The following points highlight the twenty salient features of agaricus.
1. The primary mycelium produced by the germination of basidiospores is of short duration. It consists of cells each with a haploid (n) nucleus (A).
2. Soon as a result of hyphal fusions of compatible strains, a binucleate condition established in one of the cells of the primary mycelium (B).
3. The binucleate (dikaryotised) cell by repeated divisions usually without and rarely with clamp connections forms a new mycelium with binucelate (n+n) cells. It is the secondary or dikaryotic mycelium.
4. The secondary mycelium is abundant, and long-lived (perennial). It produces mushrooms (basidiocarps) year after year (C).
5. As in the other Basidiomycetes, the mushrooms lack specialised sex organs.
6. Plasmogamy which brings the elements of a sexual union together is accomplished by somatogamous copulation (hyphal fusions) of compatible strains.
7. Karyogamy or the actual fusion of two nuclei of the dikaryon is delayed and does not occur till the formation of the basidium.
8. The plus and minus strain nuclei of dikaryons co-exist in all the cells of the secondary mycelial strands and the fruiting bodies (basidiocarps) which it bears.
9. The basidiocarps arise as tiny, white apical swellings on the branches of the underground mycelial strands called rhizomorphs (C).
10. Each enlarges to form a ‘button’ which comes above ground and rapidly expands into a mushroom.
11. The mature mushroom is a massive structure consisting of a stalk-like stipe supporting at its top a broad umbrella-shaped cap, the pileus. More than half way up, the stipe bears a membranous ring, the annulus.
12. From the undersurface of the pileus hang down plates of fungal tissue in a vertical position. These are the gills (D).
13. The surface of the gill, on both sides, is covered with a fertile layer, the hymenium (E).
14. The hymenium consists of a closely packed palisade-like layer of club-shaped cells, the basidia. