Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
79
‘comb’ or ‘grappling apparatus’ (Fig. 15C) (Tepper,
1882a, b). This structure appears from the ovary
wall late in the development of the flower, although
its initiation is present prior to fertilization (see
McConchie and Knox, 1989a). An abscission layer
forms immediately below the comb and allows
the release of the seedling, which is free until its
grappling apparatus becomes caught in a substratum
such as the fibrous base of Posidonia plant, algal
turf or sand. Roots develop subsequently, and the
grappling apparatus remains on the plant for another
6–12 months (Fig. 16F).
In Thalassodendron, the innermost bracts continue to develop after fertilization, and eventually enclose the carpels to form a false fruit (Cohen, 1939).
Normally only one carpel is fertile, and a seed from
this carpel germinates on the parent plant (see Kuo
and Kirkman, 1987, 1990). Young seedlings at first
have an aril-like structure that disappears at a later
stage of seedling development. The seedling produces, firstly, an aberrant seedling leaf and a scarious seedling sheath, then several true foliage leaves
and finally several root primordia (Fig. 15D) (Isaacs,
1969; den Hartog, 1970). The mature seedling detaches from the parent; separates from its protecting
bract and sinks to the bottom. The remaining pericarps finally wash onto the shore.
5. Types of Seeds
Kuo and Kirkman (1996) classified three main
types of functional seeds in relation to nutrient
storage, seed coat structure and germination in
seagrasses (see also Orth et al., Chapter 5). Type a.
Seeds with hard pericarp or a distinct seed coat, e.g.
Zostera, Phyllospadix, Heterozostera, Cymodocea,
Syringodium and Halodule. However, seeds of both
Syringodium and Halodule have thicker and harder
coats than the seeds of the other genera and may
have a longer dormancy. Type b. Seeds with a
membranous coat, a modified pistil, as in Posidonia,
and without a distinct dormancy, e.g. Enhalus,
Thalassia and Posidonia. Type c. Seeds that neither
have a normal distinct seed coat nor dormancy, but
germinate on the parent plants (viviparous reproduction), e.g. Thalassodendron and Amphibolis:
this unusual reproduction has not been reported in
other aquatic plants. Histochemically, in Types a
and b, seeds store large amount of starch and little
protein in the hypocotyledonous tissue to be used
during germination and seedling development (Fig.
14E, F, J and M). On the other hand, Type c seeds do
not store starch and proteins, but the embryo obtains
the necessary nutrients for seedling development
direct from the parent plants through specialized
transfer cells at the interface of seed and the parental
tissue (Fig. 15F–I) (Kuo and Kirkman, 1996). These
transfer cells have extended wall ingrowths and a
plasmalemma, and are always associated with nutrient or solute transfer. The Type a seed has a distinct
dormancy; while the Types b and c seeds do not.
The accumulation of nutrient including nitrogen, phosphorus and other macro- and microelements in Posidonia spp. (Hocking et al., 1980) and
in Enhalus (Monta˜ no et al., 1999) are in general
comparable with those of terrestrial plants. Furthermore, there are numerous electron dense globoid
crystals present in all tissues of the embryo of Z.
capricorni and these contain P, Mg, K, Ca, Na
and Cl (West et al., 1992) (see also Romero et al.,
Chapter 11).
6. Seed Numbers in the Annual
and Perennial Seagrasses
As mentioned above only a few species of seagrasses
are known as annual species, and the number of
seeds produced by the annual and perennial plants
varies greatly. For example, monoecious annual
H. decipiens from Toro Point, Panama, had 13,500
seeds m
−2 (McMillan, 1988), while the same species
at Hardy Inlet, Western Australia, produced 176,880
seeds m
−2 (Kuo and Kirkman, 1995). The dioecious annual H. tricostata at Fitzroy Island, in the
Great Barrier Reef produced 70,000 seeds m
−2
(Kuo et al., 1993). In comparison, the dioecious
perennial H. ovalis produced about 480 seeds m
−2
at Whitfords near Perth (Kuo and Kirkman, 1992).
Plants of the annual Z. marina in Gulf of California, north western Mexico, have lifetime of
about four months; they allocate all their resources
into a seed production of up to 184,000 seeds
m
−2 while the perennial Z. marina in Baja California along the Pacific coast produces only 6300
seeds m
−2 (Meling-Lopez and Ibarra-Obando, 1999,
2000).
