Haploid basidiospores germinate in a suitable environment and grow into short-lived haploid mycelia.
Undifferentiated hyphae from two haploid mycelia of opposite mating type undergo plasmogamy,
creating a dikaryotic mycelium that grows faster than, and ultimately crowds out, the parent haploid mycelium. The mycelium of the mushroom illustrated here (Cortinarius) forms mycorrhizae with trees. Environmental cues such as rain, temperature changes, and, for mycorrhizal species, seasonal changes in the plant host,
induce the dikaryotic mycelium to form compact masses that develop into mushrooms. Cytoplasm streaming in from the mycelium and from the attached mycorrhizae swells the hyphae of mushrooms, causing them to "pop up" overnight. The dikaryons of basidiomycetes are long-lived, generally producing a new crop of basiocarps (mushrooms, in this case) each year.
Karyogamy occurs in the terminal dikaryotic cells that line the surfaces of the gills (SEM at left).
Each cell swells to form a diploid basidium, which rapidly undergoes meiosis and yields four haploid nuclei.
The basidium then grows four appendages (sterigmata), and one haploid nucleus enters each appendage and develops into a basidiospore.
When mature, the basidiospores are ejected from the sterigmata (by releasing surface tension) in a process known as ballistospory. After the spores drop below the cap they are dispersed by the wind.