Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
73
bisexual (hermaphrodite), each with three stamens
and a female flower. The female flower consists of an
oblong ovary containing one ovule (Fig. 11F) and a
disc-like stigma with irregular lobes (Fig. 11E), but
no perianth (Fig. 11D). The mature stamens split
longitudinally and release thread-like pollen grains
(Fig. 12H) and the remaining thecae persist after
pollination.
5. Cymodoceaceae
The flowers of the Cymodoceaceae are usually
solitary, and terminal on erect shoots (Halodule, Cymodocea) or their branches (Amphibolis,
Thalassodendron). They are arranged as distinct
cymose inflorescences in Syringodium (Fig. 11C).
Each inflorescence consists of two morphologically
identical flowers, which develop in the same way, except for the male flower in Thalassodendron, which
has two developmentally different but morphologically identical flowers (Fig. 12G). The naked flowers
are enclosed by leaf-like bracts. The male flower has
two anthers fused back-to-back at the same height in
all genera, while those in Halodule are at an unequal
height (Fig. 12F). The female flower is sessile or
with a short pedicel, has two free ovaries, each with
a long simple style (Fig. 13F) (Halodule) or a short
style which extends into two to three slender stigmas
(Fig. 11G). The interpretation of floral morphology
and structure in the Cymodoceaceae has been discussed by Kuo and McComb (1989) and McConchie
and Knox (1989a).
6. Ruppiaceae
The inflorescence of Ruppia is formed as a terminal
spike, which is enclosed by two subopposite foliage
leaves. The peduncle elongates at anthesis, in most
species elevating the inflorescence to or near the water surface. The inflorescence consists of two (rarely
more) flowers located on opposite sides of the inflorescence axis. Each flower has two stamens with
two thecae, and a gynoecium with (2-) 4 or more (up
to 16) carpels, each bearing a single ovule (Lacroix
and Kemp, 1997).
B. Floral Development
There are only a few studies on developmental
morphology and anatomy of reproductive structures
in seagrasses. Soros-Pottruff and Posluszny (1994)
found that the developmental sequence is similar
in the dioecious Phyllospadix and the monoecious
Zostera. Initiation and development of the reproductive structures are acropetal along the length of the
spadix. However, the female flower of Phyllospadix
also contains the male floral units, which remain
as minute residual structures; in contrast, there are
no female reproductive units on the male plants.
These authors concluded that, based on the more
specialized habitat of Phyllospadix, possible pollination mechanisms with well-developed retinaculae
and the presence of residual stamens on female inflorescences, the genus appears to be more primitive than Zostera. Tomlinson and Posluszny (1978)
recorded that inflorescences in Syringodium filiforme are unique among the family Cymodoceaceae.
It has an initially racemose and subsequently extensively cymose development, with terminal units referred to as ‘flowers.’ These authors suggested that
its sympodial inflorescence is comparable with that
of the Zannichelliaceae, although it is more variable
and complex in the latter.
C. Floral Structure
1. Stigma Structure
According to McConchie and Knox (1989a), stigmas of the seagrasses belong to the ‘dry type,’
without free fluid on the surface; they have a hydrated proteinaceous surface layer and pellicle, functioning as a regulator of adhesion by hydration.
However, the same authors (McConchie and Knox,
1989b) considered the stigma of Posidonia as belonging to the ‘wet type,’ although a secretion is
not optically detectable, and a pellicle-like layer
is present. Furthermore, Enhalus, Thalassia and
Halophila have papillate stigmata with surface pellicles that possess esterase activity, with acid phosphatase accumulating below the cuticle. Zostera
and Amphibolis are non-papillate, but A. antarctica has a thin pellicle covering the surface of the
stigma.
2. Pollen Structure and Pollination
Pollination in all seagrasses is hydrophilous. The mature stamens of seagrasses form longitudinal slits
73
bisexual (hermaphrodite), each with three stamens
and a female flower. The female flower consists of an
oblong ovary containing one ovule (Fig. 11F) and a
disc-like stigma with irregular lobes (Fig. 11E), but
no perianth (Fig. 11D). The mature stamens split
longitudinally and release thread-like pollen grains
(Fig. 12H) and the remaining thecae persist after
pollination.
5. Cymodoceaceae
The flowers of the Cymodoceaceae are usually
solitary, and terminal on erect shoots (Halodule, Cymodocea) or their branches (Amphibolis,
Thalassodendron). They are arranged as distinct
cymose inflorescences in Syringodium (Fig. 11C).
Each inflorescence consists of two morphologically
identical flowers, which develop in the same way, except for the male flower in Thalassodendron, which
has two developmentally different but morphologically identical flowers (Fig. 12G). The naked flowers
are enclosed by leaf-like bracts. The male flower has
two anthers fused back-to-back at the same height in
all genera, while those in Halodule are at an unequal
height (Fig. 12F). The female flower is sessile or
with a short pedicel, has two free ovaries, each with
a long simple style (Fig. 13F) (Halodule) or a short
style which extends into two to three slender stigmas
(Fig. 11G). The interpretation of floral morphology
and structure in the Cymodoceaceae has been discussed by Kuo and McComb (1989) and McConchie
and Knox (1989a).
6. Ruppiaceae
The inflorescence of Ruppia is formed as a terminal
spike, which is enclosed by two subopposite foliage
leaves. The peduncle elongates at anthesis, in most
species elevating the inflorescence to or near the water surface. The inflorescence consists of two (rarely
more) flowers located on opposite sides of the inflorescence axis. Each flower has two stamens with
two thecae, and a gynoecium with (2-) 4 or more (up
to 16) carpels, each bearing a single ovule (Lacroix
and Kemp, 1997).
B. Floral Development
There are only a few studies on developmental
morphology and anatomy of reproductive structures
in seagrasses. Soros-Pottruff and Posluszny (1994)
found that the developmental sequence is similar
in the dioecious Phyllospadix and the monoecious
Zostera. Initiation and development of the reproductive structures are acropetal along the length of the
spadix. However, the female flower of Phyllospadix
also contains the male floral units, which remain
as minute residual structures; in contrast, there are
no female reproductive units on the male plants.
These authors concluded that, based on the more
specialized habitat of Phyllospadix, possible pollination mechanisms with well-developed retinaculae
and the presence of residual stamens on female inflorescences, the genus appears to be more primitive than Zostera. Tomlinson and Posluszny (1978)
recorded that inflorescences in Syringodium filiforme are unique among the family Cymodoceaceae.
It has an initially racemose and subsequently extensively cymose development, with terminal units referred to as ‘flowers.’ These authors suggested that
its sympodial inflorescence is comparable with that
of the Zannichelliaceae, although it is more variable
and complex in the latter.
C. Floral Structure
1. Stigma Structure
According to McConchie and Knox (1989a), stigmas of the seagrasses belong to the ‘dry type,’
without free fluid on the surface; they have a hydrated proteinaceous surface layer and pellicle, functioning as a regulator of adhesion by hydration.
However, the same authors (McConchie and Knox,
1989b) considered the stigma of Posidonia as belonging to the ‘wet type,’ although a secretion is
not optically detectable, and a pellicle-like layer
is present. Furthermore, Enhalus, Thalassia and
Halophila have papillate stigmata with surface pellicles that possess esterase activity, with acid phosphatase accumulating below the cuticle. Zostera
and Amphibolis are non-papillate, but A. antarctica has a thin pellicle covering the surface of the
stigma.
2. Pollen Structure and Pollination
Pollination in all seagrasses is hydrophilous. The mature stamens of seagrasses form longitudinal slits