A. Sporangia borne in cones
1. Creeping, leafy, moss-like plants, cones quadrangular in cross-section Selaginella 114

2. Erect, jointed, rush-like, leafless except for collar-like sheath at nodes, cones round in cross-section Equisetum 109
B. Sporangia borne in clusters on a special stalk from basal part of plant Botrychium 62

C. Sporangia borne on underside of leaf in clusters called sort
1. Indusium (cover over sporangia) absent
Underside of fronds white or yellow with a powder Pityrogramma 94

Underside of fronds without any powder
Fronds deeply pinnately lobed or pinnately compound Polypodium 96

Fronds bi- or tri-pinnately compound Athyrium americanum 70

2. Indusium present, sori marginal or on underside of leaf
a. SORI MARGINAL, covered by curled leaf margin or marginal indusium
(1) Stalks light or straw-colored, except at base
Fronds of 2 kinds: Taller fertile and shorter sterile, plants less than 1 foot high Cryptogramma 80

Fronds all alike, plants 1 1/2 to 4 feet high Pteridium 103
(2) Stalks dark-colored (except Pellaea andromedaefolia with tan to reddish stalks)
Delicate pinnae, midrib or pinnae not evident veins nearly all the same size and forking; grow in damp, protected places Adiantum 66

Thicker pinnae, (except Pellaea breweri) midrib on pinnae or lobes always visible, sori more or less continuous, in dry exposed places.
Fronds and stalks scaly or woolly or both (except two species), indusia interrupted, or if continuous segments, bead-like Cheilanthes 73

Fronds not scaly or wooly indusium continuous, bearing sporangia on surface of pinnae Pellaea 86

b. SORI NOT MARGINAL, each covered with a special indusium
(1) Sori round or kidney shaped, plants rarely over 2 feet high
Indusium scale-like, attached to vein below sporangium, reflexed or somewhat deciduous in older fronds Cystopteris 83

Indusium saucer or fringe-like, attached centrally to a stalk beneath the sporangia Woodsia 105

Indusium shield-shaped, attached centrally above the sporangia
Indusia orbicular without a sinus Polystichum 100

Indusia kidney-shaped, or orbicular with a narrow sinus Dryopteris 83

(2) Sori oblong or linear, tall ferns, usually over 2 feet high
Sori in rows, parallel to midrib on lobes of pinnae Woodwardia 107

Sori in rows, oblique midrib of pinnules or lobes of pinnae Athyrinnz 70

[click to enlarge]

Fig. 7 Split nodule revealing a preserved extinct horsetail, Annularia, found in Mazon Creek flora of Illinois, from the Coal age (Carboniferous Period). Specimen courtesy of Ethel Doerer.

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