Know Your Natives – Marginal Wood Fern

Marginal wood fern (Dryopteris marginalis) of the Wood Fern (Dryopteridaceae) family, an evergreen, twice-cut fern, occurs throughout much of the eastern U.S. from Texas and Minnesota to the Atlantic Coast, but is mostly absent from the Gulf Coastal Plain and unknown from Louisiana and Florida.  In Arkansas, it has been documented to occur in the northwestern half of the state in the Interior Highlands.  The genus name is based on the Greek words for “oak” and “fern” which relates to occurrence of wood ferns among oak forests.  The specific epithet relates to the location of sori (spore clusters) near the margins of the leaf divisions.  Other common names include marginal shield fern, evergreen wood fern and leather-leaf wood fern.  This fern favors areas with damp to dry soils found in partially to fully shaded, rocky ravines, stream banks and north-facing slopes.

This medium-large fern has an elongate, reclined and compact rootstock, with dense, shallow and branching fibrous roots that grow directly from the rootstock.  New fronds grow from the center of the rootstock in late winter as tightly curled fiddleheads (or croziers).  Fiddleheads are densely covered with light-brown, soft scales and filaments, especially near their bases.  Scales and filaments, mostly absent along the leaf rachis (the midrib of the compound leaf), remain on the stipe (leaf stalk) of mature leaves.  The stipe and rachis, yellowish-green overall, with the stipe having a reddish base, have a central upper groove along the flattened upper side and a convex lower side.  The plant has a vase-like overall shape with an airy appearance.  Old fronds (an alternative term for fern leaves) wither around the rootstock, with the old stipes persisting for several years.

photo-1-january-11Photo 1:  Rootstock, as seen in this mid-January photo, bears green fronds which will decline when new fronds appear in late winter .  Previous year’s stipes remain.  Fibrous roots have been removed.

photo-2-april-6Photo 2:  In early April, fiddleheads grow from center of rootstock.  Previous year’s and older stipes can be seen.

Stipes are one-fourth to one-third as long as the fronds: fronds are up to 18 inches long and 6 inches wide, with stipes being about 8 inches long.  Plants, with fronds ascending and arching, may have a height and width each of about 2 feet.  Fronds are a bluish-green above and a lighter yellowish-green below.  Fronds have linear-lanceolate, quickly tapering pinnae (leaflets) that are mostly opposite lower on the rachis, changing to alternate higher toward the apex.  The leathery, glabrous pinnae, with tips arching toward frond apex, are longest mid-frond and slightly shorter toward the frond base (a semi-tapered fern).  Separation of pinnae, greatest at frond base, decreases toward frond tip as pinna length quickly shortens to form an acuminate frond tip.  Pinnae approaching frond tip are lobe-like.  Lower pinnae have very short petiolules (stalks) that are perpendicular to the rachis, with upper, short pinnae becoming sessile without overlap.  Pinnae are divided (a twice-divided fern) into oblong pinnules (sub-leaflets or secondary leaflets) which have rounded tips and margins that vary from slightly crenulate to well lobed near the base of the rachis.  Fronds may have 15 or so easily distinguishable pairs of pinnae before the pinnae become indistinct at the frond apex.  Similarly, pinnae may have 15 or so easily distinguishable pairs of pinnules before they become indistinct at the pinna tip. Main and secondary veins of pinnules, purplish on the underside, are of about equal size, nearly straight and forked.   Main veins of pinnules extend onto pinna midribs.

photo-3-january-11Photo 3:  In early January, fronds remain green.  All fronds of a plant grow from center of rootstock without any off-sets.

Fertile and sterile fronds are both photosynthetic and have the same appearance when viewed from above.  However, fertile fronds bear fruit-dots or sori* (singular, sorus) on the undersides of their upper (or distal) pinnae. Each sorus is a cluster of spore-producing sporangia.  From one to five sori are located along the margins of pinnules, except on those pinnules near the frond apex that are more lobe-like. Single sori are located at sinuses between the lobes.  Sori, which develop in late spring, are at first green, but then become dark brown.  They are covered by umbrella-like shields called indusia (singular, indusium).  Indusia are attached to the pinnule at the sori’s depressed center, loosely extending over many sporangia.  Each sporangium or spore case produces 50+ spores.  The depressed center of a sorus along with a depressed side cause the round sorus to appear reniform (kidney-shaped).  As the spore cases grow, they push the indusium aside.  When air moisture is low, spore cases pop open and dust-like spores are dispersed into the air.**

photo-4-marginal-fmid-right-1Photo 4:  Upper surfaces of an infertile frond (left) and a fertile frond (right) appear the same.  Lower side of a fertile frond, which has already dispersed its spores, is shown in the middle.  As shown, pinnae may be opposite or alternate.  Photo taken in mid-July.

photo-5-marginal-yPhoto 5:  Sori are located at margins of pinnules or singly at sinuses between lobe-like pinnules, as shown.  Note venation and slightly crenulate margins.  Photo taken in mid-July.

In a garden or natural areas, this evergreen, medium-large fern with its nice form and good texture can serve as an accent plant that provides interest throughout the year.  The plant does not colonize or spread by rootstock.  Dead fronds, spread around the rootstock, quickly disintegrate and tougher stipes, which disintegrate in two years, are not especially noticeable.  Marginal wood fern does well in partial to full shade.  It is not a preferred food choice of deer or rabbits.  Once established, it can survive dry periods.

photo-6-may-15Photo 6:  Marginal wood fern is an excellent accent plant for a shady garden.

In Arkansas, 12 taxa in the wood fern family (Dryopteridaceae) have been documented outside of cultivation, of which nine (including several of hybrid origin) are native.  Of the natives, only two species are widespread evergreen ferns, namely the subject species marginal wood fern (Dryopteris marginalis) and Christmas fern (Polystichum acrostichoides).  Christmas fern is a distinctive, once-cut fern with darker green and more leathery pinnae.

*   Sori – Greek for “heaps”.
**  With dispersal of spores, the above-ground “diploid sporophyte phase” of a fern’s life cycle (referred to as “alternation of generations”) concludes.  In the soil, spores germinate to produce a prothallus as the “haploid gametophyte phase”.  The prothallus produces mobile sperm gametes and attached egg gametes.  With fertilization of the egg, a diploid zygote may develop into a new diploid sporophyte plant.

Article and photographs by ANPS member Sid Vogelpohl

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