Potamogetonaceae

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Potamogetonaceae

Description

Perennial or annual aquatic herbs, rarely semi-terrestrial. Leaves alternate (distichous) or subopposite or rarely in whorls of 3 (not in Malesian species), simple, entire or dentate, often heteromorphic (in Potamogeton); Inflorescence a pedunculate spike, simple (in Potamogeton) or umbel-like (in Ruppia) in fruit. Flowers small, bisexual, dimerous, trimerous, or (generally) tetramerous. Fruit drupaceous or achene-like, or rarely (not in Malesia) baccate;

Distribution

all over the world: present
Three genera, with c. 80 species all over the world The non-Malesian genus is the monotypic Groenlandia Gay.

Morphology

Both vegetative and generative morphology have been reviewed by Tomlinson (1982). Potamogeton has a complex modular organisation being difficult to describe both in terms of classical and functional morphology. Some terms concerning shoot types, branching patterns, and longevity used in the descriptions need some explanation (see also Kadono & Wiegleb 1989; Wiegleb & Brux 1991). Two different shoot types are distinguished, namely the ‘vertical shoot’ (incl. also the lateral ‘renewal shoots’) and the ‘horizontal shoot’. Both are differentiated by leaf insertion, vegetative anatomy, and their potential to bear spikes (Tomlinson 1982). This differentiation is visible in all species, despite the fact that some species can change their growth pattern in an opportunistic way (like e.g. P. oxyphyllus). The term ‘stem’ is used for the main axis of the vertical shoot. In contrast to Tomlinson the term ‘rhizome’ is avoided as it is morphologically incorrect. Instead the term ‘horizontal shoot’ is used throughout. In Malesian species it is mainly found as ‘lower horizontal shoot’ being mostly stoloniferous, rarely rhizomatous, or developed as complex of differently shaped parts.

A complete account of the branching pattern is avoided as it would require an extensive explanation. An expression like ‘stem of vertical shoot unbranched’ indicates that the respective species does not produce renewal shoots below the pseudo-opposite involucral leaves. But it may produce e.g. turion-bearing ‘upper horizontal shoots’ serving for vegetative reproduction and short-range dispersal. In accordance with Tomlinson (1982) the term ‘inflorescence’ refers to the ‘spike-peduncle unit’ in total. This contrasts with Hagström’s (1916) use of this term who considered the whole architecture of a plant individual.

Undifferentiated expressions like ‘annual’ or ‘perennial’ for species are avoided. The terms used always relate to the above-ground and below-ground parts separately. Additional information on seasonality is given, if available. For the description of winter buds, a simplified system following Hutchinson (1975) is used, distinguishing among turions (mostly on vertical shoots), tubers, and multiple complexes (both mostly on horizontal shoots).

Recently, Sorsa (1988) reviewed the pollen morphology of Potamogeton and Groenlandia.

Taxonomy

Affiliation and subdivision of Potamogetonaceae have largely differed. Engler classified the family with his Helobiae. It is now generally agreed to be classified with the Potamogetonales (Najadales, Zosterales), from which Alismatales (incl. Hydrocharitales) are excluded. Cladistic analyses (Dahlgren et al. 1985) showed that Potamogetonaceae (incl. Ruppiaceae) are closely associated with Zosteraceae and Posidoniaceae, while Zannichelliaceae and Cymodoceaceae are more closely allied with Najadaceae. Juncaginaceae and Scheuchzeriaceae form a distinct third clade. Recent molecular evidence indicates however a close relationship between Potamogeton subg. Coleogeton and Zannichellia (Les et al. 1996; Kubitzki 1998).

Potamogetonaceae s.s. are subdivided into 2 subfamilies (Tomlinson 1982), which have also been regarded as separate families:

Subfamily Potamogetonoideae: Differentiation of shoot system into horizontal and vertical shoots mostly well developed; branching mainly sympodial; leaves diverse, often heterophyllous; stipules mostly axillary, independent of the blade; leaf trace system from the stele consisting of at least 3 separate vascular bundles; flowers usually more than 2 per spike; stamens and carpels usually 4; sepaloid tepal adnate to the stamen connective; peduncle not elongating in fruit; carpel stalk not elongating after pollination.

Subfamily Ruppioideae. Differentiation of the shoot system poor; branching monopodial; leaves uniformly linear, with an attached sheathing base; leaf trace system from the stele consisting of a single bundle; flowers 2 per spike; each with 2 stamens and few to several carpels; tepals absent; peduncle elongated; carpel stalk often elongating after pollination.

Uses

Some species are cultivated for decorative purposes in artificial ponds, some are used as aquarium plants. In tropical areas Potamogeton, like other aquatic plants, is used as green manure.