Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
75
in the freshwater aquatics Vallisneria, Lagarosiphon
and Hydrilla in the same family. Ruppia cirrhosa
and R. aff. tuberosa release individual boomerangshaped pollen grains from submerged anthers to the
water surface for pollination. However, R. maritima
achieves a submerged pollination at the air-water interface of a gaseous bubble supplied from the air
channels of the plant itself (Verhoeven, 1979). The
pollen grain of Ruppia possesses both exine and
intine as in terrestrial plants; the exine is reticulate,
and not well developed over the ends of the
grain.
Halophila and Thalassia have ellipsoid to spherical pollen grains forming chains within mucilage
tubes, up to 80 µm long in Halophila, to achieve
the same effect as filamentous pollen in the remaining seagrass genera. The pollen grain of Halophila
possesses both exine and intine e.g. that of H. decipiens has a thin and unornamented exine covering a
massive stratified intine (Pettitt and Jermy, 1975).
The remaining seagrass genera all have filamentous pollen (see Table 2, Ackerman, Chapter 4) but
the disposition of the filamentous pollen within the
anther loculus is different among genera. Pollen ultrastructural development in Z. marina, Amphibolis
and Thalassodendron has been described by Pettitt
and Jermy (1975) and Ducker et al. (1978), respectively. The latter authors observed that the first pollen
mitosis occurs at the end of the young spore period
immediately preceding the vacuolate period, in contrast to many terrestrial pollen events.
The pollen filaments lie parallel to the longitudinal axis of the anther in Zostera; irregularly
coiled in Halodule; coiled but in no particular
orientation in Thalassodendron; and coiled like a
spring in Amphibolis (Ducker et al., 1977). Furthermore, the length of the filamentous pollen varies
with the genus, Zostera—2700 µm; Phyllospadix—
1000 µm; Posidonia—450 ± 140 µm and bowshaped; Halodule—1000 µm; and Amphibolis—
2840 ± 590 µm. Pollen of A. griffithii has a fine
tip without branching and that of A. antarctica has a
forked tip (see McConchie and Knox, 1989a).
The pollen wall of Zostera capricorni, Heterozostera nigricaulis (as H. tasmanica) and Phyllospadix torreyi has two microfibrillar layers with
a clear demarcation. H. nigricaulis (as H. tasmanica) has an additional tubular inclusion in the outer
wall layer. On the other hand, the pollen wall of Posidonia australis is not clearly stratified (layered). The
mature pollen of P. australis has globular and tubular inclusions occurring in the outer region of the
wall (Pettitt, 1984). The microfibrillar intine wall,
ca. 0.6 µm, is covered with scattered lipids which,
together with mucilage, may be important agents
for regulating cohesion and releasing and dispersing pollen in seawater (McConchie et al., 1982).
5. Pollen Structure
All seagrass pollen is trinucleate regardless of
whether it is granular or filamentous, and its nuclei
are located centrally. Posidonia australis contains an
elongated vegetative nucleus invariably in close apposition to the two ovoid sperm cells (Fig. 12K and
L). These cells are bounded by irregular periplasm
containing vesicles and their cytoplasm resembles
that of the surrounding vegetative cells (Ducker
et al., 1978).
The pollen from both granular and filamentous
types is released from the submerged anthers and
‘searches’ through currents and waves (Verduin
et al., 1996) for a submerged stigma. Cox (1988)
and Ackerman (1995) discussed the two or threedimensional ‘search’ by the pollen, particular in intertidal seagrass populations (see also Ackerman,
Chapter 4). It is possible that pollination can also take
place at the water surface in some intertidal seagrass
genera such as Halophila, Phyllospadix and Zostera,
because their mature reproductive organs often float
at the water surface and pollen appears to be released
during the highest tidal range.
6. Pollen–Stigma Interactions
For several species of the genera Halodule and
Zostera, pollen germination may occur before the
pollen contacts the stigma (Yamashita, 1976). On
the other hand, Ackerman (1993) supported De
Cock’s (1978) observations that the pollen of Z. marina would not germinate without a ‘stigmatic factor.’ McConchie and Knox (1989a) described the
interaction between the pollen and the stigma surface
in Zostera, Posidonia and Amphibolis. Ultrastructurally, the proteinaceous film covering the pollen
and the secretion from the stigma surface coalesce
at each contact point, where the cuticle covering the
stigma frequently becomes detached. Pollen germination then proceeds near but never at the point of
stigma adhesion and penetrates the stigma between
the receptive stigma cells (McConchie and Knox,
1989a,b). The stigma forms a collar of wall material
that is raised to around the pollen tube in Amphibolis. On the other hand, in Posidonia the pollen tube is
75
in the freshwater aquatics Vallisneria, Lagarosiphon
and Hydrilla in the same family. Ruppia cirrhosa
and R. aff. tuberosa release individual boomerangshaped pollen grains from submerged anthers to the
water surface for pollination. However, R. maritima
achieves a submerged pollination at the air-water interface of a gaseous bubble supplied from the air
channels of the plant itself (Verhoeven, 1979). The
pollen grain of Ruppia possesses both exine and
intine as in terrestrial plants; the exine is reticulate,
and not well developed over the ends of the
grain.
