Panarctic Flora

240116-17 The Potamogeton pusillus aggregate P. berchtoldii, P. pusillus

Geography: Circumboreal-polar.

Notes: These two species are probably not very closely related in spite of subspecific treatment by some authors. Potamogeton pusillus s. is distinguished from P. berchtoldii by several non-overlapping characters (see Dandy 1980; Preston 1995): winter buds axillary vs. terminating long or short lateral branches; young stipule sheaths closed and tubular in lower part vs. open and convolute; leaves long mucronate vs. short mucronate; lateral veins inconspicuous and midvein not bordered by bands of hyaline cells vs. lateral veins conspicuous and midvein bordered by bands of hyaline cells; nodal glands absent or feebly developed vs. conspicuous; and fruits 2 mm vs. 2-2.5 mm. We have observed no overlap in these characters in European and North American material. Some sterile hybrids are reported from Europe (e.g., Hagström 1916; Hylander 1953b) but no fertile transitional forms have been documented.

The two species have, however, been much confused. Haynes and Hellquist (2000c) treated them as two largely sympatric varieties but did not justify this solution. They did not emphasize the same characters in their text as European authors do, and their annotations on some herbarium specimens (ALA, CAN) suggest that other features than those above have been important when they went for the varietal solution. Preston (1995) presented convincing arguments for why they are two species and did not report hybrids between P. berchtoldii and P. pusillus in the British Isles (both common there). We can see no reason not to accept two species.