On this basis, with other interesting morphological comparisons, Brefeld erected his hypothesis, now untenable, that the Ascomycetes and Basidiomycetes diverge from the Zygomycetes, the former having particularly specialized the ascus (sporangial) mode of reproduction, the latter having specialized the conidial (indehiscent one-spored sporangiole) mode.
At the time of sporangial formation the protoplasm with numerous nuclei streams into the swollen end of the sporangiophore and there becomes cut off by a cell-wall to form the sporangium.
The escape of zoospores is effected by the degeneration of the sporangial wall (Chaetophora), or by a pore (Cladophora), a slit (Pediastrum), or a circular fracture (Oedogonium).
The sporangial wall, consisting of several layers of cells, encloses a cavity containing numerous oval spores (pollen-grains).
The tapetal layer is partly formed from the sporangial wall and partly as a layer covering the trabeculae.