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..
Leafy plants of various habit, the rootstocks horizontal and often elongate, or shorter and erect, the leaf-blades simple, once or several times pinnate or pinnatifid, or decompound, coiled in vernation. Sporanges borne on the under surface of the foliaceous leaf-blades, or upon slender or contracted, partially foliose or non-foliose leaves or parts of leaves, or, as in most of our species, in clusters (sori) upon the backs of the leaf-blades; distinctly stalked, provided with an incomplete vertical ring of thickened cells (the annulus), and opening transversely. Sori either with or without a membranous covering (indusium). Pro-thallia green.
About 145 genera and 4500 or more species of very wide geographic distribution. This family includes by far the greater number of living ferns.
Leaves strongly dimorphous, the fertile ones with divisions greatly contracted, brownish, berrylike or necklace-like.
Sterile blades deeply pinnatifid; veins freely anastomosing.
Sterile blades deeply 2-pinnatifid; veins free.
Leaves mostly uniform; if dimorphous, the fertile blades flat, the divisions green, not as above.
Sori dorsal upon the veins, not marginal.
Indusium wholly or partially inferior.
Indusium wholly inferior, the divisions stellate or spreading.
Indusium attached by its base at one side of the sorus, hood-shaped, withering.
Indusium, if present, superior.
Stipes jointed to the rootstock; indusia wanting.
Stipes continuous with the rootstock (not jointed); indusia present in most species.
Indusium (present, in our species) orbicular-peltate, centrally attached.
Indusium, if present, orbicular-reniform, attached at its sinus.
Sori oblong to linear.
Sori in chain-like rows parallel to the midrib and rachises.
Leaves uniform; veins free between the sori and margin.
Leaves dimorphous; veins of sterile blade freely anastomosing.
Sori oblique to the midribs or irregularly disposed.
Veins free; sori all oblique to the midribs.
Sori confluent in pairs; indusia single, contiguous, appearing double.
Sori single on the outer side of veinlet, or crossing it and recurved.
Sori straight or slightly curved; leaves mostly evergreen.
Sori usually curved, often crossing the veinlet and recurved; leaves herbaceous.
Veins freely anastomosing; sori variously disposed.
Sori borne at or very near the margin.
Sporanges borne within a special cup-shaped indusium.
Sporanges not borne within a special cup-shaped indusium.
Sori without indusia, somewhat protected by the revolute leaf-margin.
Sori with indusia formed entirely or in part by the revolute or reflexed more or less modified leaf-margins.
Sori distinct, borne on the under side of the reflexed lobes.
Sori wholly or partially confluent.
Sori borne on a vein-like receptacle connecting the ends of the free veinlets;
Sori borne at or near the ends of the free veinlets; indusia single.
Leaves uniform or nearly so.
Sori confluent, forming a wide submarginal band; segments smooth or
Sori distinct or contiguous; segments usually pubescent, tomentose or scaly.