96
Josef Daniel Ackerman
flowers, consisting as a single ovulate carpel with a
short bifid stigma, are arranged linearly on one side
of the spadix and these are bounded on either side
by one thecae of each pair of the staminate flower
(den Hartog, 1970; Tomlinson, 1982; Fig. 12C and
D, Chapter 3).
V. Pollen
Pollen characteristics are among the most unique
features in seagrasses (Table 1). As in the pollen
of other aquatic plants, there is a reduction in pollen
ultrastructure and in exine ornamentation, which is
absent in a number of taxa (Table 2; Sculthorpe,
1967; Pettitt and Jermy, 1975; McConchie, 1982;
Pettitt, 1984; McConchie and Knox, 1989a). In the
Zosteraceae there are two (or three) stratified microfibrillar layers in the pollen wall, slight stratification in the pollen wall of the Cymodoceaceae, and no
stratification in the pollen wall of the Posidoniaceae
(McConchie, 1982; Pettitt, 1984; McConchie and
Knox, 1989a). Associated with these ultrastructures
are gross pollen morphological patterns that include
spherical, ellipsoidal, and filiform (filamentous or
confervoid) shapes (den Hartog, 1970; Tomlinson,
1982; Ackerman, 2000), the latter extending to 5
mm lengths (∼20 µm diameter) in Amphibolis with
curved or forked tips (Ducker and Knox, 1976; McConchie and Knox, 1989a). Systematically, spherical pollen shapes are restricted to Enhalus and Thalassia, and ellipsoidal pollen shapes are found in
Halophila, which are all in the Hydrocharitaceae,
whereas filiform pollen are found in the remaining
10 genera, which are in the Cymodoceaceae, Posidoniaceae, and Zosteraceae (i.e. Potamogetonales of
Tomlinson, 1982; Table 2; Fig. 12, Chapter 3).
Notwithstanding these differences in pollen
shape, there is a strong tendency for pollen to be
transported as filaments in the submarine pollinated
genera (Thalassia and Halophila; Ackerman, 1995,
2000). In Thalassia, spherical pollen can be linked
in mucilaginous chains (present inside the thecae)
or can germinate precociously, which leads to a filamentous shape as first noted by Bowman (1922; also
see Pascasio and Santos, 1930). In Halophila, four
ellipsoidal pollen grains are contained and transported within a filamentous structure (Balfour, 1879;
Kausik and Rao, 1942). An examination of pollen
development is useful in this context as it provides insight into the nature of pollen with filamentous shapes (Ackerman, 1995). Monocotyledons undergo successive pollen development, which leads
to “cross T” configured tetrads in Thalassia, “linear” tetrads in Thalassia and Halophila, and “square
isobilateral” tetrads in the Zosteraceae and Cymodoceaceae (presumably, Posidonia is also of the latter
type; Fig. 2; Pettitt, 1984; Iwanami et al., 1988). The
interesting aspect of the development in Halophila is
that the linearly arranged tetrad forms the functional
unit (i.e. a pollinium). It is not known whether there is
any difference in the contribution of pollen from the
cross T or linear tetrads of Thalassia to the filamentous chains, but chains contain more than four pollen
grains (i.e. >1 tetrad). An examination of the development of filiform pollen (Fig. 3) is equally revealing
in that the origin of filiform pollen is polyphyletic
(Ackerman, 1995, 2000). In this case, filiform pollen
arises from the elongation of microspores prior
to reductive division in the Zosteraceae (Fig. 3;
Rosenberg, 1901a; Pettitt and Jermy, 1975; Stewart
and R¨ udenberg, 1980), but following reductive division in the Cymodoceaceae (Fig. 3; Yamashita,
1976; Pettitt, 1981; Pettitt et al., 1981; Pettitt, 1984).
Lastly, it is instructive to note that the orientation of
filiform pollen within anthers varies systematically,
with parallel orientation in the Zosteraceae (Dudley,
1893; Rosenberg, 1901a; Pettitt and Jermy, 1975;
Ackerman, 1993), and spiral or irregular orientation
in the Cymodoceaceae (Bornet, 1864; Pettitt, 1976;
Yamashita, 1976; Ducker et al., 1978). It would be
insightful to ascertain which of these latter two patterns occurs in the Posidoniaceae. Regardless, the
data indicate that filiform pollen morphologies are
convergent in the seagrasses either as filamentous
(i.e. filiform) or functionally filamentous structures
(Ackerman, 1995, 2000).
VI. Reproductive Patterns
Patterns in the reproduction of seagrasses may provide insight into the cues that induce floral development, trigger flowering, and control the flowering process (i.e. phenology) within individuals and
populations. These patterns will vary with biotic
factors related to the genetics, physiology (growth
regulators), and conditions of the local population,
and with abiotic factors related to light (photoperiod, red to far red ratios), temperature, and latitude
(Pettitt, 1984; Rathcke and Lacey, 1985; see Smith
and Walker, 2002). Lastly, geographic location, ease
Josef Daniel Ackerman
flowers, consisting as a single ovulate carpel with a
short bifid stigma, are arranged linearly on one side
of the spadix and these are bounded on either side
by one thecae of each pair of the staminate flower
(den Hartog, 1970; Tomlinson, 1982; Fig. 12C and
D, Chapter 3).
V. Pollen
Pollen characteristics are among the most unique
features in seagrasses (Table 1). As in the pollen
of other aquatic plants, there is a reduction in pollen
ultrastructure and in exine ornamentation, which is
absent in a number of taxa (Table 2; Sculthorpe,
1967; Pettitt and Jermy, 1975; McConchie, 1982;
Pettitt, 1984; McConchie and Knox, 1989a). In the
Zosteraceae there are two (or three) stratified microfibrillar layers in the pollen wall, slight stratification in the pollen wall of the Cymodoceaceae, and no
stratification in the pollen wall of the Posidoniaceae
(McConchie, 1982; Pettitt, 1984; McConchie and
Knox, 1989a). Associated with these ultrastructures
are gross pollen morphological patterns that include
spherical, ellipsoidal, and filiform (filamentous or
confervoid) shapes (den Hartog, 1970; Tomlinson,
1982; Ackerman, 2000), the latter extending to 5
mm lengths (∼20 µm diameter) in Amphibolis with
curved or forked tips (Ducker and Knox, 1976; McConchie and Knox, 1989a). Systematically, spherical pollen shapes are restricted to Enhalus and Thalassia, and ellipsoidal pollen shapes are found in
Halophila, which are all in the Hydrocharitaceae,
whereas filiform pollen are found in the remaining
10 genera, which are in the Cymodoceaceae, Posidoniaceae, and Zosteraceae (i.e. Potamogetonales of
Tomlinson, 1982; Table 2; Fig. 12, Chapter 3).
Notwithstanding these differences in pollen
shape, there is a strong tendency for pollen to be
transported as filaments in the submarine pollinated
genera (Thalassia and Halophila; Ackerman, 1995,
2000). In Thalassia, spherical pollen can be linked
in mucilaginous chains (present inside the thecae)
or can germinate precociously, which leads to a filamentous shape as first noted by Bowman (1922; also
see Pascasio and Santos, 1930). In Halophila, four
ellipsoidal pollen grains are contained and transported within a filamentous structure (Balfour, 1879;
Kausik and Rao, 1942). An examination of pollen
development is useful in this context as it provides insight into the nature of pollen with filamentous shapes (Ackerman, 1995). Monocotyledons undergo successive pollen development, which leads
to “cross T” configured tetrads in Thalassia, “linear” tetrads in Thalassia and Halophila, and “square
isobilateral” tetrads in the Zosteraceae and Cymodoceaceae (presumably, Posidonia is also of the latter
type; Fig. 2; Pettitt, 1984; Iwanami et al., 1988). The
interesting aspect of the development in Halophila is
that the linearly arranged tetrad forms the functional
unit (i.e. a pollinium). It is not known whether there is
any difference in the contribution of pollen from the
cross T or linear tetrads of Thalassia to the filamentous chains, but chains contain more than four pollen
grains (i.e. >1 tetrad). An examination of the development of filiform pollen (Fig. 3) is equally revealing
in that the origin of filiform pollen is polyphyletic
(Ackerman, 1995, 2000). In this case, filiform pollen
arises from the elongation of microspores prior
to reductive division in the Zosteraceae (Fig. 3;
Rosenberg, 1901a; Pettitt and Jermy, 1975; Stewart
and R¨ udenberg, 1980), but following reductive division in the Cymodoceaceae (Fig. 3; Yamashita,
1976; Pettitt, 1981; Pettitt et al., 1981; Pettitt, 1984).
Lastly, it is instructive to note that the orientation of
filiform pollen within anthers varies systematically,
with parallel orientation in the Zosteraceae (Dudley,
1893; Rosenberg, 1901a; Pettitt and Jermy, 1975;
Ackerman, 1993), and spiral or irregular orientation
in the Cymodoceaceae (Bornet, 1864; Pettitt, 1976;
Yamashita, 1976; Ducker et al., 1978). It would be
insightful to ascertain which of these latter two patterns occurs in the Posidoniaceae. Regardless, the
data indicate that filiform pollen morphologies are
convergent in the seagrasses either as filamentous
(i.e. filiform) or functionally filamentous structures
(Ackerman, 1995, 2000).
VI. Reproductive Patterns
Patterns in the reproduction of seagrasses may provide insight into the cues that induce floral development, trigger flowering, and control the flowering process (i.e. phenology) within individuals and
populations. These patterns will vary with biotic
factors related to the genetics, physiology (growth
regulators), and conditions of the local population,
and with abiotic factors related to light (photoperiod, red to far red ratios), temperature, and latitude
(Pettitt, 1984; Rathcke and Lacey, 1985; see Smith
and Walker, 2002). Lastly, geographic location, ease