sorus 
Cluster of small spore-producing structures covering the underside of a pinna.
adventitious roots 
Roots that grow out of the rhizome, enabling the fern to anchor itself to the soil and absorb water and mineral salts from it.
rhizome 
Stem usually found underground that grows horizontally, occasionally vertically, out of which adventitious fronds and roots grow.
fiddlehead 
Immature fern frond; its coiled tip is shaped like the head of a fiddle.
frond 
Fern leaf, originating at the rhizome, that bears sori and is especially adapted to capture light and perform photosynthesis.