60
J. Kuo and C. den Hartog
distinguish the old dark living rhizomes from dead
material. In the Banda Sea, the mats were up to ca.
70 cm thick, and living rhizomes were only found
in the upper 10 cm (Brouns, 1985). Further studies
on the structure of the underground rhizome-root
systems of the various seagrass species are recommended.
C. The Foliage Leaf
The foliage leaf is produced either from the rhizome nodes, normally from the upper side in
Enhalus, Halophila, Posidonia and the Zosteraceae
or from the apex of erect stems in Thalassia and the
Cymodoceaceae. The foliage usually forms a unit of
several leaves, and normally is referred to as a shoot
containing different developmental stages. Each leaf
consists of a basal leaf sheath and a distal leaf blade.
1. Leaf Sheath Morphology and Anatomy
The leaf sheaths are clearly differentiated from leaf
blades, and enclose the young, developing leaves in
all seagrass genera, which have strapped leaves. The
leaf sheaths are usually covered by sediment at least
at the base. Old ribbon leaf blades usually abscise at
the junction between blade and sheath where there is
a mechanical weakness in the structure of the epidermis and the structure and distribution of fiber cells
(Tomlinson, 1972; Kuo, 1978), with the exception
of Amphibolis and Thalassodendron where the leaf
is shed in its totality, blade and sheath remaining
together (den Hartog, 1970). A tongue-like structure known as the ligule is produced as an adaxial outgrowth at the junction of sheath and blade in
all seagrasses except those in the Hydrocharitaceae.
Zostera subgenus Zostera in the Zosteraceae has a
tubular leaf sheath, which becomes ruptured with
age. Leaf sheaths in all other genera are normally
‘open,’ i.e. split vertically, with opposite margins
overlapping. The degree of this opening can sometimes be a useful diagnostic feature.
The general anatomy of the leaf sheath is similar
to that of the blade, but detailed ultrastructures show
marked differences (Fig. 5C and D). Furthermore,
the structure of the outer and inner epidermis is significantly different (Fig. 5G and H). In contrast to
the blade, the cuticle of the sheath epidermis appears
as a thin, uniform, electron dense layer that is nonporous (Posidonia) (Kuo, 1978) (Fig. 5F) or lacks
subcuticular cavities (Zostera, Heterozostera, Phyllospadix) (Kuo and Stewart, 1995) (Fig. 5H). The
epidermis of the sheath lacks cell wall ingrowths,
but has a large central vacuole with a thin peripheral cytoplasm, and chloroplasts, if present, occur
only sparsely in the outer epidermal cells. The vascular bundles of the leaf sheath are similar to those
in the blade, but there are no wall ingrowths in the
vascular sheath cells and phloem parenchyma cells
(in Zostera and Heterozostera), suggesting that in
the leaf sheath there is little exchange of metabolites
between the mesophyll and vascular tissue. Air lacunae and fiber bundles are more pronounced in the
sheath than those in the blade (Fig. 5C and D). The
fiber bundles of the sheath in Posidonia are lignified
(Fig. 5E) and remain on the rhizome surfaces long
after tissues of the leaf sheath have rotted away (Kuo,
1978).
Leaf litter from Posidonia australis and other
species may accumulate on beaches, as high as 2 m
along some parts of southern Australian coastline
during the winter and, often, it is washed back into
the ocean by late spring. In other locations, permanent deposits of up to 3 m in thickness occur. There
is no doubt that these accumulations play an important role in marine food chains. Furthermore, the finally detached sheath fibers of Posidonia are rolled
by wave action to form ‘marine balls’ or ‘Posidonia balls’ of different sizes. Fiber balls have been
recorded from Mediterranean beaches since ancient
times, and are commonly observed along southern
Australian coastlines. Posidonia fibers have accumulated on the ocean floor in such quantities in Spencer
and Gulf St. Vincent in South Australia, that they
were harvested for a time (1905–1915) to make grain
bags, paper and insulation material (Winterbottom,
1917; Ried and Smith, 1919).
In contrast to the leaf blade in seagrasses, the
leaf sheath probably has little function in the performance of photosynthesis or ion exchange with the
surrounding media, but is very important in protecting the apical meristem and developing leaves. Tyerman (1989) showed that within the solution enclosed
by the sheath, there is an osmotic gradient, created
and maintained by ion uptake from the sheath.
2. Leaf Blade Morphology
In contrast to terrestrial monocotyledons, the
surface of seagrass leaf blades is simple and naked
(Fig. 6A–C and E–G) and only unicellular hairs
occur on the surface of epidermal cells in certain species of Halophila, e.g. H. decipiens (Fig.
