Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
57
lamella restricts penetration, but fungi and bacteria
occur in the lumen of exodermal and outer cortical cells. By analogy with terrestrial plants, it
was suggested that rhizosphere organisms may be
involved in nutrient uptake and nitrogen fixation
in seagrasses. On the other hand, Nielsen et al.
(1999) concluded that vesicular-arbuscular mycorrhizae cannot colonize the roots of Zostera marina
and Thalassia testudinum, probably due to the effect
of low oxygen supply and high salinity in marine
sediments.
B. Rhizome and Stem
1. Morphology of Rhizome and Stem
Seagrass rhizomes are usually herbaceous, cylindrical to laterally compressed, and monopodially or
irregularly branched; however, in Amphibolis and
Thalassodendron (Fig. 1I), the rhizome branches
sympodially and becomes woody. Rhizomes are almost always buried in sediment (the exceptions being Thalassodendron spp. and Phyllospadix spp.)
and the persistent, fibrous remains of old leaf sheaths
usually cover the rhizome of Enhalus and Posidonia
(Fig. 1A and E), and to some extent Zostera, Heterozostera, Phyllospadix and Halodule. The lengths of
rhizome internodes are relatively long in most of the
genera, but they are extremely short in Phyllospadix
(Barnabas, 1994b). Erect stems resulting from sympodial branching of rhizomes are present in all genera in the Cymodoceaceae (Fig. 1F–I), some species
of Heterozostera (H. nigricaulis and H. chilensis)
in the Zosteraceae, and Thalassia and some species
of Halophila (Fig. 1B), in the Hydrocharitaceae.
Erect stems form at each rhizome node in most
genera, but only form at certain rhizome nodes in
Amphibolis, Thalassodendron (Fig. 1I), Thalassia
and Heterozostera. Among the above-mentioned
genera, only Amphibolis has many branched stems,
the others have few branches. Erect stems have numerous nodes representing leaf scars and can last
for many years. In the genus Halophila, sections
Microhalophila, Spinulosae, Americanae (Fig. 1B)
and Tricostatae, all have distinct elongated stems
but section Halophila (Fig. 1C) has very short erect
stems. Erect stems of Cymodocea, Halodule, Thalassodendron, Amphibolis, Heterozostera, Thalassia
and section Spinulosae of Halophila are rigid; while
those of Syringodium and most of the Halophila
species are soft and hegceous.
2. Anatomy of Rhizome and Stem
The internodes of rhizomes and erect stems are similar in transverse section, and have a similar tissue
arrangement as those in roots. The epidermis is normally distinct and covered by a cuticle, and may
contain tannin cells (Fig. 4C). Walls of epidermal
cells are usually thickened and lignified. The exodermis is distinct and usually has more than one
layer of cells with a thickened wall and a suberized
middle lamella (Fig. 5A and B) (e.g. Posidonia and
Halophila). Cortical parenchyma cells are larger in
the outer region than in the inner one. Sometimes
these two regions are very distinct, for example,
in Amphibolis and Thalassodendron. Cells of the
outer cortical parenchyma have thick, lignified walls,
while those of the inner cortical parenchyma have
thin, non-lignified walls. In this cortical tissue, large
lacunae may develop, as in Enhalus, Syringodium,
Cymodocea (Fig. 4C, D and H) Zostera and
Halophila and the number of lacunae appears to be
constant within genera. Unlike root tissues, beside
the central stele, there are always two or more vascular bundles and numerous fiber bundles scattered
among the outer cortical tissues in the rhizome and
the erect stem (Fig. 4A–D). In addition, starch accumulates in the cortical parenchyma cells in some
genera, e.g. Enhalus (Fig. 4G), Thalassia, Zostera
and Halodule, but not in others (Amphibolis and
Thalassodendron).
A conspicuous endodermis encloses the central
stele and may become suberized (e.g. Posidonia,
Halodule) or develop thickened and lignified walls
(e.g. Amphibolis, Thalassodendron). The central
stele has one or more central protoxylem elements
surrounded by sieve tubes, but the pericycle is not
distinct.
Underground rhizomes are a common feature
of many monocotyledons and account for distinctive ecological features (Harper, 1977). The main
functions of the rhizome in seagrasses are for anchoring, mechanical support, nutrient storage, and
regulation and maintenance of vegetative growth.
Tomlinson (1974) considered that vegetative propagation of rhizomes is probably of greater importance
in the maintenance and spread of seagrasses than
seed production.
The rhizomes of the larger species may, together
with roots and leaf sheath remains, form dense mats.
