疏脉苍山蕨 shu mai cang shan jue

Asplenium paucivenosum f. minus Bir; Ceterach paucivenosa Ching; Ceterachopsis paucivenosa (Ching) Ching.

Plants 15-25 cm tall. Rhizome erect, short; scales brown, triangular, margins entire or sparsely dentate. Fronds herbaceous, clustered; stipe short; lamina narrowly elliptic-obovate, 11-16 × 1.5-6 cm, gradually narrowed to base, pinnatipartite, apex acute; segments 6-13 pairs, alternate, perpendicular to midrib (rachis), basal segments semicircular, upper ones oblong to triangular-ovate, apex obtuse, middle segments 1-2.5 × 1-1.5 cm, entire or slightly sinuate, glaucous or green becoming brown with distinct hyaline narrow margin after drying. Veins faintly visible, nothocatadromous (anadromous base pattern but several middle pinnae with their basal vein pair catadromous), average guard cell length 79-93 µm. Sori short, subelliptic to ensiform-semilunulate, 2-8 × 1-2 mm, oblique, usually 2 or 3 pairs per segment, in middle between costa and margin, on acroscopic secondary veinlets; indusia grayish brown, semi-elliptic to ensiform-semilunulate, thinly membranous, entire, persistent. Spores with lophate-reticulate perispore, average exospore length 45-51 µm. Plants sexual tetraploid: 2n = 144.

On rocky cliffs in mixed forests by streamsides; 2000-2700 m. Xizang, Yunnan [Bhutan, N India, Nepal].

Asplenium paucivenosum is a tetraploid species similar to the diploid A. dalhousiae but has much larger spores (and stomata) with a mean exospore length over 40 µm (less than 35 µm in A. dalhousiae) and a different, more fenestrate outer perispore. According to Nakaike (Ferns Nepal 2: 45, 1986), it was confused with A. dalhousiae by Itô (in H. Hara, Fl. E. Himal. 1: 487. 1966; Fl. E. Himal. 2: 214. 1971) and Iwatsuki (in Ohashi, Fl. E. Himal. 3: 194. 1975). It is even more easily confused with A. magnificum which has a larger mean exospore length (48 µm in A. paucivenosum vs. 52 µm in A. magnificum). In critical cases, a chromosome count is needed for identification.