Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

macrosporangium
     n : a plant structure that produces megaspores [syn: {megasporangium}]
     [also: {macrosporangia} (pl)]