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POTAMOGETONACEAE

PONDWEED FAMILY

Robert F. Thorne

Annual, perennial herb, aquatic, (generally fresh to alkaline water), glabrous, from rhizomes or small, bulb-like, winter buds
Stems erect, simple to branched, cylindric or flattened
Leaves simple, cauline, alternate or in subopposite pairs; submersed thread-like to round, sessile or petioled; floating, if present, elliptic to ovate, petioled, leathery; sheath open, continuous with petiole or ± free from leaf base, generally with stipules, stipules sometimes fused, ligule-like
Inflorescence: spike or head-like, axillary or terminal, generally emergent, peduncled; bracts 0
Flower bisexual; perianth parts 0 or 4, clawed; stamens 2 or 4, if 4, each fused to base of perianth part, sessile or filament short, wide, anthers open to outside; pistils 4, ovary 1-chambered, ovule 1, style short or stigmas sessile
Fruit: drupe
Seed 1
Genera in family: 3 genera, ± 95 species: worldwide
Recent taxonomic note: Recently treated to include Zannichelliaceae [Angiosperm Phylogeny Group 1998 Ann Missouri Bot Gard 85:531–553].

POTAMOGETON

PONDWEED

Annual, perennial herb, from rhizomes or small, bulb-like, winter buds
Stem simple or branched, cylindric or flattened, rooting at lower nodes
Leaves simple, cauline, generally alternate, generally flat, generally green, margin generally entire; submersed leaves sessile or petioled, linear to round, tip rounded to acuminate, veins 1–35; floating leaves, if any, elliptic to ovate, generally petioled, leathery; stipules free or fused, sheath-like below leaf junction, free or fused (ligule-like) above leaf junction
Inflorescence: cylindric spike or head-like, axillary or terminal, floating to emergent
Flower: inconspicuous; perianth parts 4, clawed, greenish; stamens 4, attached to base of perianth, anthers generally sessile; ovule attached at chamber base, style short or stigma sessile
Fruit generally obovate, sessile, floating
Species in genus: ± 90 species: mostly temp n hemisphere
Etymology: (Greek: river neighbor, from aquatic habitat)
Reference: [Haynes 1974 Rhodora 76:564–649; 1985 Sida 11:173–188; Wieglet 1988 Feddes Repert 99:249–266]

Native

P. pusillus L.

SMALL PONDWEED


Stem < 100 cm, cylindric
Leaves: all submersed, thread-like to linear; base generally with 2 prominent glands, veins < 7; stipule tips generally becoming finely fibrous
Inflorescence ± interrupted, < 1.5 cm
Fruit < 2.5 mm; back rounded; beak short
Ecology: Shallow water, ponds, lakes, reservoirs, ditches, vernal pools, slow streams
Elevation: < 2700 m.
Bioregional distribution: North Coast, Klamath Ranges, n Sierra Nevada Foothills, High Sierra Nevada, Great Central Valley, San Francisco Bay Area, South Coast Ranges, Southwestern California (except Channel Islands), Great Basin Floristic Province, Mojave Desert
Distribution outside California: circumboreal

Native

var. pusillus


Stem generally many-branched, < 100 cm
Leaf < 7 cm, generally (0.5)2–3 mm wide; tip acute; veins 3–5; stipules 6–15 mm, fused below middle, breaking apart early
Inflorescence: spike, 6–15 mm, generally interrupted; whorls 2–4; peduncle 1.5–8 cm
Fruit: sides concave; tip beaked at top of central axis
Ecology: Common
Elevation: < 2700 m. Habitat of sp.
Bioregional distribution: North Coast, Klamath Ranges, n Sierra Nevada Foothills, High Sierra Nevada, Great Central Valley, San Francisco Bay Area, South Coast Ranges, Southwestern California (except Channel Islands), Great Basin Floristic Province, Mojave Desert
Flowering time: May–Jun

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