megasporangiate strobilus open stage megasporangiate strobilus post-fertilization immature stage A megasporangiate strobilus (seed cone) open stage in which a seed is developing after fertilization. This stage begins when a megasporangium has been fertilized and begins to swell and ends when the seed cone opens and the first seed abscises or drops. For seed cones that do not dehisce, this stage ends when the seed cone has reached its mature color.