The Collaborative International Dictionary of English v.0.48:

Parella \Pa*rel"la\, Parelle \Pa`relle\, n. [Cf. F. parelle.]
   (Bot.)
   (a) A name for two kinds of dock (Rumex Patientia and
       Rumex Hydrolapathum).
   (b) A kind of lichen (Lecanora parella) once used in dyeing
       and in the preparation of litmus.
       [1913 Webster]