Potamogeton L.

1. Potamogeton L. View in CoL 2

Leaves alternate, or the involucral ones subopposite; sheaths adnate to the leaf-base, ligulate, or free from the leaf-base and stipuliform. Carpels (l-)4, free or shortly connate at the base. Fruitlets drupaceous, with bony endocarp and softer exocarp; embryo unciform or spiral.

Hybridization is frequent within each of the two subgenera,

but does not occur between them, their methods of pollination being different. With the exception of P. x zizii (5 x 6), which often bears well-formed fruitlets, all the hybrids are sterile, entirely without fruit or with occasional malformed fruitlets. The hybrids frequently form vigorous, persistent clonal populations, and, especially in rivers and canals, may become dispersed over long distances by means of vegetative winter-buds or detached fragments. As the hybrids show varying degrees of intermediacy between the parent species it is not practicable to include them in the key, but some of the more frequent are mentioned in the text and others are included in the index; the distinguishing features of most of them are given by J. E. Dandy in C. A. Stace, Hybridization and the Flora of the British Isles 444-459. London. 1975.

The floating leaves which occur in some of the species and hybrids are always petiolate, with an expanded lamina which is usually opaque but occasionally translucent. Some of the floating-leaved species and hybrids can grow subterrestrially in driedup situations or in Sphagnum-bogs, in which case the floating leaves become aerial.

Potamogeton species are found in a wide variety of fresh-water habitats-in deep or shallow water, stagnant to fast-flowing; a few species, especially those of Subgen. Coleogeton , can flourish also in brackish water. Some of the species are more or less restricted to base-rich water, but the majority are tolerant of a considerable range of water conditions. The broad-leaved species (Sect. Potamogeton ) vary greatly in the size and shape of the leaves according to the type of water, and leaf-dimensions are of little diagnostic value in this group. 1 Leaf-sheaths adnate to the leaf-base, ligulate; leaves all submerged, narrowly linear to filiform, entire

2 Leaves acute to apiculate or mucronate at the apex; sheaths open and convolute, usually white-margined; fruitlets

3-5 mm, with a short subventral beak 22. pectinatus 2 Leaves obtuse or rounded at the apex, not mucronate; sheaths not white-margined; fruitlets 2-3 mm, with a very short, verruciform, subapical beak

3 Leaf-sheathsopenandconvolute,theloweronesinflated; spikes shortly interrupted; fruitlets 2-5-3 mm 21. vaginatus 3 Leaf-sheaths closed,tubular towards the baseat leastwhen young, the lower ones not inflated; spikes widely interrupted especially in fruit; fruitlets 2-2-75 mm 20. filiformis 1 Leaf-sheaths free or almost free from the leaf-base, forming an intrafoliar stipule

4 Floatingleavespresent,atleastinmaturefloweringplants

5 Floating leaveswitha discoloured,flexiblejoint at the topof the petiole immediately below the lamina; submerged leaves (if present) all reduced to narrowly linear, opaque, phyllodes 1. natans 5 Floatingleaveswithouta discoloured joint at the topof the petiole; submerged leaves (if present) with a translucent lamina, only occasional ones reduced to phyllodes

6 Submerged leaves present, all or most sessile

7 Stems compressed; submerged leaves up to 8 mm wide, parallel-sided, narrowly linear, with a broad band of lacunae bordering the midrib 10. epihydrus 7 Stems terete; submerged leaves often more than 8 mm wide, convex-sided, without a broad band of lacunae bordering the midrib

8 Submerged leaves acute or acuminate, usually narrowly elliptic-oblong, with minutely denticulate margin and regularly ascending secondary veins; petiole of floating leaves often longer than the lamina 6. gramineus 8 Submerged leaves rounded or obtuse, elliptical to lanceolate, with entire margin and mostly horizontal secondary veins; petiole of floating leaves shorter than the lamina 7. alpinus 6 Submerged leaves (if present) all petiolate

9 Fruitlets 3-4 mm; floating leaves elliptical or ovateelliptical, cuneate or obtuse at the base; submerged leaves lanceolate to elliptic-lanceolate, at first very minutely denticulate, often disappearing early

4. nodosus 9 Fruitlets 1 -5-2-5 mm; floating leaves ovate to lanceolate, cuneate to subcordate at the base; submerged leaves entire

