Citation: L., Sp. Pl. 126 (1753).

Synonymy: Not Applicable

Common name: Pondweeds.

Description:
Perennial rhizomatous herbs submerged in fresh or less often brackish waters or partly emergent and leaves then floating; leaves alternate and/or opposite, with an axillary stipular sheath; blade linear to orbicular.

Inflorescence an emergent few- to many-flowered spike; flowers bisexual; perianth-segments 4; anthers 4; carpels 4, free, l-celled; ovule 1; stigma along the ventral suture on a beak.

Distribution:  About 100 species, cosmopolitan.

Biology: No text

Taxonomic notes: Key taken from H. I. Aston (1973) Aquatic plants of Australia.

Key to Species:

1. Plants wholly submerged (except for the inflorescence), the leaves all sessile and more or less similar
2. Stipular sheath adnate below to the leaf base, the upper portion free and forming a distinct ligule; leaves very narrow-linear, less than 2 mm wide	P. pectinatus 4.
2. Stipular sheath almost completely free from the leaf; leaves, if linear, mostly more than 3 mm wide
3. Leaves narrow-lanceolate to linear-oblong, 5- (rarely 3-) nerved without fine intermediate nerves, the margins serrulate and usually also undulate to strongly crisped	P. crispus 2.
3. Leaves linear, 5- (rarely 3-) nerved with numerous fine longitudinal intermediate nerves, the margins entire, flat	P. ochreatus 3.
1. Plants with the upper leaves emergent and floating, petiolate, and dissimilar to the submerged foliage (the 'broad-leafed' species)
4. Floating leaves opaque, more or less leathery, with inconspicuous secondary venation
5. Floating leaves with 18-25 nerves	P. tepperi 5.
5. Floating leaves usually with 11-17 nerves	P. tricarinatus 6.
4. Floating leaves often more or less translucent, glossy, membranous, with conspicuous secondary venation	P. australiensis 1.

Author: Prepared by J.P. Jessop