Polypodium arcanum Maxon

Authority

Mickel, John T. & Smith, Alan R. 2004. The pteridophytes of Mexico. Mem. New York Bot. Gard. 88: 1-1054.

Family

Polypodiaceae

Scientific Name

Polypodium arcanum Maxon

Description

Species Description - Rhizomes creeping, 4–7 mm diam.; rhizome scales white to reddish brown with reddish brown bases, deltate, with longattenuate often flexuous-contorted tips, denticulate, especially on the long tips, 4–6 x 1.5 mm, often with a few minute hairs on the scale surfaces; fronds (7–)15–40 cm long, distant; stipes 1/3–1/2 the frond length, stramineous to brown, glabrous; rachises glabrous except for rare long hairs 0.8–1 mm and rare, clavate brown hairs 0.1–0.3 mm, mostly in the axils of the pinnae; blades pinnatisect, oblong to oblong-deltate, 6–18 cm wide; rachises glabrous or with minute hairs, usually with a few small, contorted, hair-like, reduced scales at bases of proximal pinnae; pinnae 3–26 pairs, linear-deltate, equally dilated at base, 3–9 cm x 8–18 mm, acute to acuminate, laminae chartaceous, often reddish, especially near costules; margins serrulate to serrulatecrenulate; veins netted, generally with one row of areoles on each side of costae; sori round, medial, one row on each side of costae; sporangia with minute, inconspicuous to conspicuous hairs 0.1 mm.

Discussion

Polypodium deltoideum Liebm., Mexic. Bregn. 190 (reprint 38). 1849, hom. illeg., non Sw., 1788. Lectotype (chosen by Tejero, Acta Bot. Mex. In press). Mexico. Puebla: Chinautla, Liebmann s.n. [Pl. Mex. 2598, Fl. Mex. 156]) (C!; isotype GH!).

There is considerable variation in number of pinnae in P. arcanum, and this seems to be correlated with geography. Specimens from western Mexico (Michoaca´n, Jalisco, and southwestern Oaxaca) have (8–)12–26 pairs of pinnae whereas those to the east have (3–)4–8(–10) pairs. The former have been called P. plesiosorum var. bakeri based on pinna pair number (10–29 in P. plesiosorum), but clearly resemble P. arcanum in the conspicuous whitish (to tan) rhizome scales. We believe the rhizome scales are more consistent and taxonomically reliable in this group. Polypodium arcanum is similar to P. eatonii, which occurs in Chiapas and Veracruz. The smaller scales, toothed pinna margins, narrower pinnae, and lack of cordate-clasping bases on the proximal pinnae distinguish P. arcanum. Polypodium arcanum is also very closely related to P. colpodes and P. plesiosorum, differing from the former in lacking dark stipes, and from the latter in having fewer pinnae (mostly 4–10 in the east and north, (12–26 in var. bakeri in the west) vs. 15–30(–39) in P. plesiosorum) and having larger, more spreading rhizome scales (vs. appressed and somewhat deciduous in P. plesiosorum). The blades are glabrous or with short minute hairs on the stipes and rachises and generally have sparse to rare small contorted reduced hair-like scales along the rachises at the bases of the proximal pinnae (Fig. 245K), reminiscent of the scale tips on the rhizomes of some specimens. The sporangia have short inconspicuous to conspicuous hairs 0.1 mm. Blades may have a reddish color when young.