[Greek halos, salt]

Diplanthera Thouars, Gen. Nov. Madagasc., 3. 1806, not Gleditsch 1764

Stems erect, subtended by elliptic or ovate scales. Leaves 1--4; blade flattened, apex 2--3-toothed, lacunae absent; veins 3, midvein conspicuous, widened distally, lateral veins inconspicuous, each ending in tooth. Inflorescences solitary. Flowers axillary to leafy bract, inflated sheath absent. Staminate flowers: anthers attached to axis at different levels. Pistillate flowers nearly sessile, styles not divided into stigmas. Fruits achenelike, slightly compressed, nearly globose to ovoid.

Species 6 (1 in the flora): warm oceanic waters of both hemispheres.

The taxonomy of Halodule is based almost entirely on the shape of the vegetative leaf tip; many of the species are unknown in sexual reproductive condition. Features of the leaf tip important in systematics include the presence or absence of lateral teeth, the presence or absence of a middle tooth, and the length of the middle tooth relative to the two lateral ones.

For a discussion of pollination in Halodule pinifolia (Miki) Hartog, see P. A. Cox (1988) and P. A. Cox and R. B. Knox (1989). The flowers are enclosed in a vegetative shoot that closely resembles a perianth (E. A. Johnson and S. L. Williams 1982). Staminate flowers are borne at the surface of the substrate, and pistillate flowers are produced below the substrate (P. A. Cox 1988). During low tide, the filament elongates, projecting the anther to the water surface. Following exposure to air, the anthers dehisce, releasing a cottony-like mass of threadlike pollen grains approximately 1000 m m in length. These pollen grains rapidly assemble themselves into "search vehicles" that are relatively large, ca. 5000 m m. This large size apparently increases the possibility of contacting a stigma. Apparently most search vehicles are composed of pollen from one anther. Submarine dehiscence does occur (P. A. Cox 1988); no explanation is given for what occurs afterwards.

The pistillate flower is produced below the soil surface (P. A. Cox 1988). The elongate style projects the stigma to the water surface during low tide. Possibly a relatively large "search vehicle" floating on the water surface will contact a stigma and pollination is accomplished. The fruit develops in the substrate, remaining there until released from the plant. Fruits remaining in the substrate possibly are the reason so few collections have fruits on them. They could fall off during the collection and removal of the substrate.

SELECTED REFERENCES

Cox, P. A. 1988. Hydrophilous pollination. Annual Rev. Ecol. Syst. 19: 261--280. Cox, P. A. and R. B. Knox. 1989. Two-dimensional pollination in hydrophilous plants: Convergent evolution in the genera Halodule (Cymodoceaceae), Halophila (Hydrocharitaceae), Ruppia (Ruppiaceae), and Lepilaena (Zannichelliaceae). Amer. J. Bot. 76: 164--175. Hartog, C. den. 1964. An approach to the taxonomy of the sea-grass genus Halodule Endl. (Potamogetonaceae). Blumea 12: 289--312. Johnson, E. A. and S. L. Williams. Sexual reproduction in seagrasses: Reports for five Caribbean species with details for Halodule wrightii Aschers, and Syringodium filiforme Kutz. Caribbean J. Sci. Math. 18: 61--70. 1982. McMmillan, C. 1991. Isozyme patterning iin marine spermatophytes. In: L. Triest, ed. 1988+. Isozymes In Water Plants. Opera Bot. Belg. 1+ vols. Belgium, Meise. Vol. 4,: pp. 193--200. Phillips, R. C. 1967. On species of the seagrass, Halodule, in Florida. Bull. Mar. Sci. 17: 672--676.

Lower Taxon

• Halodule wrightii Ascherson