This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Small or medium-sized ferns, growing in rocky places, the rootstocks in dense tufts. Leaves numerous, the stipes often jointed above the base and separable, the blades 1-2-pinnate or deeply 3-pinnatifid. Sori roundish, borne on the simply-forked free veins. Indusia slight and often evanescent, inferior in attachment, either roundish and soon cleft into irregularly jagged lobes, or deeply stellate, the filiform divisions concealed beneath the sporanges or inflexed and partially covering them. [Named in honor of Joseph Woods, 1776-1864, an English architect and botanist.]
Indusium small or inconspicuous, the divisions narrow or filiform.
Stipes jointed near the base; filiform divisions of the indusium more or less inflexed over the sporanges.
Blades with more or less rusty chart underneath.
Blades glabrous or nearly so.
Blades oblong-lanceolate; divisions of the indusium numerous.
W. alpin a.
Blades linear or linear-lanceolate; divisions of the indusium few.
Stipes not jointed; divisions of the indusium spreading, mostly concealed beneath the sporanges.
Puberulent, usually hispidulous; indusium deeply cleft into narrow flaccid segments.
Glabrous; indusium divided to the center into a few short whitish turgid beaded hair-like
Indusium ample; the divisions broad, early spreading.
Acrostichum ilvense L. Sp. PI. 1071. 1753.
Rootstocks short, ascending, growing in masses, the leaves closely caespitose. Stipes short, stoutish, jointed near the base, rusty chaffy with narrow filiform scales; blades lanceolate, 4'-10' long, pinnate, nearly glabrous above, more or less covered with rusty chaff beneath; pinnae crowded, sessile, pinnately parted, the crowded segments oblong, crenate; sori borne near the margins of the segments, somewhat confluent with age; indusium minute, concealed beneath the sorus, cleft into numerous filiform segments, these inflexed over the sporanges and inconspicuous.
On exposed rocks, Labrador to Alaska, south to North Carolina, Kentucky and Iowa. Ascends to 5000 ft. in New Hampshire. Also in Greenland, Europe and Asia. June-Aug. Ray's Woodsia, Oblong Woodsia.