102
Josef Daniel Ackerman
Walker, 2002). The plants appear to be protandrous
with pollen release preceding stigma receptivity
(Smith and Walker, 2002). Cavolini (1792 translated
in K¨ onig, 1805) suggested that some of the large
amounts of cotton-like pollen must contact stigmas. Indeed, dye release studies and pollen capture experiments in the field indicated that pollen
dispersal distances were greater for Posidonia australis Hooker f. (flowers at the top of canopy) than
Posidonia sinuosa (flowers within the canopy), and
that these differences were reflected in the degree
of genetic variability of the two species (Smith,
2000).
J. Heterozostera
Sexual reproduction in Heterozostera tasmanica
(Martens ex Ascherson) den Hartog (Campey et al.,
2002) was variable in time and space and was not
evident in some years. The maximum seed production was <20% over 3 years, but some seeds may
have been exported from the system by hydrodynamic processes or eaten by predators. It is likely
that pollen transport and capture is similar to that of
Nanozostera and Zostera given the similarities in the
inflorescence and floral structures (see below). Note
that Les et al. (2002) and Waycott et al. (Chapter 2)
collapse Heterzostera into Zostera.
K. Phyllospadix
There is considerable spatial variation in the location of male and female clones and female clones
were more common than males in the upper intertidal (Williams, 1995). Field observations indicate
that anthers exposed to air dehisce pollen on the water surface and pollinate stigmas of exposed female
plants, however pollen was also released underwater
during ebb tide when they pollinate submerged stigmas (Dudley, 1893). Additional information on the
nature of pollination in the exposed rocky intertidal
zone is needed.
L. Zostera and Nanozostera
Zostera marina is the best characterized seagrass
species in terms of the mechanics of pollination, and
Clavaud (1878) noted that water motion carried the
pollen. Pollen transport and capture was characterized in a laboratory flow chamber using stroboscopic
photography and in the field using pollen models and
gel-coated surfaces (Ackerman, 1997a,b, 2002). In
the lab, pollination was found to occur under smooth
and viscous conditions (lower Reynolds number)
rather than the turbulent eddies described for wind
pollination (Fig. 4B; see Niklas, 1992; Ackerman,
2000). Morphological changes during phenology,
namely the emergence of female flowers, led to an
increase in the fluid shear stress around the inflorescence (Ackerman, 1997a). Filamentous pollen
(2,700 × 7.5 µm) rotated and crossed streamlines
towards female flowers in the flow (Fig. 5; Ackerman, 1997b) due to an axial force that is directly
related to the length and aspect ratio of the pollen
(Forgacs and Mason, 1958). In contrast to spherical pollen, pollination occurs through one of three
mechanisms: (1) direct interception on stigmas; (2)
rotation within 1 / 2 a pollen length of stigmas; and
(3) by being redirected through streamlines towards
carpellate flowers (Ackerman, 1997b). Field measurements revealed that flow within the canopy at
the scale of inflorescences and flowers was similar to those used in the laboratory (Ackerman and
Okubo, 1993), and the differential capture of filamentous vs. spherical particles in the canopy support
these aforementioned pollen and transport mechanisms (Ackerman, 2002). Importantly, it was estimated that between 10
3 and 10
4 Z. marina pollen
are required to pollinate a single flower, an estimate which is similar to the measured pollen–
ovule ratio in this species (Ackerman, 1993; see
below).
VIII. Post-Pollination: Pollen Tubes,
Embryonic and Seedling Development
As in other angiosperms, following the transfer
of pollen to stigma, the pollen germinates and
a pollen tube grows through the stigma, style,
and locule to the micropyle of the ovule where
it penetrates the synergid cells (e.g. Raven et al.,
1999; Kuo and den Hartog, Chapter 3). The pollen
tube facilitates the transfer of sperm cells that
fertilize the egg cell and the endosperm nucleus.