The seagrass Posidonia oceanica in the Mediterranean Sea usually grows on the thick ‘matte’
with a thickness of up to 4 m locally, constituting
57
lamella restricts penetration, but fungi and bacteria
occur in the lumen of exodermal and outer cortical cells. By analogy with terrestrial plants, it
was suggested that rhizosphere organisms may be
involved in nutrient uptake and nitrogen fixation
in seagrasses. On the other hand, Nielsen et al.
(1999) concluded that vesicular-arbuscular mycorrhizae cannot colonize the roots of Zostera marina
and Thalassia testudinum, probably due to the effect
of low oxygen supply and high salinity in marine
sediments.
B. Rhizome and Stem
1. Morphology of Rhizome and Stem
Seagrass rhizomes are usually herbaceous, cylindrical to laterally compressed, and monopodially or
irregularly branched; however, in Amphibolis and
Thalassodendron (Fig. 1I), the rhizome branches
sympodially and becomes woody. Rhizomes are almost always buried in sediment (the exceptions being Thalassodendron spp. and Phyllospadix spp.)
and the persistent, fibrous remains of old leaf sheaths
usually cover the rhizome of Enhalus and Posidonia
(Fig. 1A and E), and to some extent Zostera, Heterozostera, Phyllospadix and Halodule. The lengths of
rhizome internodes are relatively long in most of the
genera, but they are extremely short in Phyllospadix
(Barnabas, 1994b). Erect stems resulting from sympodial branching of rhizomes are present in all genera in the Cymodoceaceae (Fig. 1F–I), some species
of Heterozostera (H. nigricaulis and H. chilensis)
in the Zosteraceae, and Thalassia and some species
of Halophila (Fig. 1B), in the Hydrocharitaceae.
Erect stems form at each rhizome node in most
genera, but only form at certain rhizome nodes in
Amphibolis, Thalassodendron (Fig. 1I), Thalassia
and Heterozostera. Among the above-mentioned
genera, only Amphibolis has many branched stems,
the others have few branches. Erect stems have numerous nodes representing leaf scars and can last
for many years. In the genus Halophila, sections
Microhalophila, Spinulosae, Americanae (Fig. 1B)
and Tricostatae, all have distinct elongated stems
but section Halophila (Fig. 1C) has very short erect
stems. Erect stems of Cymodocea, Halodule, Thalassodendron, Amphibolis, Heterozostera, Thalassia
and section Spinulosae of Halophila are rigid; while
those of Syringodium and most of the Halophila
species are soft and hegceous.
2. Anatomy of Rhizome and Stem
The internodes of rhizomes and erect stems are similar in transverse section, and have a similar tissue
arrangement as those in roots. The epidermis is normally distinct and covered by a cuticle, and may
contain tannin cells (Fig. 4C). Walls of epidermal
cells are usually thickened and lignified. The exodermis is distinct and usually has more than one
layer of cells with a thickened wall and a suberized
middle lamella (Fig. 5A and B) (e.g. Posidonia and
Halophila). Cortical parenchyma cells are larger in
the outer region than in the inner one. Sometimes
these two regions are very distinct, for example,
in Amphibolis and Thalassodendron. Cells of the
outer cortical parenchyma have thick, lignified walls,
while those of the inner cortical parenchyma have
thin, non-lignified walls. In this cortical tissue, large
lacunae may develop, as in Enhalus, Syringodium,
Cymodocea (Fig. 4C, D and H) Zostera and
Halophila and the number of lacunae appears to be
constant within genera. Unlike root tissues, beside
the central stele, there are always two or more vascular bundles and numerous fiber bundles scattered
among the outer cortical tissues in the rhizome and
the erect stem (Fig. 4A–D). In addition, starch accumulates in the cortical parenchyma cells in some
genera, e.g. Enhalus (Fig. 4G), Thalassia, Zostera
and Halodule, but not in others (Amphibolis and
Thalassodendron).
A conspicuous endodermis encloses the central
stele and may become suberized (e.g. Posidonia,
Halodule) or develop thickened and lignified walls
(e.g. Amphibolis, Thalassodendron). The central
stele has one or more central protoxylem elements
surrounded by sieve tubes, but the pericycle is not
distinct.
Underground rhizomes are a common feature
of many monocotyledons and account for distinctive ecological features (Harper, 1977). The main
functions of the rhizome in seagrasses are for anchoring, mechanical support, nutrient storage, and
regulation and maintenance of vegetative growth.
Tomlinson (1974) considered that vegetative propagation of rhizomes is probably of greater importance
in the maintenance and spread of seagrasses than
seed production.
The rhizomes of the larger species may, together
with roots and leaf sheath remains, form dense mats.
The seagrass Posidonia oceanica in the Mediterranean Sea usually grows on the thick ‘matte’
with a thickness of up to 4 m locally, constituting