The Collaborative International Dictionary of English v.0.48:

Antheridium \An`ther*id"i*um\, n.; pl. Antheridia. [Anther + ?
   (a Gr. diminutive ending).] (Bot.)
   The male reproductive apparatus in the lower plants,
   consisting of a cell or other cavity in which spermatozoids
   are produced; -- called also spermary. --
   An`ther*id"i*al, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antheridium
    n 1: the male sex organ of spore-producing plants; produces
         antherozoids; equivalent to the anther in flowers