62
J. Kuo and C. den Hartog
usually more pronounced on the abaxial than the
adaxial surface. Unicellular short hairs/spines or
‘marginal teeth’ often occur in the leaf blade margins (Fig. 6D); in particular, toward the leaf apex as
in Cymodocea, Thalassodendron, Phyllospadix (‘fin
cells’), Thalassia and most species of Halophila.
These features are often taken to be taxonomically
diagnostic.
The shape of epidermal cells in surface view
may differ between seagrass species, and has been
used as a species identification character in certain
genera; e.g. in Posidonia (Fig. 6A–C), Phyllospadix
(Fig. 6E–G), Heterozostera and Halophila (Kuo,
1978; Kuo and McComb, 1989; Kuo et al., 1988;
Kuo and Stewart, 1995; Cambridge and Lambers,
1998; Kuo, 2005). The appearance of the leaf
apex has also been used as an important diagnostic
feature for identifying certain seagrass genera
including Zostera, Halodule and Amphibolis (den
Hartog, 1970). However, the reliability of this
feature has been questioned for identifying species
of Halodule (Phillips and Me˜ nez, 1988).
3. Leaf Blade Anatomy and Ultrastructure
Anatomically, the arrangement of tissues and cells
in the adaxial and the abaxial sides of a strap
leaf blade is identical, but the position of xylem
elements (adaxial side) in the vascular bundles can
usually be used to determine which side is which
(Fig. 8A and B).
The most obvious differences in anatomical structure of seagrasses from those of terrestrial plants are:
(a) a lack of stomata (Fig. 6A–G); (b) extremely thin
cuticle (Fig. 6H–P); (c) epidermis acting as the major photosynthetic site; and (d) reduced water conducting elements. Otherwise, other cell types and
tissues may be similar or slightly different among
different seagrass groups. The leaf blade anatomy of
seagrasses does not correspond to either the typical
C 3 or C 4 anatomy of terrestrial plants.
a. Leaf Cuticle
Ultrastructurally, there are three distinct types of
cuticle in seagrasses and they appear to be genus
specific. (a) The cuticle appears as a thin, uniform
electron transparent layer, 0.1–0.2 µm thick, as in
the case of Halophila, Thalassodendron and Syringodium (Fig. 6I, N and O). (b) It may appear
as a thin, electron transparent layer, 0.05 µm thick,
with ‘subcuticular cavities’ at or near the junction between the cuticle and the outer most tangential cell
walls as in Enhalus, Zostera, Heterozostera, Phyllospadix and Cymodocea (Fig. 6H, J, K and M). (c)
The cuticle appears porous in texture and 0.5 µm
thick, as in the case of Thalassia, Posidonia and
Amphibolis (Fig. 6L and P). Despite marked differences in their appearance, the functions of the cuticle
may be very similar (see below).
b. Leaf Epidermis
Epidermal cells are normally small, about 10–30 µm
in diameter. Histochemically and ultrastructurally,
the walls are different. The thicker outer tangential
walls mainly consist of polysaccharides and protein
with little cellulose, and are never lignified. They
appear as two distinct zones: the outer one is more
packed with microfibers and the inner one has a
lamellar appearance (Fig. 6H–P). Whether there is
any functional difference in these two zones has yet
to be established. The radial wall is tapering from the
outer walls toward the inner ones and mainly consists
of cellulose. The inner tangential walls have a uniform lamellar appearance and wall ingrowths may
occur in certain species (Fig. 6I; see below).
A distinctive feature of the epidermis of certain
seagrasses is the presence of wall ingrowths, a characteristic of transfer cells in many terrestrial plants
associated with a convoluted plasmalemma with
a relatively large surface area (Gunning and Pate,
1968; Gunning, 1972; Pate and Gunning, 1972)
(Figs. 6I and 7A, C and E). This peculiar structure
was speculated to have an association with osmoregulation and nutrient uptake in seagrasses (Jagels,
1973; Doohan and Newcomb, 1976). However, such
structures are normally more pronounced on the inner than the outer tangential wall, even though they
are absent in certain genera e.g. Posidonia (Fig. 7D),
Amphibolis and Syringodium (Fig. 7G). These facts
suggest that the epidermal wall ingrowths, reminiscent of transfer cells may not be associated with
nutrient uptake and osmoregulation in seagrasses.
