Polypodium is a taxonomically difficult genus of ca. 75-100 species. Most are confined to the northern hemisphere. Numerous tropical species were formerly included in Polypodium but are currently accommodated in various other genera (see Page & Bennell 1986). Species delimitation is most of the time critical. In addition to morphological features cytological studies are often required to distinguish among taxa. A fourth taxon might have been overlooked, Polypodium cambricum L. (syn.: P. australe Fée, P. vulgare subsp. serrulatum F.W. Schultz ex Arcang.), native in large parts of western Europe. It is readily separated cytologically (diploid 2n = 74) and furthermore distinguished by leaf blades less than twice as long as wide, pinnae narrowly acute and often serrate and sporangia mixed with hairs > 0,5 mm long (Stace 1997; see also photos below). It was recently confirmed from an old city wall in Brugge, possibly as a neglected native species (although it is cultivated in parts of Europe as well; see Page & Bennell 1986).
Annulus of the sporangium reddish-brown, with (8-)12(-18) indurated cells, with 2-3(-4) basal cells between the proximal end of the annulus and the sporangium stalk === 1. Polypodium hesperium
Annulus of the sporangium pale yellowish to reddish brown, with 4-14 indurated cells, with 1-3(-4) basal cells between the proximal end of the annulus and the sporangium stalk (native) === P. interjectum and P. vulgare
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Literature:
Haufler C.H., Windham M.D., Lang F.A. & Whitmore S.A. (1993) Polypodium. In: Flora of North America Editorial Committee (ed.), Flora of North America north of Mexico, vol. 2. Oxford University Press, New York-Oxford: 315-323.
Page C.N. & Bennell F.M. (1986) Polypodium. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 33-35. Cambridge University Press, Cambridge.
Stace C. (1997) New flora of the British Isles, 2nd ed.: XXVII + 1130 p. Cambridge University Press.