The reproductive organs of seagrasses are generally highly reduced, which reflects both their evolutionary origin and the specialized nature of these plants for submerged abiotic pollination (Fig. 1;
Arber, 1920; Sculthorpe, 1967; den Hartog, 1970; Tomlinson, 1982; McConchie and Knox, 1989a; Ackerman, 1995, 2000; Kuo and den Hartog, 2001; Kuo and den Hartog, Chapter 3). Nine of the thirteen genera are dioecious with largely solitary male and female flowers, and the remaining four genera (note that Halophila has both dioecious and monoecious species) are monoecious with either perfect flowers (Posidonia) or unisexual flowers in cymose inflorescences (Halophila) or in flattened inflorescences with spathe-spadix arrangements (Zosteraceae; Table 2; see also Fig. 12C and D, Kuo and den Hartog, Chapter 3). With the exception of Enhalus and Thalassia, which share many features with their freshwater relatives (e.g. Vallisneria), the flowers are drab rather than showy, and perianth parts are absent or bract like in appearance (den Hartog, 1970; Tomlinson, 1982). For example, the solitary unisexual flowers of Cymodoceae are "naked", being composed essentially of two united anthers on a filamentous stalk in the case of staminate flowers, and two free carpels each with two slender styles (three in Amphibolis) (see Fig. 12, Chapter 3).
The spatial location and orientation of the reproductive organs is critical from the perspective of submarine pollination. Specifically, the fluid dynamic release and/or capture of pollen will vary according to whether anthers or carpels are located basally or distally on the plants, given the velocity gradient near the bottom and the plant canopy (i.e. benthic and canopy boundary layers; Okubo et al., 2002). In this situation, there is an advantage to elevating anthers away from the bottom to facilitate pollen dispersal in the faster moving fluid, while placing stigmas close to the bottom or within axils of bracts or vegetative material to facilitate the reduction and redirection of water flow for pollen capture (cf. Niklas, 1992). This appears to be the case in a number of genera like Thalassia, Halophila, and Cymodocea, and reaches an extreme in Am-phibolis where some anthers can detach and float to the surface where pollen can disperse through the water column (McConchie and Knox, 1989b). This latter example should not be confused with the obligate surface pollination of Enhalus, where detached free-floating male flowers encounter female flowers on long spirally coiled peduncles (den Hartog, 1970; Tomlinson, 1982). The situation becomes more complex when reproductive organs of diocecious (Syringodium, Phyllospadix) or monoecious genera (Posidonia, Nanozostera, Zostera) are organized into inflorescences that extend into or above the top of the plant canopy. In these cases, the fluid dynamically induced movements of the plants are likely to affect the dispersal and capture of pollen through waving sweep-like motions (i.e. monami; Ackerman and Okubo, 1993) analogous to wind pollination in grasses and other herbaceous plants
(Niklas, 1992; Ackerman, 2000). Recognizing this, it should be noted that much research is needed to confirm these patterns, which extend from fluid dynamic principles in wind-pollinated plants (Niklas, 1992; Ackerman, 2000).
Staminate and carpellate flowers appear to be organized into a number of different patterns that do not appear to match with systematic patterns (i.e. Hydrocharitaceae vs. other seagrass families) or patterns in pollen morphology (spherical vs. filamentous; see below; Table 2). In the first type, the carpellate inflorescence of Enhalus and Tha-lassia are trimerous with subtending perianth parts, which are easily identified as "flowers" by the non-specialists (Fig. 1A). The solitary carpellate flowers have long bifid stigmas, which are papillate (for additional information pertaining to properties of stigmas see McConchie and Knox, 1989a; Table 2). The second type of floral arrangement includes a number of genera (Halophila, Amphibolis, Cymod-ocea, Halodule, and Thalassodendron) with solitary, highly-reduced "naked" flowers that can be on short pedicels close to the seafloor. As noted above, the staminate flowers are usually stalked with two united anthers (three in Halophila), and the carpellate flowers usually have two carpels each with a long slender bifid stigma (Halodule has one stigma; Amphibolis has three stigmas), which is non-papillate (Halophila is papillate; Tomlinson, 1982). It is important to note that the stigmas, which are the receptive surfaces of the styles, may be long; i.e. ~9 mm in Amphibolis and ~30 mm in Cymodocea (Tomlinson, 1982; McConchie and Knox, 1989b; see Fig. 11, Chapter 3). A third pattern is seen in Posidonia, which has a racemose inflorescence with perfect flowers consisting of a single disk-shaped irregularly lobed stigma and three stamens (Tomlinson, 1982; McConchie and Knox, 1989b; see Fig. 11, Chapter 3). It is important to note that this type of stigmatic surface is distinct among the seagrass, which have slender stigmas, and is more reminiscent of some freshwater species (Table 2). The final pattern is seen in the remaining genera in which unisexual flowers are borne on long branched cymose inflorescences (Syringodium; Fig. 1C and D) or on long branched inflorescences with a spathe-spadix configuration (Heterozostera, Phyllospadix, Nanozostera, and Zostera; Fig. 1E and F). In Syringodium the stalked staminate flowers have two united anthers and the carpellate flowers have two free carpels each with a bifid stigma. In the Zosteraceae, the carpellate 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 (denHartog, 1970; Tomlinson, 1982; Fig. 12C and D, Chapter 3).
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