Potamogeton filiformis, Pers. Potamogeton vaginatus, Turcz. Potamogeton pectinatus, L.

20. P. filiformis Pers. View in CoL , Syn. Pl. 1: 152 (1805).

Leaves up to 1 -5 mm wide but usually less than 1 mm, obtuse or rounded; sheaths closed, tubular towards the base at least when young. Spikes widely interrupted, especially in fruit. Fruitlets 2-2-75 mm, with a very short, verruciform, subapical beak. 2/2 = 78. Chiefly near the sea and often in brackish water. N. Europe and the Alps, extending very locally southwards toN.E. Spain, S. Italy and Crna Gora. Au Br Da Fa Fe Ga Ge Hb He Hs Hu Is It Ju No Po Rs (N, B, C) Su. P. xsuecicus K. Richter, Pl. Eur. 1: 15 (1890) (20 x 22) is a fairly frequent hybrid which can be confused with either 20 or 22.

21. P. vaginatus Turcz. View in CoL , Bull. Soc. Nat. Moscou 27(2): 66 (1854).

Leaves up to 2-5 mm wide but mostly less than 1 mm, obtuse or rounded; sheaths open, convolute, the lower ones loose and inflated with a more or less reduced lamina. Spikes shortly interrupted. Fruitlets 2-5-3 mm, with a very short, verruciform, subapical beak. 2n = c. 88. Usually in brackish water. E. Sweden and W. Finland. Fe Su. (N. Asia, North America.)

22. P. pectinatus L. View in CoL , Sp. Pl. 127 (1753) View Cited Treatment .

Leaves usually less than 2 mm wide (sometimes wider on young non-flowering branches), gradually acute to apiculate or mucronate; sheaths open, convolute, usually white-margined. Spikes at first cylindrical, later becoming more or less interrupted. Fruitlets 3-5 mm, with a short, subventral beak. 2/2 = 78. Base-rich or brackish water. Almost throughout Europe. All except Az Fa Is Sb.