The Collaborative International Dictionary of English v.0.48:

Calyptra \Ca*lyp"tra\ (k[.a]*l[i^]p"tr[.a]), n. [NL., fr. Gr.
   kaly`ptra a covering for the head, fr. kaly`ptein to cover.]
   (Bot.)
   A little hood or veil, resembling an extinguisher in form and
   position, covering each of the small flasklike capsules which
   contain the spores of mosses; also, any similar covering
   body.
   [1913 Webster]

WordNet (r) 3.0 (2006):

calyptra
    n 1: the hood or cap covering the calyx of certain plants: e.g.,
         the California poppy