British ferns (Filicopsida)


L. Watson and M. J. Dallwitz

Cystopteris diaphana (Bory) Blasdell

Sporophyte. The rhizomes short, rather stout; ascending, or erect; bearing scales (these thin, brown, lanceolate). Plants with no clear distinction into fertile and sterile leaves.

Leaves aggregated terminally; to (5–)10–35(–45) cm long; persistent; circinnate; compound; complexly divided; bipinnate with conspicuously divided pinnules to tripinnate with undivided ultimate pinnules (the pinnules not parallel-sided, deeply pinnatifid, ultimate segments sharply toothed, serrated, or bluntly lobed). Pinnae 5–15 on each side of the leaf (? - “up to 15”). The petioles shorter than the blades to about as long as the blades (from a third as long, slender and brittle, dark brown at the base and paler above, with a few scales at the base and usually a few hair-like scales above). Leaf blades in outline ovate, or lanceolate. The longest pinnae near the base of the blade to around the middle of the blade; 1–4 cm long. The adjacent pinnae approximated but not strongly overlapping to distant from one another and not at all overlapping (the separation becoming less marked acropetaly). The venation of the lamina open. The veins of the ultimate pinnules mostly terminating in the notches between the teeth.

The sporangia superficial; protected; aggregated in sori. The sori sub-orbicular (small, each on a receptacle with a vascular strand from the vein, borne in two rows, one on either side of the midrib of the pinnule); remaining discrete at maturity; with a true indusium. The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed; whitish, ovate, exceeding the sorus. The mature spores echinate; without a perispore.

Distribution and habitat. On base-rich substrates. Native to the British Isles, or naturalized (a widespread warm temperate and tropical species, British status uncertain). Recorded only from Cornwall and south Devon.

Vice-county records. Britain: West Cornwall, East Cornwall, South Devon.

Classification. Family Polypodiaceae (C.T.W.); Cystopteridaceae (Swale and Hassler); Woodsiaceae (Stace). Order Athyriales (Swale and Hassler).

Comments. See Murphy and Rumsey, 2005.

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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2004 onwards. British ferns (Filicopsida). Version: 4th January 2012.’.