Chapter 4 Seagrass Sexual Reproduction
95
monoecious species) are monoecious with either
perfect flowers (Posidonia) or unisexual flowers in
cymose inflorescences (Halophila) or in flattened
inflorescences with spathe-spadix arrangements
(Zosteraceae; Table 2; see also Fig. 12C and D, Kuo
and den Hartog, Chapter 3). With the exception of
Enhalus and Thalassia, which share many features
with their freshwater relatives (e.g. Vallisneria), the
flowers are drab rather than showy, and perianth parts
are absent or bract like in appearance (den Hartog,
1970; Tomlinson, 1982). For example, the solitary
unisexual flowers of Cymodoceae are “naked”, being composed essentially of two united anthers on
a filamentous stalk in the case of staminate flowers,
and two free carpels each with two slender styles
(three in Amphibolis) (see Fig. 12, Chapter 3).
The spatial location and orientation of the reproductive organs is critical from the perspective
of submarine pollination. Specifically, the fluid dynamic release and/or capture of pollen will vary according to whether anthers or carpels are located
basally or distally on the plants, given the velocity
gradient near the bottom and the plant canopy (i.e.
benthic and canopy boundary layers; Okubo et al.,
2002). In this situation, there is an advantage to elevating anthers away from the bottom to facilitate
pollen dispersal in the faster moving fluid, while
placing stigmas close to the bottom or within axils
of bracts or vegetative material to facilitate the reduction and redirection of water flow for pollen capture (cf. Niklas, 1992). This appears to be the case
in a number of genera like Thalassia, Halophila,
and Cymodocea, and reaches an extreme in Amphibolis where some anthers can detach and float
to the surface where pollen can disperse through
the water column (McConchie and Knox, 1989b).
This latter example should not be confused with
the obligate surface pollination of Enhalus, where
detached free-floating male flowers encounter female flowers on long spirally coiled peduncles (den
Hartog, 1970; Tomlinson, 1982). The situation becomes more complex when reproductive organs of
diocecious (Syringodium, Phyllospadix) or monoecious genera (Posidonia, Nanozostera, Zostera) are
organized into inflorescences that extend into or
above the top of the plant canopy. In these cases, the
fluid dynamically induced movements of the plants
are likely to affect the dispersal and capture of pollen
through waving sweep-like motions (i.e. monami;
Ackerman and Okubo, 1993) analogous to wind
pollination in grasses and other herbaceous plants
(Niklas, 1992; Ackerman, 2000). Recognizing this,
it should be noted that much research is needed to
confirm these patterns, which extend from fluid dynamic principles in wind-pollinated plants (Niklas,
1992; Ackerman, 2000).
Staminate and carpellate flowers appear to be
organized into a number of different patterns that
do not appear to match with systematic patterns
(i.e. Hydrocharitaceae vs. other seagrass families)
or patterns in pollen morphology (spherical vs. filamentous; see below; Table 2). In the first type,
the carpellate inflorescence of Enhalus and Thalassia are trimerous with subtending perianth parts,
which are easily identified as “flowers” by the nonspecialists (Fig. 1A). The solitary carpellate flowers have long bifid stigmas, which are papillate (for
additional information pertaining to properties of
stigmas see McConchie and Knox, 1989a; Table 2).
The second type of floral arrangement includes a
number of genera (Halophila, Amphibolis, Cymodocea, Halodule, and Thalassodendron) with solitary,
highly-reduced “naked” flowers that can be on short
pedicels close to the seafloor. As noted above, the
staminate flowers are usually stalked with two united
anthers (three in Halophila), and the carpellate flowers usually have two carpels each with a long slender
bifid stigma (Halodule has one stigma; Amphibolis
has three stigmas), which is non-papillate (Halophila
is papillate; Tomlinson, 1982). It is important to note
that the stigmas, which are the receptive surfaces of
the styles, may be long; i.e. ∼9 mm in Amphibolis and ∼30 mm in Cymodocea (Tomlinson, 1982;
McConchie and Knox, 1989b; see Fig. 11, Chapter 3). A third pattern is seen in Posidonia, which
has a racemose inflorescence with perfect flowers
consisting of a single disk-shaped irregularly lobed
stigma and three stamens (Tomlinson, 1982; McConchie and Knox, 1989b; see Fig. 11, Chapter 3).
