British ferns (Filicopsida)


L. Watson and M. J. Dallwitz

Adiantum raddianum K. Presl

“Delta Maiden-hair Fern”.

Sporophyte. The rhizomes creeping; bearing scales (densely covered with dark brown, lanceolate-attenutate scales). Plants with no clear distinction into fertile and sterile leaves.

Leaves distributed along the rhizomes (but close together); to 15–45 cm long; complexly divided; tripinnate with undivided ultimate pinnules, or tripinnate with conspicuously divided ultimate pinnules, or 4-pinnate (the rachides and rachillae dark reddish brown to blackish and shining like the petiole, pinnae and pinnules stalked, the ultimate pinnules lacking a midrib, not articulate, wedge- or fan-shaped, symmetrical or not, crenately lobed distally with broad, rounded or truncate lobes which on the fertile pinnules are recurved, the sterile margins with each vein ending in a small notch or sinus). The petioles about as long as the blades to longer than the blades (wiry, black and shining); vascularised via a single strand. Leaf blades in outline broadly ovate, or ovate-triangular, or lanceolate, or lanceolate-triangular (to 30 cm broad). The venation of the lamina open (dichotomously branching to the distal lobes of the segments).

The sporangia marginal; aggregated in sori (the sori borne along the distal margins of the pinnules). The sori borne towards the ends of the veins, under the recurved margins of the lobes; remaining discrete at maturity; with only a false indusium derived exclusively from the lamina margin (this in the form of a very specialized, pale or whitish indusoid flap). The false indusia interrupted on the individual blade segments; reniform-orbicular. Paraphyses absent. The mature spores trilete; without a perispore.

Distribution and habitat. On base-rich substrates. Marginally naturalized (native to Central and South America and the Caribbean). Recorded only from the London area.

Vice-county records. Britain: Middlesex.

Classification. Family Polypodiaceae (C.T.W.); Adiantaceae (Swale and Hassler); Adiantaceae (Stace). Order Pteridales (Swale and Hassler).

Comments. A temporary description, requiring editing re differences between this species and A. capillus-veneris.

Illustrations. • A. raddianum (photo, F. and K. Starr).

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

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