6D), H. stipulacea and H. capricorni. These hairs are
J. Kuo and C. den Hartog
distinguish the old dark living rhizomes from dead
material. In the Banda Sea, the mats were up to ca.
70 cm thick, and living rhizomes were only found
in the upper 10 cm (Brouns, 1985). Further studies
on the structure of the underground rhizome-root
systems of the various seagrass species are recommended.
C. The Foliage Leaf
The foliage leaf is produced either from the rhizome nodes, normally from the upper side in
Enhalus, Halophila, Posidonia and the Zosteraceae
or from the apex of erect stems in Thalassia and the
Cymodoceaceae. The foliage usually forms a unit of
several leaves, and normally is referred to as a shoot
containing different developmental stages. Each leaf
consists of a basal leaf sheath and a distal leaf blade.
1. Leaf Sheath Morphology and Anatomy
The leaf sheaths are clearly differentiated from leaf
blades, and enclose the young, developing leaves in
all seagrass genera, which have strapped leaves. The
leaf sheaths are usually covered by sediment at least
at the base. Old ribbon leaf blades usually abscise at
the junction between blade and sheath where there is
a mechanical weakness in the structure of the epidermis and the structure and distribution of fiber cells
(Tomlinson, 1972; Kuo, 1978), with the exception
of Amphibolis and Thalassodendron where the leaf
is shed in its totality, blade and sheath remaining
together (den Hartog, 1970). A tongue-like structure known as the ligule is produced as an adaxial outgrowth at the junction of sheath and blade in
all seagrasses except those in the Hydrocharitaceae.
Zostera subgenus Zostera in the Zosteraceae has a
tubular leaf sheath, which becomes ruptured with
age. Leaf sheaths in all other genera are normally
‘open,’ i.e. split vertically, with opposite margins
overlapping. The degree of this opening can sometimes be a useful diagnostic feature.
The general anatomy of the leaf sheath is similar
to that of the blade, but detailed ultrastructures show
marked differences (Fig. 5C and D). Furthermore,
the structure of the outer and inner epidermis is significantly different (Fig. 5G and H). In contrast to
the blade, the cuticle of the sheath epidermis appears
as a thin, uniform, electron dense layer that is nonporous (Posidonia) (Kuo, 1978) (Fig. 5F) or lacks
subcuticular cavities (Zostera, Heterozostera, Phyllospadix) (Kuo and Stewart, 1995) (Fig. 5H). The
epidermis of the sheath lacks cell wall ingrowths,
but has a large central vacuole with a thin peripheral cytoplasm, and chloroplasts, if present, occur
only sparsely in the outer epidermal cells. The vascular bundles of the leaf sheath are similar to those
in the blade, but there are no wall ingrowths in the
vascular sheath cells and phloem parenchyma cells
(in Zostera and Heterozostera), suggesting that in
the leaf sheath there is little exchange of metabolites
between the mesophyll and vascular tissue. Air lacunae and fiber bundles are more pronounced in the
sheath than those in the blade (Fig. 5C and D). The
fiber bundles of the sheath in Posidonia are lignified
(Fig. 5E) and remain on the rhizome surfaces long
after tissues of the leaf sheath have rotted away (Kuo,
1978).
Leaf litter from Posidonia australis and other
species may accumulate on beaches, as high as 2 m
along some parts of southern Australian coastline
during the winter and, often, it is washed back into
the ocean by late spring. In other locations, permanent deposits of up to 3 m in thickness occur. There
is no doubt that these accumulations play an important role in marine food chains. Furthermore, the finally detached sheath fibers of Posidonia are rolled
by wave action to form ‘marine balls’ or ‘Posidonia balls’ of different sizes. Fiber balls have been
recorded from Mediterranean beaches since ancient
times, and are commonly observed along southern
Australian coastlines. Posidonia fibers have accumulated on the ocean floor in such quantities in Spencer
and Gulf St. Vincent in South Australia, that they
were harvested for a time (1905–1915) to make grain
bags, paper and insulation material (Winterbottom,
1917; Ried and Smith, 1919).
In contrast to the leaf blade in seagrasses, the
leaf sheath probably has little function in the performance of photosynthesis or ion exchange with the
surrounding media, but is very important in protecting the apical meristem and developing leaves. Tyerman (1989) showed that within the solution enclosed
by the sheath, there is an osmotic gradient, created
and maintained by ion uptake from the sheath.
2. Leaf Blade Morphology
In contrast to terrestrial monocotyledons, the
surface of seagrass leaf blades is simple and naked
(Fig. 6A–C and E–G) and only unicellular hairs
occur on the surface of epidermal cells in certain species of Halophila, e.g. H. decipiens (Fig.
6D), H. stipulacea and H. capricorni. These hairs are