Thelypteris palustris
Thelypteris palustris Schott var. pubescens (G. Lawson) Fernald
(sp.: Latin: paluster, marshy or swampy, presumably in reference to the habitat; var.: Latin: pubens, downy or softly hairy, possibly for the hairs on the costae and veins on the underside of the blades or the often pubescent indusia)
Local names: southern marsh fern, marsh fern, eastern marsh fern
Rhizomes long-creeping, leaves monomorphic or nearly so, dying back in winter; petioles straw-colored above base; leaf blades 10–40(–55) cm long, the lower surface with sparse to dense hairs on costae (= midveins of pinnae) and sometimes other veins; pinnae deeply pinnatifid, to within 1 mm of the costae (rarely pinnate), the lower pinnae commonly slightly shorter than middle pinnae; lateral veins of ultimate leaf segments often forked, usually extending to margin of blade tissue above sinuses between ultimate segments; margins of fertile ultimate segments entire, usually turned downward; sori medial; indusial often with pubescence; 2n = 70 (Smith 1993a). Terrestrial in low woods, along streams, and in swamps and bogs; scattered in the Pineywoods (e.g., Polk Co., T. Starbuck 403b, BRIT; Titus Co., P.A. Amerson 1111, BRIT) and Post Oak Savannah (e.g., Leon Co., Orzell & Bridges 6522, BRIT, TEX-LL—“Acid seep forest…Sparta Sand”); also n Gulf Prairies and Marshes (e.g., Colorado Co., Correll & J.R. Peace 34950, TEX-LL); se Canada and throughout e U.S. w to ND, OK, and TX; also e Asia and Europe (circumboreal). Sporulating late summer and fall. [Dryopteris thelypteris (L.) A. Gray var. pubescens (G. Lawson) Weath., Thelypteris palustris var. haleana Fernald] The distribution of this fern is a classic example of a disjunction caused by vicariance (= a once more widespread species becoming fragmented with survivors in only parts of the former area). It occurs broadly in Europe, e North America, and e Asia (including Japan). In fact, 40% of the fern species of the northernmost island of Japan (Hokkaido) and northeastern North America are common to both regions—a remarkable holdover from the Tertiary Period when the Earth’s climate was much warmer, North America, Europe, and Asia were connected by land bridges, and a continuous circumboreal forest occurred broadly in the Northern Hemisphere. Much has changed since that time, and Thelypteris palustris and a variety of other plants have survived in only limited portions of their ancient range (Kato & Iwatsuki 1983; Moran 2008) (see page 32 and Fig. 35 for further information). The plants in North America and e Asia are var. pubescens, while those in Europe and w Asia are var. palustris. Sterile specimens of this species can be confused with sterile specimens of cinnamon fern (Osmundastrum cinnamomeum) because of the similar way in which the lateral veins of the ultimate leaf segments are forked. However, cinnamon fern has a persistent tuft of tan to orangish tomentum (“arm pit hairs”) where each pinna attaches to the rachis, a feature lacking in Thelypteris palustris.