Electronic Flora of South Australia Family Fact Sheet


Phylum Magnoliophyta

Seagrasses are flowering plants adapted to living completely submerged in the marine environment. In his comprehensive monograph "Seagrasses of the World", den Hartog (1970) described 12 genera and 49 species. Eight of these genera are represented in southern Australia, and two additional genera, Ruppia and Lepilaena are also discussed here. These latter two genera, which frequently grow in close association, are generally not recognised as seagrasses. They are unique in tolerating and growing successfully in a wide range of salinities from fresh water through to hypersaline inland waters. In southern Australia both genera have species occurring in marine habitats and hence are included here with the purely marine genera.

Taxonomic notes: South-western Western Australia is particularly rich in seagrass species and since 1970 two new species of Posidonia have been described from this region (Cambridge & Kuo 1979). It is anticipated that a forthcoming paper by the same authors will increase the number still further.

Traditionally the characters used in classifying flowering plants are the floral and fruiting parts (e.g. petals, sepals, stamens, fruits and seeds) and these are used here in the various synoptic keys. However, flowers and fruits of most seagrasses are not often collected and when found they are mostly small and inconspicuous.

Alternative vegetative characters with which to identify the seagrasses have been examined. Seagrass leaf apices are very useful diagnostically. Considerable variation in leaf apex shape can be found within a species or even on one plant, but none-the-less if used in conjunction with other features this character is valuable taxonomically. Longitudinal air lacunae or air canals are present in the leaves of all seagrasses and their size and arrangement may be characteristic for a particular species. Separating the air canals are blocks of cells or diaphragms, some running longitudinally and others transversely. Vascular tissue may pass through these, e.g. the transverse veins in Zosteraceae. Minute scale-like structures, the squamules (squamulae intravaginales), are found in the axils of foliage and scale leaves of all seagrasses (and most other non-marine aquatics as well). Their exact function is not known but it is probable that they are primarily secretory (see Tomlinson 1982, p. 46). The number, shape and arrangement of squamules is constant for any one genus and may vary between species. Other useful vegetative characters include leaf venation, blade L/B* ratio, the nature of the sheath and the number and arrangement of roots.

The key on p. 58, based entirely on vegetative characters, enables sterile material to be identified to genus. Later taxonomic keys are based on both vegetative and floral characters, and these in conjunction with the illustrations enable specific determinations to be made. The illustrations have been drawn either from fresh specimens or liquid preserved material. Many of the species have been maintained in culture for considerable periods of time at the University of Adelaide.

Seagrasses are all monocotyledons. Traditionally flowering plants as a whole have been known as Angiospermae but following the more modern classification they are here referred to the Division Magnoliophyta (Cronquist, Takhtajan & Zimmermann 1966) which is divided into two natural Classes, the Liliopsida (= Monocotyledonae), and the Magnoliopsida ( = Dicotyledonae). Within the Liliopsida there are five major groups or subclasses and the seagrasses belong within the subclass Alismatidae Takhtajan (= Helobiae).

The system of classification adopted for the seagrasses in this Flora follows Cronquist (1981), in conformity with the treatment used in "Flora of Australia" (see Kanis 1981, p. 82), except that within the subclass Alismatidae the circumscription of orders and families follows Tomlinson (1982, p. 10). Accordingly Ruppia is included in Potamogetonaceae, and not in the segregate family Ruppiaceae, and the order Potamogetonales is used to embrace all the seagrass families except Hydrocharitaceae. This latter highly specialized and distinctive family is placed in the separate order Hydrocharitales. In both Cronquist and Tomlinson the subclass Alismatidae is essentially that of Takhtajan (1966) but Cronquist defines 4 orders and Tomlinson 5, within the subclass. The families to which the seagrasses belong are fairly clearly circumscribed but as Tomlinson observes "the task of assembling a totally acceptable arrangement of them into higher categories ... seems impossible".

* L/B i.e. length/breadth ratio.

** Department of Botany, University of Adelaide.


CAMBRIDGE, M.L. & KUO, J. (1979). Two new species of seagrasses from Australia, Posidonia sinuosa and P. angustifolia (Posidoniaceae). Aquat. Bot, 6, 307–328.