10 Floating leaves opaque, the petiole shorter or longer than the lamina; submerged leaves lanceolate, clearly differentiated from the floating leaves, often disappearing early; fruitlets 2-2-5 mm, red or reddishbrown 2. polygonifolius 10 Floating leaves translucent, the petiole not longer than the lamina; submerged leaves similar to the floating leaves but narrower; fruitlets 1 -5-1- 75 mm, greenish

3. coloratus 4 Floatingleavesabsent

11 Carpels 4, shortly connate at the base, the beak in fruit as long as or longer than the body of the carpel; leaves often undulate 19. crispus 11 Carpels 4(-l), free, the beak in fruit much shorter than the body of the carpel

12 Leaves convex-sided, narrowly elliptic-oblong to elliptical or ovate; stems terete; rhizome creeping

13 Leaves acute or + cuspidate (occasionally reduced to phyllodes), the secondary veins regularly ascending; peduncles ± thickened upwards

14 Leaves petiolate, elliptical or obovate-elliptical to narrowly elliptic-oblong, ± distinctly serrulate; fruitlets 3 -5-4 mm 5. lucens 14 Leaves (at least the lower ones) sessile, narrowly elliptic-oblong to narrowly elliptical or oblanceolate-oblong, minutely denticulate; fruitlets 2-3 mm

6. gramineus 13 Leaves obtuse or rounded and often ± cuculiate, sessile,

the secondary veins mostly horizontal; peduncles not thickened upwards 15 Leaves cuneate at the base, not or only slightly amplexicaul, linear-lanceolate to elliptic-oblong,

entire; plant often tinged with red 7. alpinus 15 Leaves (at least the lower) rounded to cordate and

± amplexicaul at the base

16 Stipules large, conspicuous, ± fibrous-persistent; leaves

(at least the lower) rounded and semiamplexicaul at the base, lanceolate to ovate-oblong, entire; fruitlets

4-5 mm 8. praelongus

16 Stipules small, membranous, evanescent; leaves cordate and amplexicaul at the base, ovate to lanceolate, minutely denticulate; fruitlets 3-3-5 mm

9. perfoliatus 12 Leaves parallel-sided, linear, not more than 5 mm wide,

sessile, entire; stems compressed or very slender;

rhizome absent 17 Leaves appearing many-veined (3 or 5 vascular veins interspersed with numerous fine, vein-like sclerenchymatous strands) 18 Leaves with 5 vascular veins and very numerous sclerenchymatous strands, the midrib closely bordered by lacunae especially towards the base; spikes cylindrical, usually shorter than the peduncle

17. compressus 18 Leaves with 3 vascular veins and fairly numerous sclerenchymatous strands, the midrib widely bordered by lacunae especially towards the base; spike usually few-flowered, subcapitate, shorter than or almost equalling the peduncle 18. acutifolius 17 Leaves 3- to 5(-7)-veined, without sclerenchymatous strands 19 Stipules closed, tubular in the lower part at least when young; fruitlets 1 - 5-2 mm

20 Leaves usually more than 2 mm wide, mostly 5-veined and with conspicuous nodal glands; stipules usually becoming 2-fid; winter-buds compressed and strongly ribbed at the base 11. friesii

20 Leaves usually less than 2 mm wide, mostly 3-veined,

the nodal glands usually inconspicuous or absent;

stipules not becoming 2-fid

21 Leaves rigid, gradually narrowed to a fine, almost bristle-like point; lateral veins conspicuous; stipules firm, strongly veined, fibrous-persistent; winterbuds strongly ribbed at the base 12. rutilus

21 Leaves not rigid, mucronate; lateral veins inconspicuous; stipules delicately veined, not fibrouspersistent; winter-buds not or finely ribbed

13. pusillus 19 Stipules open, convolute

22 Carpels 1-3; leaves not more than 1 - 5 mm wide, 3-

veined, the midrib prominent and thick especially towards the base 16. trichoides

22 Carpels 4; leaves 3- to 5-veined, with ±conspicuous nodal glands

23 Fruitlets 3-4 mm, with a sharp dorsal keel; leaves mostly more than 2 mm wide, 3- to 5-veined, the lateral veins faint, the midrib with or without a narrow border of lacunae 14. obtusifolius

23 Fruitlets 2-2-5 mm, with a rounded dorsal keel;

leaves mostly less than 2 mm wide, usually 3-veined,

the lateral veins fine but distinct, the midrib with a border of lacunae, at least towards the base

15. berchtoldii