Furthermore, it has been reported that plasmodesmata are present at earlier stages of leaf development, but disappear in the mature leaf epidermis of
Z. marina and T. testudinum (Jagels, 1983). These
observations indicate that there appears to be no cytoplasmic continuity (plasmodesmata) between the
epidermal and underlying mesophyll cells in those
species possessing wall ingrowths. This implies
J. Kuo and C. den Hartog
usually more pronounced on the abaxial than the
adaxial surface. Unicellular short hairs/spines or
‘marginal teeth’ often occur in the leaf blade margins (Fig. 6D); in particular, toward the leaf apex as
in Cymodocea, Thalassodendron, Phyllospadix (‘fin
cells’), Thalassia and most species of Halophila.
These features are often taken to be taxonomically
diagnostic.
The shape of epidermal cells in surface view
may differ between seagrass species, and has been
used as a species identification character in certain
genera; e.g. in Posidonia (Fig. 6A–C), Phyllospadix
(Fig. 6E–G), Heterozostera and Halophila (Kuo,
1978; Kuo and McComb, 1989; Kuo et al., 1988;
Kuo and Stewart, 1995; Cambridge and Lambers,
1998; Kuo, 2005). The appearance of the leaf
apex has also been used as an important diagnostic
feature for identifying certain seagrass genera
including Zostera, Halodule and Amphibolis (den
Hartog, 1970). However, the reliability of this
feature has been questioned for identifying species
of Halodule (Phillips and Me˜ nez, 1988).
3. Leaf Blade Anatomy and Ultrastructure
Anatomically, the arrangement of tissues and cells
in the adaxial and the abaxial sides of a strap
leaf blade is identical, but the position of xylem
elements (adaxial side) in the vascular bundles can
usually be used to determine which side is which
(Fig. 8A and B).
The most obvious differences in anatomical structure of seagrasses from those of terrestrial plants are:
(a) a lack of stomata (Fig. 6A–G); (b) extremely thin
cuticle (Fig. 6H–P); (c) epidermis acting as the major photosynthetic site; and (d) reduced water conducting elements. Otherwise, other cell types and
tissues may be similar or slightly different among
different seagrass groups. The leaf blade anatomy of
seagrasses does not correspond to either the typical
C 3 or C 4 anatomy of terrestrial plants.
a. Leaf Cuticle
Ultrastructurally, there are three distinct types of
cuticle in seagrasses and they appear to be genus
specific. (a) The cuticle appears as a thin, uniform
electron transparent layer, 0.1–0.2 µm thick, as in
the case of Halophila, Thalassodendron and Syringodium (Fig. 6I, N and O). (b) It may appear
as a thin, electron transparent layer, 0.05 µm thick,
with ‘subcuticular cavities’ at or near the junction between the cuticle and the outer most tangential cell
walls as in Enhalus, Zostera, Heterozostera, Phyllospadix and Cymodocea (Fig. 6H, J, K and M). (c)
The cuticle appears porous in texture and 0.5 µm
thick, as in the case of Thalassia, Posidonia and
Amphibolis (Fig. 6L and P). Despite marked differences in their appearance, the functions of the cuticle
may be very similar (see below).
b. Leaf Epidermis
Epidermal cells are normally small, about 10–30 µm
in diameter. Histochemically and ultrastructurally,
the walls are different. The thicker outer tangential
walls mainly consist of polysaccharides and protein
with little cellulose, and are never lignified. They
appear as two distinct zones: the outer one is more
packed with microfibers and the inner one has a
lamellar appearance (Fig. 6H–P). Whether there is
any functional difference in these two zones has yet
to be established. The radial wall is tapering from the
outer walls toward the inner ones and mainly consists
of cellulose. The inner tangential walls have a uniform lamellar appearance and wall ingrowths may
occur in certain species (Fig. 6I; see below).
A distinctive feature of the epidermis of certain
seagrasses is the presence of wall ingrowths, a characteristic of transfer cells in many terrestrial plants
associated with a convoluted plasmalemma with
a relatively large surface area (Gunning and Pate,
1968; Gunning, 1972; Pate and Gunning, 1972)
(Figs. 6I and 7A, C and E). This peculiar structure
was speculated to have an association with osmoregulation and nutrient uptake in seagrasses (Jagels,
1973; Doohan and Newcomb, 1976). However, such
structures are normally more pronounced on the inner than the outer tangential wall, even though they
are absent in certain genera e.g. Posidonia (Fig. 7D),
Amphibolis and Syringodium (Fig. 7G). These facts
suggest that the epidermal wall ingrowths, reminiscent of transfer cells may not be associated with
nutrient uptake and osmoregulation in seagrasses.
Furthermore, it has been reported that plasmodesmata are present at earlier stages of leaf development, but disappear in the mature leaf epidermis of
Z. marina and T. testudinum (Jagels, 1983). These
observations indicate that there appears to be no cytoplasmic continuity (plasmodesmata) between the
epidermal and underlying mesophyll cells in those
species possessing wall ingrowths. This implies