The main rib or vein of a segment, pinnule, pinna, or frond; a midvein.    From Wordnik.com.  [The Fern Lover's Companion A Guide for the Northeastern States and Canada] Reference

The indusium whitish and sometimes herbaceous, formed of the reflexed margin of the lobes or of the whole pinnule.    From Wordnik.com.  [The Fern Lover's Companion A Guide for the Northeastern States and Canada] Reference

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.    From Wordnik.com.  [The Fern Lover's Companion A Guide for the Northeastern States and Canada] Reference

pinnules on the inferior side of the pinnæ often elongated, especially the lower pair, the pinnule nearest the rachis being usually the longest, at least in the lowest pinnæ.    From Wordnik.com.  [The Fern Lover's Companion A Guide for the Northeastern States and Canada] Reference

The indusium is formed of the altered margin of the pinnule, at first reflexed to the midrib, giving it a pod-like appearance, but at length opening out flat and exposing the sporangia.    From Wordnik.com.  [The Fern Lover's Companion A Guide for the Northeastern States and Canada] Reference

Each leaflet is called a pinnule.    From Wordnik.com.  [Wikibooks - Recent changes [en]] Reference