The Collaborative International Dictionary of English v.0.48:

Macrosporangium \Mac`ro*spo*ran"gi*um\, n. [NL. See Macro-,
   and Sporangium.] (Bot.)
   A sporangium or conceptacle containing only large spores; --
   opposed to microsporangium. Both are found in the genera
   Selaginella, Isoctes, and Marsilia, plants remotely
   allied to ferns.
   [1913 Webster]

WordNet (r) 3.0 (2006):

macrosporangium
    n 1: a plant structure that produces megaspores [syn:
         megasporangium, macrosporangium]