Source : Webster's Revised Unabridged Dictionary (1913)

Involucrum \In`vo*lu"crum\, n.; pl. L. {Involucra}, E.
   {Involucrums}. [L. See {Involucre}.]
   1. (Bot.) See {Involucre}.

   2. (Zo["o]l.) A sheath which surrounds the base of the lasso
      cells in the Siphonophora.

Source : WordNet®

involucra
     See {involucre}

involucra
     See {involucrum}

involucre
     n : a highly conspicuous bract or bract pair or ring of bracts
         at the base of an inflorescence
     [also: {involucra} (pl)]