CRONQUIST, A.J. (1981). An Integrated System of Classification of Flowering Plants. (Columbia University Press: New York.)

CRONQUIST, A.J., TAKHTAJAN, A. & ZIMMERMANN, W. (1966). On the higher taxa of Embryobionta. Taxon 15, 129–134.

DEN HARTOG, C. (1970). The seagrasses of the World. Verh. k. ned. Akad. Wet. Afd. Natuurk., ser. 2, 59(1), 1–275 (-1–31 Plates).

KANIS, A. (1981). An Introduction to the system of classification used in the flora of Australia. In Flora of Australia, Vol. 1, Introduction, pp. 72–112. (Bureau of Flora and Fauna: Canberra.)

TAKHTAJAN, A. (1966). Systema et phylogenia magnoliophytorum. (Nauka: Moscow.)

TOMLINSON, P.B. (1982). Helobiae (Alismatidae) including the seagrasses. Vol. VII. Anatomy of the Monocotyledons (ed. C.R. Metcalfe). (Clarendon Press: Oxford.)

The Marine Benthic Flora of Southern Australia Part I complete list of references.

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
Part I
©Board of the Botanic Gardens and State Herbarium, Government of South Australia


1. Leaf with a distinct broad blade and narrow petiole, without a definite sheathing base, auricles absent


1. Leaf more or less the same breadth throughout its length, not petiolate, with a well-developed sheathing base, auricles present, one either side of the sheath


2. Ligule absent


2. Ligule present at junction of blade and sheath


3. Leaf blade more or less terete, or strongly convex on one or both faces


3. Leaf blade flat, or somewhat concavo-convex, with upper and lower faces parallel


4. Leaf blade soft and succulent, in cross section showing a central vascular bundle surrounded by a ring of 5–7 air canals and an outer ring of 7–10 peripheral vascular bundles

Syringodium isoetifolium

4. Leaf blade tough and fibrous; in cross section showing a row of 3–9 longitudinal vascular bundles

Posidonia ostenfeldii group

5. Leaf blade less than 1 mm broad with a single median vein


5. Leaf blade 1–15 mm broad, in cross section showing a row of 3 or more longitudinal veins


6. Leaf blade and sheath falling together, leaving a clean scar on the stem


6. Leaf blade falling first, sheath remaining persistent on the stem for some time and always with some hairy fibres remaining, never leaving a clean scar (except in Halodule uninervis).


7. Leaf margin entire, apex bi-dentate or concave; roots usually at every node, branched and often somewhat coiled


7. Leaf margin with numerous truncated denticulations, apex rounded; roots at every fourth internode, unbranched, not coiled


8. Leaf apex tri-dentate


8. Leaf apex rounded or truncate, with or without a notch, not dentate


9. Base of leaf sheath finally falling and leaving a clean scar; leaf blade with 3 longitudinal veins, the 2 lateral ones marginal and each ending in a lateral tooth, not uniting with the midvein, transverse veins absent, tannin cells numerous and appearing as dark spots scattered on the blade

Halodule uninervis

9. Base of leaf sheath remaining as persistent hairy fibres on the stem; leaf blade with 3 longitudinal veins, the 2 lateral ones submarginal and uniting with the midvein near the apex, transverse veins connecting the lateral veins to the midvein at more or less regular intervals, tannin cells absent

Zostera mucronata

10. Leaf blade with 3 longitudinal veins, the 2 lateral ones submarginal (so that the blade appears single-veined to the naked eye), or the lateral veins, if not submarginal, at least much closer to the margin than to the midvein


10. Leaf blade with 5 or more longitudinal veins


11. Rhizome in cross section showing 2 lateral vascular bundles, one either side of the central vascular bundle


11. Rhizome in cross section showing 6–12 cortical vascular bundles and a central vascular bundle


12. Leaf blade with 7–20 longitudinal veins, uniting near the apex, tannin cells numerous, appearing as dark linear spots either side of each transverse vein


12. Leaf blade with 5 longitudinal veins, the veins either side of the midvein united to it near the leaf apex, the marginal veins joining the inner laterals some distance from the apex, tannin cells absent

Zostera capricorni

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