It is important to note that this type of stigmatic
surface is distinct among the seagrass, which have
slender stigmas, and is more reminiscent of some
freshwater species (Table 2). The final pattern is seen
in the remaining genera in which unisexual flowers
are borne on long branched cymose inflorescences
(Syringodium; Fig. 1C and D) or on long branched
inflorescences with a spathe-spadix configuration
(Heterozostera, Phyllospadix, Nanozostera, and
Zostera; Fig. 1E and F). In Syringodium the stalked
staminate flowers have two united anthers and the
carpellate flowers have two free carpels each with
a bifid stigma. In the Zosteraceae, the carpellate
95
monoecious species) are monoecious with either
perfect flowers (Posidonia) or unisexual flowers in
cymose inflorescences (Halophila) or in flattened
inflorescences with spathe-spadix arrangements
(Zosteraceae; Table 2; see also Fig. 12C and D, Kuo
and den Hartog, Chapter 3). With the exception of
Enhalus and Thalassia, which share many features
with their freshwater relatives (e.g. Vallisneria), the
flowers are drab rather than showy, and perianth parts
are absent or bract like in appearance (den Hartog,
1970; Tomlinson, 1982). For example, the solitary
unisexual flowers of Cymodoceae are “naked”, being composed essentially of two united anthers on
a filamentous stalk in the case of staminate flowers,
and two free carpels each with two slender styles
(three in Amphibolis) (see Fig. 12, Chapter 3).
The spatial location and orientation of the reproductive organs is critical from the perspective
of submarine pollination. Specifically, the fluid dynamic release and/or capture of pollen will vary according to whether anthers or carpels are located
basally or distally on the plants, given the velocity
gradient near the bottom and the plant canopy (i.e.
benthic and canopy boundary layers; Okubo et al.,
2002). In this situation, there is an advantage to elevating anthers away from the bottom to facilitate
pollen dispersal in the faster moving fluid, while
placing stigmas close to the bottom or within axils
of bracts or vegetative material to facilitate the reduction and redirection of water flow for pollen capture (cf. Niklas, 1992). This appears to be the case
in a number of genera like Thalassia, Halophila,
and Cymodocea, and reaches an extreme in Amphibolis where some anthers can detach and float
to the surface where pollen can disperse through
the water column (McConchie and Knox, 1989b).
This latter example should not be confused with
the obligate surface pollination of Enhalus, where
detached free-floating male flowers encounter female flowers on long spirally coiled peduncles (den
Hartog, 1970; Tomlinson, 1982). The situation becomes more complex when reproductive organs of
diocecious (Syringodium, Phyllospadix) or monoecious genera (Posidonia, Nanozostera, Zostera) are
organized into inflorescences that extend into or
above the top of the plant canopy. In these cases, the
fluid dynamically induced movements of the plants
are likely to affect the dispersal and capture of pollen
through waving sweep-like motions (i.e. monami;
Ackerman and Okubo, 1993) analogous to wind
pollination in grasses and other herbaceous plants
(Niklas, 1992; Ackerman, 2000). Recognizing this,
it should be noted that much research is needed to
confirm these patterns, which extend from fluid dynamic principles in wind-pollinated plants (Niklas,
1992; Ackerman, 2000).
Staminate and carpellate flowers appear to be
organized into a number of different patterns that
do not appear to match with systematic patterns
(i.e. Hydrocharitaceae vs. other seagrass families)
or patterns in pollen morphology (spherical vs. filamentous; see below; Table 2). In the first type,
the carpellate inflorescence of Enhalus and Thalassia are trimerous with subtending perianth parts,
which are easily identified as “flowers” by the nonspecialists (Fig. 1A). The solitary carpellate flowers have long bifid stigmas, which are papillate (for
additional information pertaining to properties of
stigmas see McConchie and Knox, 1989a; Table 2).
The second type of floral arrangement includes a
number of genera (Halophila, Amphibolis, Cymodocea, Halodule, and Thalassodendron) with solitary,
highly-reduced “naked” flowers that can be on short
pedicels close to the seafloor. As noted above, the
staminate flowers are usually stalked with two united
anthers (three in Halophila), and the carpellate flowers usually have two carpels each with a long slender
bifid stigma (Halodule has one stigma; Amphibolis
has three stigmas), which is non-papillate (Halophila
is papillate; Tomlinson, 1982). It is important to note
that the stigmas, which are the receptive surfaces of
the styles, may be long; i.e. ∼9 mm in Amphibolis and ∼30 mm in Cymodocea (Tomlinson, 1982;
McConchie and Knox, 1989b; see Fig. 11, Chapter 3). A third pattern is seen in Posidonia, which
has a racemose inflorescence with perfect flowers
consisting of a single disk-shaped irregularly lobed
stigma and three stamens (Tomlinson, 1982; McConchie and Knox, 1989b; see Fig. 11, Chapter 3).
It is important to note that this type of stigmatic
surface is distinct among the seagrass, which have
slender stigmas, and is more reminiscent of some
freshwater species (Table 2). The final pattern is seen
in the remaining genera in which unisexual flowers
are borne on long branched cymose inflorescences
(Syringodium; Fig. 1C and D) or on long branched
inflorescences with a spathe-spadix configuration
(Heterozostera, Phyllospadix, Nanozostera, and
Zostera; Fig. 1E and F). In Syringodium the stalked
staminate flowers have two united anthers and the
carpellate flowers have two free carpels each with
a bifid stigma. In the Zosteraceae, the carpellate