Halophila and Thalassia have ellipsoid to spherical pollen grains forming chains within mucilage
tubes, up to 80 µm long in Halophila, to achieve
the same effect as filamentous pollen in the remaining seagrass genera. The pollen grain of Halophila
possesses both exine and intine e.g. that of H. decipiens has a thin and unornamented exine covering a
massive stratified intine (Pettitt and Jermy, 1975).
The remaining seagrass genera all have filamentous pollen (see Table 2, Ackerman, Chapter 4) but
the disposition of the filamentous pollen within the
anther loculus is different among genera. Pollen ultrastructural development in Z. marina, Amphibolis
and Thalassodendron has been described by Pettitt
and Jermy (1975) and Ducker et al. (1978), respectively. The latter authors observed that the first pollen
mitosis occurs at the end of the young spore period
immediately preceding the vacuolate period, in contrast to many terrestrial pollen events.
The pollen filaments lie parallel to the longitudinal axis of the anther in Zostera; irregularly
coiled in Halodule; coiled but in no particular
orientation in Thalassodendron; and coiled like a
spring in Amphibolis (Ducker et al., 1977). Furthermore, the length of the filamentous pollen varies
with the genus, Zostera—2700 µm; Phyllospadix—
1000 µm; Posidonia—450 ± 140 µm and bowshaped; Halodule—1000 µm; and Amphibolis—
2840 ± 590 µm. Pollen of A. griffithii has a fine
tip without branching and that of A. antarctica has a
forked tip (see McConchie and Knox, 1989a).
The pollen wall of Zostera capricorni, Heterozostera nigricaulis (as H. tasmanica) and Phyllospadix torreyi has two microfibrillar layers with
a clear demarcation. H. nigricaulis (as H. tasmanica) has an additional tubular inclusion in the outer
wall layer. On the other hand, the pollen wall of Posidonia australis is not clearly stratified (layered). The
mature pollen of P. australis has globular and tubular inclusions occurring in the outer region of the
wall (Pettitt, 1984). The microfibrillar intine wall,
ca. 0.6 µm, is covered with scattered lipids which,
together with mucilage, may be important agents
for regulating cohesion and releasing and dispersing pollen in seawater (McConchie et al., 1982).
5. Pollen Structure
All seagrass pollen is trinucleate regardless of
whether it is granular or filamentous, and its nuclei
are located centrally. Posidonia australis contains an
elongated vegetative nucleus invariably in close apposition to the two ovoid sperm cells (Fig. 12K and
L). These cells are bounded by irregular periplasm
containing vesicles and their cytoplasm resembles
that of the surrounding vegetative cells (Ducker
et al., 1978).
The pollen from both granular and filamentous
types is released from the submerged anthers and
‘searches’ through currents and waves (Verduin
et al., 1996) for a submerged stigma. Cox (1988)
and Ackerman (1995) discussed the two or threedimensional ‘search’ by the pollen, particular in intertidal seagrass populations (see also Ackerman,
Chapter 4). It is possible that pollination can also take
place at the water surface in some intertidal seagrass
genera such as Halophila, Phyllospadix and Zostera,
because their mature reproductive organs often float
at the water surface and pollen appears to be released
during the highest tidal range.
6. Pollen–Stigma Interactions
For several species of the genera Halodule and
Zostera, pollen germination may occur before the
pollen contacts the stigma (Yamashita, 1976). On
the other hand, Ackerman (1993) supported De
Cock’s (1978) observations that the pollen of Z. marina would not germinate without a ‘stigmatic factor.’ McConchie and Knox (1989a) described the
interaction between the pollen and the stigma surface
in Zostera, Posidonia and Amphibolis. Ultrastructurally, the proteinaceous film covering the pollen
and the secretion from the stigma surface coalesce
at each contact point, where the cuticle covering the
stigma frequently becomes detached. Pollen germination then proceeds near but never at the point of
stigma adhesion and penetrates the stigma between
the receptive stigma cells (McConchie and Knox,
1989a,b). The stigma forms a collar of wall material
that is raised to around the pollen tube in Amphibolis. On the other hand, in Posidonia the pollen tube is