The Collaborative International Dictionary of English v.0.48:

Litmus \Lit"mus\, n. [D. lakmoes; lak lacker + moes a thick
   preparation of fruit, pap, prob. akin to E. meat: cf. G.
   lackmus. See Lac a resinous substance.] (Chem.)
   A dyestuff extracted from certain lichens (Roccella
   tinctoria, Lecanora tartarea, etc.), as a blue amorphous
   mass which consists of a compound of the alkaline carbonates
   with certain coloring matters related to orcin and orcein.
   [1913 Webster]

   Note: Litmus is used as a dye, and being turned red by acids
         and restored to its blue color by alkalies, is a common
         indicator or test for acidity and alkalinity.
         [1913 Webster]

   Litmus paper (Chem.), unsized paper saturated with blue or
      red litmus, -- used in testing for acids or alkalies.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Roccellic \Roc*cel"lic\, a. [F. roccellique, fr. roccelle
   archil, It. & NL. roccella, fr. It. rocca a rock, because
   archil grows on rock.] (Chem.)
   Pertaining to, or designating, a dibasic acid of the oxalic
   series found in archil (Roccella tinctoria, etc.), and
   other lichens, and extracted as a white crystalline substance
   C17H32O4.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Archil \Ar"chil\ (?; 277), n. [OF. orchel, orcheil, It. orcella,
   oricello, or OSp. orchillo. Cf. Orchil.]
   1. A violet dye obtained from several species of lichen
      (Roccella tinctoria, etc.), which grow on maritime rocks
      in the Canary and Cape Verd Islands, etc. --Tomlinson.
      [1913 Webster]

   2. The plant from which the dye is obtained. [Written also
      orchal and orchil.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

Roccella tinctoria
    n 1: a source of the dye archil and of litmus [syn: roccella,
         Roccella tinctoria]