SUMMARY: Young non-germinating sporangia of contain nuclei and cytoplasmic organelles, similar in appearance to those in other fungi, and a conspicuous central and other vacuoles. The sporangial wall consists of an outer homogeneous layer and an inner vesicular layer, prominent in the apical region. Peripheral cytoplasmic vacuoles are frequent, particularly where the vesicular layer is well developed. In old sporangia, which lose the ability to germinate indirectly (i.e. to produce zoospores), there is no vesicular layer and there is a change in the nature of the storage material of the vacuoles. When young sporangia are stimulated to indirect germination by low-temperature treatment the central storage vacuole disappears and there is a marked elongation of mitochondrial cristae, consistent with an increased rate of respiration. The sporangial wall becomes three-layered with the development of an inner homogeneous layer which separates the plasma membrane from an increasingly prominent vesicular layer. In the region of the papilla the vesicular layer shows a striking increase in thickness; it is suggested that this layer and the inner homogeneous layer play an important part in zoospore discharge.


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