[Illustration]

1. Sori on the edge of a pinnule terminating a vein; sporangia at the base
of a long, bristle-like receptacle surrounded by a cup-shaped indusium.

Filmy Fern. _Trichomanes_.

[Illustration]

2. Indusium formed by the reflexed margin of the pinnules.

(1) Sporangia on a continuous line; fronds large, ternate; indusium narrow.
Bracken. Brake. _Pteris_.

[Illustration]

(2) Sporangia in oblong sori under a reflexed tooth of a pinnule; indusium
broad; rachis dark and shining. Maidenhair. _Adiantum_.

[Illustration]

(3) Sori in roundish or elongated masses.

Indusium broad, nearly continuous, fronds mostly smooth, somewhat leathery,
pinnate. Rock species. Cliff brakes. _Pellæa_.

[Illustration]

Indusium narrow, seldom continuous, formed by the margin of separate lobes