2. Asplenium septentrionale (Linnaeus) Hoffmann, Deutschl. Fl. 2: 12. 1796.
叉叶铁角蕨 cha ye tie jiao jue
Acrostichum septentrionale Linnaeus, Sp. Pl. 2: 1068. 1753; Acropteris septentrionalis (Linnaeus) Link; Amesium sasakii Hayata; A. septentrionale (Linnaeus) Newman; Asplenium sasakii (Hayata) Makino & Nemoto; A. septentrionale var. sasakii (Hayata) C. Christensen.
Plants 8-15 cm tall. Rhizome shortly creeping to ascending, apex scaly; scales dark brown, narrowly triangular to subulate, entire to denticulate, at base fimbriate. Fronds caespitose; stipe shiny castaneous only at base, abruptly green toward rachis, 6-10 cm, 2-3 × as long as lamina, with unicellular glands or subglabrous, apex 2- or 3-forked; segments linear, 2-3 × 0.1-0.15(-0.25) cm, base gradually decurrent onto stipe, minutely forked again at apex. Fronds without distinct rachis, green, other veins slender and subparallel to central axis, 1 veinlet per ultimate segment. Fronds herbaceous-leathery, grass-green. Sori 1-5 per segment, linear, 1-2 cm, on acroscopic veins and close to costa, at maturity covering entire surface; indusia brown, linear, membranous, free margin often with unicellular glands, entire-sinuate, opening toward main vein (costa), persistent but reflexed and covered by sporangia at maturity. Spores with lophate perispore, average exospore length 39-44 µm. Plants sexual tetraploid: 2n = 144.
In crevices of non-calcareous (often granitic) rocks in open or partially shaded situations; 1100-4100 m. Shaanxi, Shanxi, Taiwan, Xinjiang, Xizang [Afghanistan, India, Kashmir, Kazakhstan, Kyrgyzstan, W Mongolia, Nepal, Pakistan, Russia, Tajikistan; NW Africa, SW Asia, Europe, North America].
Asplenium septentrionale is the autotetraploid that originated by chromosome doubling in its diploid ancestor A. caucasicum. In the absence of a chromosome count, it can best be distinguished by its larger mean exospore length (more than 37 µm) and its broader lamina.
Asplenium septentrionale often hybridizes with other Asplenium species in Europe wherever they co-occur; one of these, A. ×heufleri Reichardt (A. septentrionale × A. quadrivalens), was recently found in Xinjiang.