Anatomy and Structure of Australian Seagrasses

Seagrasses are monocotyledonous angiosperms, and as with terrestrial angiosperms, they have vegetative organs (roots, rhizomes and shoots with leaf sheaths and leaf blades), and reproductive organs (flowers, fruits and seeds). They have adapted to a marine environment in a saline medium, and have rather simple tissues and cell types such as a thin cuticle and epidermal cells with concentrated chloroplasts but lack stomata in the leaves. Within the vascular bundles, the cell walls of vascular bundle sheath cells are either lignified, suberized or have wall ingrowths. The number and size of xylem elements are much reduced in seagrasses. Phloem cells have thin or nacreous wall and/or thick walled sieve elements. Whether these structural variations are significant in solute translocation remains to be determined. Seagrass rhizomes are usually herbaceous, but some become woody. They are either monopodially or sympodially branched, with adventitious roots. Roots may be branched and bear roots hairs, depending on the substratum. Air lacunae are continuous within all vegetative and reproductive organs, with regular septa interrupting the air lacunae. Unusual apoplastic fungal hyphae grow in the intercellular spaces of living leaf tissue of the subtidal Zostera muelleri . Seagrasses are monoecious or dioecious plants with hydrophilous pollination. They have unusual filamentous pollen or pollen grains that form long chains. Fruits and seeds have either a period of dormancy or germinate as they are being released. Unlike seeds of other seagrasses, the embryos of Amphibolis and Thalassodendron do not store starch but instead obtain nutrients required for the prolonged development phase of the viviparous seedlings directly from the parent plant through ‘transfer cells’. Morphological and anatomical organization of both vegetative and reproductive organs vary among Australian seagrass taxonomic groups, reflecting their different evolutionary origins, as well as providing a means of indentifying genera and species with classical taxonomy.