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..
Mainly woodland ferns, commonly of upright habit, the fertile and sterile leaves usually similar, not jointed to the rootstock. Blades 1-3-pinnate or dissected, with veins free in northern species, uniting occasionally or even freely in some of the southern. Sori round or rarely elliptical in outline, borne upon the veins, indusiate or non-indusiate, the indusium (if present) in northern species orbicular-reniform, fixed at its sinus; sporanges numerous.
A genus of several hundred species, widely distributed in the tropics, its limits variously understood. Besides the following, some 13 species occur in the southern and western United States. Type species: Polypodium Filix-mas L.
Indusia present (§ Eudryopteris).
Texture membranous; veins simple or once forked.
Lower pinnae gradually and conspicuously reduced.
Lower pinnae scarcely reduced.
Veins once or twice forked.
Texture firmer, sometimes subcoriaceous; veins freely forked.
Blades 2-pinnatifid or 2-pinnate; segments not spinulose.
Leaves small; rachis commonly chaffy throughout.
Leaves larger, 1 1/2º-5º high; rachis naked or deciduously chaffy.
Indusia flat, thin.
Blades narrow, linear-oblong to lanceolate; sori nearly medial.
Blades broader, narrowly oblong, ovate or triangular ovate; sori near midvein.
Apex attenuate; pinnae broadest at base; sori 3-7 pairs. 6.
Apex short-acuminate, often abruptly so; pinnae broadest above the base;
sori 6-10 pairs.
Indusia convex, firm.
Sori near the margin.
Sori near the midvein.
Blades 2-pinnate to 3-pinnate; segments spinulose or mucronate.
Blades ovate-lanceolate, triangular, or broadly oblong, usually not narrowed below.
Indusia glabrous or nearly so; pinnae usually somewhat oblique to the rachis, the lowest broadly and unequally ovate to triangular.
Pinnules flat, decurrent; sori terminal on the veinlets; scales pale brownish.
Pinnules concave, some not decurrent; sori mostly subterminal; scales dark brownish.
Indusia glandular; pinnae usually at right angles, the lowest unequally lanceolate to
Blades elongate-lanceolate, usually narrowed below.
Indusia wanting (§ Phegopteris).
Basal pinnae sessile or partially adnate: rachis more or less alate.
Blades usually longer than broad; rachis and midveins freely chaffy; under surfaces pilose.
Blades usually broader than long; rachis and midveins scarcely scaly; under surfaces
Basal pinnae long-stalked; rachis not alate.
Blades nearly horizontal, glabrous or nearly so, subternate, the basal pinnae approaching
the terminal portion in size.
Blades suberect, copiously glandular, triangular-ovate, the basal pinnae considerably smaller
than the terminal portion.