As indicated above, post-pollination processes are
complicated in seagrasses by the complete submergence of reproductive organs in the marine environment (McConchie and Knox, 1989a). Consequently,
pollen and stigmas have a waterproof adhesive that
Josef Daniel Ackerman
Walker, 2002). The plants appear to be protandrous
with pollen release preceding stigma receptivity
(Smith and Walker, 2002). Cavolini (1792 translated
in K¨ onig, 1805) suggested that some of the large
amounts of cotton-like pollen must contact stigmas. Indeed, dye release studies and pollen capture experiments in the field indicated that pollen
dispersal distances were greater for Posidonia australis Hooker f. (flowers at the top of canopy) than
Posidonia sinuosa (flowers within the canopy), and
that these differences were reflected in the degree
of genetic variability of the two species (Smith,
2000).
J. Heterozostera
Sexual reproduction in Heterozostera tasmanica
(Martens ex Ascherson) den Hartog (Campey et al.,
2002) was variable in time and space and was not
evident in some years. The maximum seed production was <20% over 3 years, but some seeds may
have been exported from the system by hydrodynamic processes or eaten by predators. It is likely
that pollen transport and capture is similar to that of
Nanozostera and Zostera given the similarities in the
inflorescence and floral structures (see below). Note
that Les et al. (2002) and Waycott et al. (Chapter 2)
collapse Heterzostera into Zostera.
K. Phyllospadix
There is considerable spatial variation in the location of male and female clones and female clones
were more common than males in the upper intertidal (Williams, 1995). Field observations indicate
that anthers exposed to air dehisce pollen on the water surface and pollinate stigmas of exposed female
plants, however pollen was also released underwater
during ebb tide when they pollinate submerged stigmas (Dudley, 1893). Additional information on the
nature of pollination in the exposed rocky intertidal
zone is needed.
L. Zostera and Nanozostera
Zostera marina is the best characterized seagrass
species in terms of the mechanics of pollination, and
Clavaud (1878) noted that water motion carried the
pollen. Pollen transport and capture was characterized in a laboratory flow chamber using stroboscopic
photography and in the field using pollen models and
gel-coated surfaces (Ackerman, 1997a,b, 2002). In
the lab, pollination was found to occur under smooth
and viscous conditions (lower Reynolds number)
rather than the turbulent eddies described for wind
pollination (Fig. 4B; see Niklas, 1992; Ackerman,
2000). Morphological changes during phenology,
namely the emergence of female flowers, led to an
increase in the fluid shear stress around the inflorescence (Ackerman, 1997a). Filamentous pollen
(2,700 × 7.5 µm) rotated and crossed streamlines
towards female flowers in the flow (Fig. 5; Ackerman, 1997b) due to an axial force that is directly
related to the length and aspect ratio of the pollen
(Forgacs and Mason, 1958). In contrast to spherical pollen, pollination occurs through one of three
mechanisms: (1) direct interception on stigmas; (2)
rotation within 1 / 2 a pollen length of stigmas; and
(3) by being redirected through streamlines towards
carpellate flowers (Ackerman, 1997b). Field measurements revealed that flow within the canopy at
the scale of inflorescences and flowers was similar to those used in the laboratory (Ackerman and
Okubo, 1993), and the differential capture of filamentous vs. spherical particles in the canopy support
these aforementioned pollen and transport mechanisms (Ackerman, 2002). Importantly, it was estimated that between 10
3 and 10
4 Z. marina pollen
are required to pollinate a single flower, an estimate which is similar to the measured pollen–
ovule ratio in this species (Ackerman, 1993; see
below).
VIII. Post-Pollination: Pollen Tubes,
Embryonic and Seedling Development
As in other angiosperms, following the transfer
of pollen to stigma, the pollen germinates and
a pollen tube grows through the stigma, style,
and locule to the micropyle of the ovule where
it penetrates the synergid cells (e.g. Raven et al.,
1999; Kuo and den Hartog, Chapter 3). The pollen
tube facilitates the transfer of sperm cells that
fertilize the egg cell and the endosperm nucleus.
As indicated above, post-pollination processes are
complicated in seagrasses by the complete submergence of reproductive organs in the marine environment (McConchie and Knox, 1989a). Consequently,
pollen and stigmas have a waterproof adhesive that