macrosporangium(n = noun.plant) megasporangium - a plant structure that produces megaspores;
is a kind of sporangium, spore case, spore sac

   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]