Chapter 19: Homobasidiomycetes

Agaricales are unified by their basidiocarps, which typically feature a pileus, lamellate hymenium, and stipe, though considerable morphological variation exists across the order, from conventionally gilled agarics to puffballs and sequestrate fungi adapted to specific environments. The gilled structure fundamentally maximizes surface area for basidiospore production, with spore print coloration ranging from white and pink through brown and black, providing essential taxonomic characters for species identification. Key cytological features include clamp connections, dolipore septa, and dikaryotic mycelia that define these organisms at the cellular level. The chapter surveys major genera and their defining characteristics: Agaricus encompasses cultivated varieties like white button mushrooms and portobellos, characterized by saprotrophic nutrition and variable spore print colors; Lepiota and Macrolepiota produce parasol mushrooms with significant toxicity variation; Coprinus species demonstrate remarkable autodigestion of their gills; Cortinarius represents the largest agaric genus with ectomycorrhizal associations; Russula and Lactarius exhibit brittle-fleshed bodies with distinctive chemical properties; and Cantharellus and Clitocybe illustrate further ecological and morphological diversity. Agaricales occupy multiple ecological roles as saprotrophs recycling dead organic matter, as ectomycorrhizal partners with forest trees, and as plant pathogens exemplified by Armillaria root rot. The chapter addresses biochemical phenomena including spore dispersal mechanisms via Buller's drop, bioluminescence in certain taxa, and specialized fruit body forms adapted to arid conditions. Notable toxic species including Amanita phalloides and hallucinogenic varieties like Psilocybe receive attention alongside culinary and medicinal species. Evolutionary patterns revealed through fossil evidence and molecular analysis demonstrate repeated convergence of similar fruit body forms across distantly related lineages, underscoring the extraordinary adaptive radiation and human cultural significance of Agaricales.