[adj] covered with fibrils more or less evenly disposed
FIBRILLOSE is a 10 letter word that starts with F.
Webster's 1913 Dictionary
Definition:
\Fi*bril"lose\ (? or ?), a.
Covered with hairlike appendages, as the under surface of
some lichens; also, composed of little strings or fibers; as,
fibrillose appendages.
WordNet Dictionary