64
J. Kuo and C. den Hartog
that nutrient movement is apoplastic in these seagrasses (see Kuo and McComb, 1989). On the other
hand, a well-developed system of plasmodesmata
connects adjacent epidermal and mesophyll cells
in Enhalus (Fig. 7A), Posidonia spp., and also in
Zostera muelleri and Phyllospadix spp. suggesting
that symplastic communication in these species is
possible (see Kuo and McComb, 1989; Kuo and
Stewart, 1995).
The epidermis of the leaf blade is the major site
of photosynthesis for seagrasses and some fresh
water plants, in contrast to terrestrial plants (see
also Larkum et al., Chapter 14). In all species
of seagrasses, the epidermis has high concentrations of chloroplasts, contains mitochondria, lipid
droplets, dictyosomes, endoplasmic reticulum and
microbodies; small starch grains are present in
Thalassodendron and Amphibolis. The vacuoles of
Enhalus (Fig. 7A), Thalassia (Fig. 7B and E), Posidonia (Fig. 7D), Halodule, Cymodocea, Amphibolis
and Thalassodendron contain a polyphenolic substance (tannin) but those of Zostera, Heterozostera
and Phyllospadix (Fig. 7C and F) do not have
tannin.
Various cytoplasmic components, including paramural bodies, endoplasmic reticulum, dictyosomes
and microtubules were observed during the initiation and subsequent development of wall ingrowths
in the epidermis of Zostera leaves (Barnabas et al.,
1982; Jagels, 1983). However, the precise relationship between the presence of these organelles and
the synthesis of wall ingrowths remains uncertain
(see Kuo and McComb, 1989).
c. Leaf Fiber Cells
The occurrence of prominent fiber cells in the leaf
blade is of particular interest (Fig. 8B–E). They are
present as strands of various sizes in the leaves of
certain genera but absent from others. The distribution and the size of these fiber strands may even
differ among species within a genus, e.g. in Zostera
and Posidonia and can be used as species identification characters. Fiber strands are normally abundant
near the leaf margins, associated with longitudinal
vascular bundles and also present in the hypodermis
adjacent to air lacunae (Fig. 8C and D). A fiber strand
usually consists of several fiber cells with thickened
but not lignified walls (Fig. 8D and E). Thus, together with the thick walled epidermal cells, they
could provide tensile strength but retain a degree of
flexibility, allowing the strap-shaped leaf blades to
withstand vigorous wave action. In Posidonia, fiber
strands are present in the hypodermal and subhypodermal layers of those species having flattened
blades, but also occur deep in the mesophyll in
species with biconvex to terete blades. In contrast,
in the family Cymodoceaceae, fiber strands are only
present in Cymodocea and are absent from the remaining four genera (Fig. 8I–K). Despite the fact
that Thalassodendron and Amphibolis often grow
in high-energy environments, their leaves are usually carried on the tips of long, flexible stems and
their blades are extremely thin but strengthened
with thick-walled bundle sheath cells that compensate for the absence of fiber strands. In contrast, both Halodule and Syringodium have enlarged
aerenchyma systems and normally inhabit sheltered
areas. Fibrous strands are also reduced in leaf blades
of Halophila, Ruppia and Lepilaena, but all these
genera have thin leaf blades with few mesophyll cells
and few air lacunae.
d. Leaf Mesophyll and Air Lacunae
The mesophyll tissue of seagrasses is homogeneous
with thin-walled but highly vacuolated parenchyma
cells (Figs. 8A and B and 9A–C). The thin peripheral cytoplasm contains few chloroplasts with small
starch grains. The mesophyll cells surround air lacunae of varying size; in some genera there are prominent and regularly arranged (about 3–5) air lacunae
separating longitudinal vascular bundles, e.g. in
Heterozostera, Phyllospadix Zostera (Figs. 8A and
9A and B), Thalassia and Enhalus. Air lacunae are
present in Posidonia (Figs. 8B and 9C), but less
prominent than in the above genera. In the leaves
of Halophila, Amphibolis and Thalassodendron lacunae are present though minute in size (Fig. 8I and
J). It is generally assumed that aerenchyma is developed to increase the internal gas space of aquatic
plants as they may grow in oxygen poor substrata
(see also Borum et al., Chapter 10).
