The Collaborative International Dictionary of English v.0.48:

Involucrum \In`vo*lu"crum\, n.; pl. L. Involucra, E.
   Involucrums. [L. See Involucre.]
   [1913 Webster]
   1. (Bot.) See Involucre.
      [1913 Webster]

   2. (Zool.) A sheath which surrounds the base of the lasso
      cells in the Siphonophora.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Involucre \In"vo*lu`cre\ (?; 277), n. [L. involucrum a covering,
   wrapper, fr. involvere to wrap up, envelop: cf. F. involucre.
   See Involve.] (Bot.)
   (a) A whorl or set of bracts around a flower, umbel, or head.
   (b) A continuous marginal covering of sporangia, in certain
       ferns, as in the common brake, or the cup-shaped
       processes of the filmy ferns.
   (c) The peridium or volva of certain fungi. Called also
       involucrum.
       [1913 Webster]