Stems erect or ascending. Leaves arching, 5--18 dm; bulblets absent. Petiole 1/8--1/4 length of leaf, densely scaly; scales red-brown to dark brown or nearly black, gradually diminishing in size distally. Blade linear-lanceolate, 1-pinnate, base slightly narrowed. Pinnae narrowly lanceolate, straight to falcate, not overlapping, pinnae of shade-growing plants in 1 plane, those of sun-growing plants twisted or contorted, 1--15 cm; base ± cuneate, auricles well developed; margins serrulate-spiny with teeth ascending; apex acuminate with subapical teeth same size as apical tooth; microscales ovate-lanceolate to linear-lanceolate, with contorted projections, dense, on abaxial surface only. Indusia ciliate. Spores light yellow. 2 n = 82. Terrestrial, forest floor, only occasionally on rock, in mesic coniferous to moist, mixed evergreen forests; 0--2200 m; B.C.; Calif., Idaho, Mont., Oreg., S.Dak., Wash.; Mexico on Guadalupe Island; naturalized in Europe. One of the most abundant ferns in the western flora (rivaled only by Pteridium ), Polystichum munitum also is of significant economic importance. Enormous quantities of leaves are gathered for backgrounds in funeral wreaths and other floral displays; the evergreen leaves keep well in cold storage and are exported to Europe. It is extensively used in landscaping, the trade being mainly in wild-collected plants. Polystichum munitum appears to be most closely related to P . imbricans based on morphologic (D. H. Wagner 1979) and electrophoretic (P. S. Soltis et al. 1990) analyses. The chloroplast DNA of P . imbricans , however, is divergent (G. Yatskievych et al. 1988), suggesting a chloroplast origin independent of the nuclear genome. That Polystichum munitum is related to P . acrostichoides is supported by data from chloroplast DNA analysis (G. Yatskievych et al. 1988) but contradicted by data from electrophoretic studies (P. S. Soltis et al. 1990).
Polystichum munitum can be distinguished from P . imbricans by its persistent, wide (the largest wider than 1 mm) distal petiolar scales; such scales of P . imbricans are less than 1 mm wide and fall off early.
From an evolutionary standpoint, Polystichum munitum is a diploid progenitor of P . andersonii , P . californicum , P . setigerum , and, perhaps, P . scopulinum . Hybrids with all except P . setigerum have been reported, all triploid, attesting to its parental role in the tetraploids (see discussion under each). Hybrids with P . braunii (A. Sleep and T. Reichstein 1967), P . kruckebergii (P. S. Soltis et al. 1987), P . dudleyi (W. H. Wagner Jr. 1973), and P . lemmonii (P. S. Soltis et al. 1989) also have been reported.
The population on Guadalupe Island has been called Polystichum solitarium Maxon.