76
J. Kuo and C. den Hartog
occluded by a plug of callose where it enters the pistil and the adjacent cells have callose deposited only
at the wall adjacent to the pollen tube (McConchie
and Knox, 1989a).
The entry of the pollen tube into the embryo
sac has rarely been observed in any seagrass (see
McConchie and Knox, 1989a; Ackerman, 1993).
The pollen tube of Cymodocea enters through a
micropyle and grows directly toward a synergid
(Bornet, 1864), while that of Amphibolis grows
between the cells of the nucellus before entering a
synergid laterally from below.
Flowering in some seagrass species apparently
does not occur often or has not been observed in the
field (Marb` a and Walker, 1999), and the male flowers
in some species are rare (Williams, 1995). Therefore,
the precise pollination and fertilization processes in
these species remain unknown. It is also necessary to
prove whether apomixy operates in certain seagrass
species. Furthermore, knowledge of the barriers, if
any, for preventing interspecific crossings leading to
hybridization is still lacking (see also Waycott et al.,
Chapter 2 and Ackerman, Chapter 4).
D. Fruit and Seed
After fertilization, the ovary develops into a fruit
in all flowering plants including seagrasses. In seagrasses, fruit development and fruit structure are
as diversified as their flowering patterns and floral
structures (see also Orth et al., Chapter 5). In general the seeds, ranging in the size from 0.3 to 0.5 mm
in some Halophila species to more than 1–2 cm in
Enhalus, are furnished with a nutrition reserve and
sink rather than float. The most remarkable structural feature in embryos of seagrasses is that the endosperm is short-lived and has been replaced by an
enlarged hypocotyl (Tomlinson, 1982). In some seagrasses, the hypocotyl seems not only to serve as a
nutrient storage for germination but also to provide
a stabilizing device maintaining a constant seedling
orientation.
1. Hydrocharitaceae
The floating female flowers of Enhalus are supported
by elongated peduncles, which after pollination
become coiled and retract the developing fruits
down into the seagrass canopy. The mature fruit
of Enhalus is a fleshy capsule ca. 6 cm long with
a spinous surface. The capsule dehisces to release
2–6 angular seeds covered by a thin membrane
(Fig. 13A). The embryo has a large hypocotyl, but
the radicle is not developed. The fruit of Thalassia is globose, containing several angular seeds
(Fig. 13B); the embryo has a large hypocotyl fused
with the cotyledon (Fig. 14F). The fleshy capsule of
Halophila has a persistent hypanthium (Fig. 13C)
and contains several to numerous sub-spherical
seeds, 0.5–2 mm in diameter depending on the
species (Fig. 14D and E). Each seed possesses an
embryo with a distinct leaf primordium protected
by a coiled cotyledon, and an enlarged hypocotyl
(Fig. 14D). The seed coat consists of several cell
layers, the remains of the pericarp and a two-layered
integument. The surface of the seed coat has distinct
isometric reticulation and some micro-sculptures
within the reticulation in most of the Halophila
species (Fig. 14A) (Kuo and Kirkman, 1992, 1995)
but appears as numerous fine protrusions in H.
tricostata (Fig. 14B and C) (Kuo et al., 1993).
2. Zosteraceae
The fruit of Zostera and Heterozostera (Fig. 13D), is
an achene with a scarious pericarp and a hard, fibrous
endocarp as a ‘seed coat’. The seed of these genera is
ellipsoidal and the embryo has an enlarged hypocotyl
continuous with a large cotyledon (Fig. 14I and J).
In contrast, the fruits of Phyllospadix are dark and
rhomboid with two stiff inflexible arms covered numerous stiff bristles, forming a ‘grappling apparatus’ (Fig. 14K) that provides them with an excellent means to become entangled with marine algae,
coralline algae in particular, and other organisms on
the substratum (Gibbs, 1902). During seed development, the pericarp differentiates into a soft exocarp
and mesocarp and a hard fibrous endocarp. The latter forms the main body of the two arms; it produces
a large number of long cells with thick, lignified
walled, which later become stiff inflexible bristles.
3. Posidoniaceae
In the Posidoniaceae, the fruits are soft with a
fleshy pericarp, when shed. Each fruit splits longitudinally and releases a single developing seed
(Fig. 13H and I). The embryo has an enlarged
hypocotyl, a short radicle and is covered by a thin
membrane (Fig. 14G).
