Dryopteris expansa (Spreading Wood Fern)
|Also known as:
|Northern Wood Fern
|Dryopteridaceae (Wood Fern)
|part shade, shade; cool moist woods, rocky slopes
|1 to 3 feet
|Wetland Indicator Status:
|GP: none MW: FAC NCNE: FAC
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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Leaves and stems:
Leaf is up to 36 inches long and to 12 inches wide, lanceolate to triangular in outline, twice compound though the lowest pair of branches at the base of the frond are again compound. There are may be a few glandular hairs. At the base of the stem are brown scales with a dark brown stripe. A few to several leaves grow in an asymmetrical clump.
Leaflets are lobed, sometimes divided almost to mid-nerve, and toothed, the teeth with a bristle-like tip. Veins are forked. On the first branch at the base of the leaf, the leaflet on the lower-side of the branch closest to the stem is as long as or longer than the next leaflet and 2 to 3 times as long as the leaflets opposite on the upper-side of the branch (see image enlargement). In addition, the first leaflet on the lower-side is attached closer to the second leaflet on the upper-side than the first leaflet.
The sori (group of spores) are found on the underside of the leaf. They are circular and arranged mostly in a row along each side of the leaflet center vein. There is a translucent tissue (indusium) that partly covers the spores and is attached on the inner curve.
Dryopteris species can be identified by the circular shaped sori that is covered by a translucent tissue. Dryopteris expansa is most similar to D. carthusiana. On the lowest branch of both species, the leaflet closest to the stem on the lower side of the branch is usually longer than the next leaflet. However, on D. carthusiana, the first leaflet on the lower side of the branch is attached nearly opposite the first leaflet on the upper side of the branch, where on D. expansa the first pair are far more offset. D. expansa is also similar to D. intermedia, which has the first leaflet on the lower side of the branch shorter than the next leaflet and is glandular hairy in the upper leaf. While the natural range of D. expansa overlaps the other two, it is primarily limited to the cool moist woods in the Arrowhead region, though the Herbarium also lists a record of this species found in a cave in Winona County.
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Photos courtesy Cindy Kottschade Hoffmann taken in Superior National Forest, near Lutsen and along Oberg Mountain Trail.
Have you seen this plant in Minnesota, or have any other comments about it?