The Collaborative International Dictionary of English v.0.48:

manna lichen \manna lichen\ n.
   Any of several Old World semicrustaceous or shrubby lecanoras
   that roll up and are blown about over African and Arabian
   deserts and used as food by people and animals; same as
   manna[2].
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Manna \Man"na\ (m[a^]n"n[.a]), n. [L., fr. Gr. ma`nna, Heb.
   m[=a]n; cf. Ar. mann, properly, gift (of heaven).]
   1. (Script.) The food supplied to the Israelites in their
      journey through the wilderness of Arabia; hence, divinely
      supplied food. --Ex. xvi. 15.
      [1913 Webster]

   2. (Bot.) A name given to lichens of the genus Lecanora,
      sometimes blown into heaps in the deserts of Arabia and
      Africa, and gathered and used as food; called also manna
      lichen.
      [1913 Webster]

   3. (Bot. & Med.) A sweetish exudation in the form of pale
      yellow friable flakes, coming from several trees and
      shrubs and used in medicine as a gentle laxative, as the
      secretion of Fraxinus Ornus, and Fraxinus
      rotundifolia, the manna ashes of Southern Europe.
      [1913 Webster]

   Note: Persian manna is the secretion of the camel's thorn
         (see Camel's thorn, under Camel); Tamarisk manna,
         that of the Tamarisk mannifera, a shrub of Western
         Asia; Australian, manna, that of certain species of
         eucalyptus; Brian[,c]on manna, that of the European
         larch.
         [1913 Webster]

   Manna insect (Zool), a scale insect (Gossyparia
      mannipara), which causes the exudation of manna from the
      Tamarix tree in Arabia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

manna lichen
    n 1: any of several Old World partially crustaceous or shrubby
         lecanoras that roll up and are blown about over African and
         Arabian deserts and used as food by people and animals