Air lacunae are interconnected in all vegetative
and reproductive organs of all seagrass species, i.e.
leaves, petioles, rhizomes, stems, roots and flowers
and fruits). They are interrupted by septa between organs and also at regular intervals in particular organs,
e.g. leaves by perforated septa (Fig. 9E–G). Each
septum consists of a group of small parenchyma
J. Kuo and C. den Hartog
that nutrient movement is apoplastic in these seagrasses (see Kuo and McComb, 1989). On the other
hand, a well-developed system of plasmodesmata
connects adjacent epidermal and mesophyll cells
in Enhalus (Fig. 7A), Posidonia spp., and also in
Zostera muelleri and Phyllospadix spp. suggesting
that symplastic communication in these species is
possible (see Kuo and McComb, 1989; Kuo and
Stewart, 1995).
The epidermis of the leaf blade is the major site
of photosynthesis for seagrasses and some fresh
water plants, in contrast to terrestrial plants (see
also Larkum et al., Chapter 14). In all species
of seagrasses, the epidermis has high concentrations of chloroplasts, contains mitochondria, lipid
droplets, dictyosomes, endoplasmic reticulum and
microbodies; small starch grains are present in
Thalassodendron and Amphibolis. The vacuoles of
Enhalus (Fig. 7A), Thalassia (Fig. 7B and E), Posidonia (Fig. 7D), Halodule, Cymodocea, Amphibolis
and Thalassodendron contain a polyphenolic substance (tannin) but those of Zostera, Heterozostera
and Phyllospadix (Fig. 7C and F) do not have
tannin.
Various cytoplasmic components, including paramural bodies, endoplasmic reticulum, dictyosomes
and microtubules were observed during the initiation and subsequent development of wall ingrowths
in the epidermis of Zostera leaves (Barnabas et al.,
1982; Jagels, 1983). However, the precise relationship between the presence of these organelles and
the synthesis of wall ingrowths remains uncertain
(see Kuo and McComb, 1989).
c. Leaf Fiber Cells
The occurrence of prominent fiber cells in the leaf
blade is of particular interest (Fig. 8B–E). They are
present as strands of various sizes in the leaves of
certain genera but absent from others. The distribution and the size of these fiber strands may even
differ among species within a genus, e.g. in Zostera
and Posidonia and can be used as species identification characters. Fiber strands are normally abundant
near the leaf margins, associated with longitudinal
vascular bundles and also present in the hypodermis
adjacent to air lacunae (Fig. 8C and D). A fiber strand
usually consists of several fiber cells with thickened
but not lignified walls (Fig. 8D and E). Thus, together with the thick walled epidermal cells, they
could provide tensile strength but retain a degree of
flexibility, allowing the strap-shaped leaf blades to
withstand vigorous wave action. In Posidonia, fiber
strands are present in the hypodermal and subhypodermal layers of those species having flattened
blades, but also occur deep in the mesophyll in
species with biconvex to terete blades. In contrast,
in the family Cymodoceaceae, fiber strands are only
present in Cymodocea and are absent from the remaining four genera (Fig. 8I–K). Despite the fact
that Thalassodendron and Amphibolis often grow
in high-energy environments, their leaves are usually carried on the tips of long, flexible stems and
their blades are extremely thin but strengthened
with thick-walled bundle sheath cells that compensate for the absence of fiber strands. In contrast, both Halodule and Syringodium have enlarged
aerenchyma systems and normally inhabit sheltered
areas. Fibrous strands are also reduced in leaf blades
of Halophila, Ruppia and Lepilaena, but all these
genera have thin leaf blades with few mesophyll cells
and few air lacunae.
d. Leaf Mesophyll and Air Lacunae
The mesophyll tissue of seagrasses is homogeneous
with thin-walled but highly vacuolated parenchyma
cells (Figs. 8A and B and 9A–C). The thin peripheral cytoplasm contains few chloroplasts with small
starch grains. The mesophyll cells surround air lacunae of varying size; in some genera there are prominent and regularly arranged (about 3–5) air lacunae
separating longitudinal vascular bundles, e.g. in
Heterozostera, Phyllospadix Zostera (Figs. 8A and
9A and B), Thalassia and Enhalus. Air lacunae are
present in Posidonia (Figs. 8B and 9C), but less
prominent than in the above genera. In the leaves
of Halophila, Amphibolis and Thalassodendron lacunae are present though minute in size (Fig. 8I and
J). It is generally assumed that aerenchyma is developed to increase the internal gas space of aquatic
plants as they may grow in oxygen poor substrata
(see also Borum et al., Chapter 10).
Air lacunae are interconnected in all vegetative
and reproductive organs of all seagrass species, i.e.
leaves, petioles, rhizomes, stems, roots and flowers
and fruits). They are interrupted by septa between organs and also at regular intervals in particular organs,
e.g. leaves by perforated septa (Fig. 9E–G). Each
septum consists of a group of small parenchyma