E. Seedlings
As in all angiosperms, seeds of seagrass species normally germinate after a period of dormancy. However, the length of dormancy periods in seagrass
79
‘comb’ or ‘grappling apparatus’ (Fig. 15C) (Tepper,
1882a, b). This structure appears from the ovary
wall late in the development of the flower, although
its initiation is present prior to fertilization (see
McConchie and Knox, 1989a). An abscission layer
forms immediately below the comb and allows
the release of the seedling, which is free until its
grappling apparatus becomes caught in a substratum
such as the fibrous base of Posidonia plant, algal
turf or sand. Roots develop subsequently, and the
grappling apparatus remains on the plant for another
6–12 months (Fig. 16F).
In Thalassodendron, the innermost bracts continue to develop after fertilization, and eventually enclose the carpels to form a false fruit (Cohen, 1939).
Normally only one carpel is fertile, and a seed from
this carpel germinates on the parent plant (see Kuo
and Kirkman, 1987, 1990). Young seedlings at first
have an aril-like structure that disappears at a later
stage of seedling development. The seedling produces, firstly, an aberrant seedling leaf and a scarious seedling sheath, then several true foliage leaves
and finally several root primordia (Fig. 15D) (Isaacs,
1969; den Hartog, 1970). The mature seedling detaches from the parent; separates from its protecting
bract and sinks to the bottom. The remaining pericarps finally wash onto the shore.
5. Types of Seeds
Kuo and Kirkman (1996) classified three main
types of functional seeds in relation to nutrient
storage, seed coat structure and germination in
seagrasses (see also Orth et al., Chapter 5). Type a.
Seeds with hard pericarp or a distinct seed coat, e.g.
Zostera, Phyllospadix, Heterozostera, Cymodocea,
Syringodium and Halodule. However, seeds of both
Syringodium and Halodule have thicker and harder
coats than the seeds of the other genera and may
have a longer dormancy. Type b. Seeds with a
membranous coat, a modified pistil, as in Posidonia,
and without a distinct dormancy, e.g. Enhalus,
Thalassia and Posidonia. Type c. Seeds that neither
have a normal distinct seed coat nor dormancy, but
germinate on the parent plants (viviparous reproduction), e.g. Thalassodendron and Amphibolis:
this unusual reproduction has not been reported in
other aquatic plants. Histochemically, in Types a
and b, seeds store large amount of starch and little
protein in the hypocotyledonous tissue to be used
during germination and seedling development (Fig.
14E, F, J and M). On the other hand, Type c seeds do
not store starch and proteins, but the embryo obtains
the necessary nutrients for seedling development
direct from the parent plants through specialized
transfer cells at the interface of seed and the parental
tissue (Fig. 15F–I) (Kuo and Kirkman, 1996). These
transfer cells have extended wall ingrowths and a
plasmalemma, and are always associated with nutrient or solute transfer. The Type a seed has a distinct
dormancy; while the Types b and c seeds do not.
The accumulation of nutrient including nitrogen, phosphorus and other macro- and microelements in Posidonia spp. (Hocking et al., 1980) and
in Enhalus (Monta˜ no et al., 1999) are in general
comparable with those of terrestrial plants. Furthermore, there are numerous electron dense globoid
crystals present in all tissues of the embryo of Z.
capricorni and these contain P, Mg, K, Ca, Na
and Cl (West et al., 1992) (see also Romero et al.,
Chapter 11).
6. Seed Numbers in the Annual
and Perennial Seagrasses
As mentioned above only a few species of seagrasses
are known as annual species, and the number of
seeds produced by the annual and perennial plants
varies greatly. For example, monoecious annual
H. decipiens from Toro Point, Panama, had 13,500
seeds m
−2 (McMillan, 1988), while the same species
at Hardy Inlet, Western Australia, produced 176,880
seeds m
−2 (Kuo and Kirkman, 1995). The dioecious annual H. tricostata at Fitzroy Island, in the
Great Barrier Reef produced 70,000 seeds m
−2
(Kuo et al., 1993). In comparison, the dioecious
perennial H. ovalis produced about 480 seeds m
−2
at Whitfords near Perth (Kuo and Kirkman, 1992).
Plants of the annual Z. marina in Gulf of California, north western Mexico, have lifetime of
about four months; they allocate all their resources
into a seed production of up to 184,000 seeds
m
−2 while the perennial Z. marina in Baja California along the Pacific coast produces only 6300
seeds m
−2 (Meling-Lopez and Ibarra-Obando, 1999,
2000).
E. Seedlings
As in all angiosperms, seeds of seagrass species normally germinate after a period of dormancy. However, the length of dormancy periods in seagrass