J. Kuo and C. den Hartog
occluded by a plug of callose where it enters the pistil and the adjacent cells have callose deposited only
at the wall adjacent to the pollen tube (McConchie
and Knox, 1989a).
The entry of the pollen tube into the embryo
sac has rarely been observed in any seagrass (see
McConchie and Knox, 1989a; Ackerman, 1993).
The pollen tube of Cymodocea enters through a
micropyle and grows directly toward a synergid
(Bornet, 1864), while that of Amphibolis grows
between the cells of the nucellus before entering a
synergid laterally from below.
Flowering in some seagrass species apparently
does not occur often or has not been observed in the
field (Marb` a and Walker, 1999), and the male flowers
in some species are rare (Williams, 1995). Therefore,
the precise pollination and fertilization processes in
these species remain unknown. It is also necessary to
prove whether apomixy operates in certain seagrass
species. Furthermore, knowledge of the barriers, if
any, for preventing interspecific crossings leading to
hybridization is still lacking (see also Waycott et al.,
Chapter 2 and Ackerman, Chapter 4).
D. Fruit and Seed
After fertilization, the ovary develops into a fruit
in all flowering plants including seagrasses. In seagrasses, fruit development and fruit structure are
as diversified as their flowering patterns and floral
structures (see also Orth et al., Chapter 5). In general the seeds, ranging in the size from 0.3 to 0.5 mm
in some Halophila species to more than 1–2 cm in
Enhalus, are furnished with a nutrition reserve and
sink rather than float. The most remarkable structural feature in embryos of seagrasses is that the endosperm is short-lived and has been replaced by an
enlarged hypocotyl (Tomlinson, 1982). In some seagrasses, the hypocotyl seems not only to serve as a
nutrient storage for germination but also to provide
a stabilizing device maintaining a constant seedling
orientation.
1. Hydrocharitaceae
The floating female flowers of Enhalus are supported
by elongated peduncles, which after pollination
become coiled and retract the developing fruits
down into the seagrass canopy. The mature fruit
of Enhalus is a fleshy capsule ca. 6 cm long with
a spinous surface. The capsule dehisces to release
2–6 angular seeds covered by a thin membrane
(Fig. 13A). The embryo has a large hypocotyl, but
the radicle is not developed. The fruit of Thalassia is globose, containing several angular seeds
(Fig. 13B); the embryo has a large hypocotyl fused
with the cotyledon (Fig. 14F). The fleshy capsule of
Halophila has a persistent hypanthium (Fig. 13C)
and contains several to numerous sub-spherical
seeds, 0.5–2 mm in diameter depending on the
species (Fig. 14D and E). Each seed possesses an
embryo with a distinct leaf primordium protected
by a coiled cotyledon, and an enlarged hypocotyl
(Fig. 14D). The seed coat consists of several cell
layers, the remains of the pericarp and a two-layered
integument. The surface of the seed coat has distinct
isometric reticulation and some micro-sculptures
within the reticulation in most of the Halophila
species (Fig. 14A) (Kuo and Kirkman, 1992, 1995)
but appears as numerous fine protrusions in H.
tricostata (Fig. 14B and C) (Kuo et al., 1993).
2. Zosteraceae
The fruit of Zostera and Heterozostera (Fig. 13D), is
an achene with a scarious pericarp and a hard, fibrous
endocarp as a ‘seed coat’. The seed of these genera is
ellipsoidal and the embryo has an enlarged hypocotyl
continuous with a large cotyledon (Fig. 14I and J).
In contrast, the fruits of Phyllospadix are dark and
rhomboid with two stiff inflexible arms covered numerous stiff bristles, forming a ‘grappling apparatus’ (Fig. 14K) that provides them with an excellent means to become entangled with marine algae,
coralline algae in particular, and other organisms on
the substratum (Gibbs, 1902). During seed development, the pericarp differentiates into a soft exocarp
and mesocarp and a hard fibrous endocarp. The latter forms the main body of the two arms; it produces
a large number of long cells with thick, lignified
walled, which later become stiff inflexible bristles.
3. Posidoniaceae
In the Posidoniaceae, the fruits are soft with a
fleshy pericarp, when shed. Each fruit splits longitudinally and releases a single developing seed
(Fig. 13H and I). The embryo has an enlarged
hypocotyl, a short radicle and is covered by a thin
membrane (Fig. 14G).