctica.