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 |xm diameter) in Amphibolis with curved or forked tips (Ducker and Knox, 1976; Mc-Conchie and Knox, 1989a). Systematically, spherical pollen shapes are restricted to Enhalus and Tha-lassia, 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 filamen tous 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 Cymod-oceaceae (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 Rudenberg, 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).
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