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Pakistan | Family List | Potamogetonaceae

Potamogeton Linn., Sp. P1. 126. 1753. Gen. P1. ed. 5:61. 1754; Hook. f., Fl. Brit. Ind. 6:565. 1893; A. Bennett, Fl. Trop. Afr. 8:219. 1901; Clapham, Tutin & Warburg, Fl. Brit. Is. ed. 2. 942. 1962; Aalto, in Acta Bot. Fenn. 23. 1970; J.E. Dandy in Rechinger f., Fl. Iran. 83: 1. 1971; S. Ross-Craig, (onwards referred as SRC), Draw. Brit. Pl. part 31. 1973.

Potamogeton pusillus
Illustration

Leaves submerged or floating, alternate or imperfectly opposite, toothed or entire. Flowers ebracteate, small, tetramerous, actinomorphic, spicate in dense or interrupted ovoid or cylindric spikes (submerged water pollinated, emergent on a peduncle wind pollinated). Perianth segments concave, smooth, obovate. Stamens 4, sessile or sub-sessile. Fruitlets drupaceous, coriaceous, more or less compressed, one seeded, smooth or ornamented with warts, crests or short spines; embryo uniform or spiral.

About 90 species, mostly of fresh water habitat in temperate regions; about a dozen species are reported from Pakistan.

Taxonomically this is a difficult genus due to variation induced by water conditions resulting in differing morphological forms in the same species and the occurrence of hybrids or intermediate forms involving a group of species. Sterile hybrids, such as Potamogeton x. nitens (Potamogeton gramineus x Potamogeton perfoliatus), Potamogeton x sparganifolius (Potamogeton gramineus x Potamogeton natans), etc., and fertile hybrid, such as Potamogeton x zizii (supposed to be a hybrid of Potamogeton gramineus and Potamogeton lucens) have been treated as species by some authors. Sometimes even the fruit characters are not very reliable for identification of taxa, due to their variability.


1 Leaves narrowly linear to filiform; stipular sheath adnate to the leaf forming a sheathing leaf base   (2)
+ Leaves variable in shape; stipular sheath free form the leaf base, axillary   (3)
       
2 (1) Fruitlets 3-5 mm long; leaf apex acute or mucronate; stipular sheaths open and convolute; spikes cylindric when young   Potamogeton pectinatus
+ Fruitlets less than 3 mm long (2-2.8 mm); leaf apex obtuse or rounded; stipular sheaths tubular towards the base; spikes interrupted   Potamogeton filiformis
       
3 (1) Leaves all alike, submerged   (4)
+ Leaves mostly dimorphic. (Floating leaves coriaceous)   (7)
       
4 (3) Leaves narrowly linear (less than 3 mm wide); stipular sheaths tubular towards the base. Fruitlets c. 2 mm long.   Potamogeton pusillus
+ Leaves broadly linear to ovate (more than 3 mm wide); stipular sheaths not tubular. Fruitlets usually more than 2 mm long   (5)
       
5 (4) Leaves usually shortly petiolate; stipules 2.5-5.7 cm long (2-keeled)   Potamogeton lucens
+ Leaves sessile; stipules less than 2 cm long, caducous   (6)
       
6 (5) Leaves not amplexicaul, oblong to broadly linear, serrulate. Fruitlets 4-5 mm long. often with lateral warts and long beak   Potamogeton crispus
+ Leaves cordate-amplexicaul, ovate to lanceolate. Fruitlets 1-3.5 mm long, smooth and shortly beaked   Potamogeton perfoliatus
       
7 (3) Submerged leaves narrowly linear, sessile with a band of lacunae bordering the midrib; lamina of floating leaves up to 3.5 cm long. Spikes few-flowered   Potamogeton octandrus
+ Submerged leaves narrowly lanceolate to elliptic- oblong or reduced to narrowly linear, opaque phyll- odes; lamina of floating leaves more than 3.5 cm long. Spikes many-flowered   (8)
       
8 (7) Floating leaves with a discoloured joint at the top of the petiole; submerged leaves if present, reduced to narrowly linear, opaque phyllodes not more than 2 mm broad   Potamogeton natans
+ Floating leaves without a discoloured joint at the top of the petiole; submerged leaves, if present, narrowly lanceolate to elliptic-oblong   (9)
       
9 (8) Submerged leaves petiolate, lanceolate or broadly lanceolate-oblong   Potamogeton nodosus
+ Submerged leaves sessile, linear-lanceolate to elliptic-oblong   (10)
       
10 (9) Apices of submerged leaves rounded or obtuse, margin entire   Potamogeton alpinus
+ Apices of submerged leaves or acuminate, margin minutely denticulate   Potamogeton gramineus

  • List of lower taxa


     

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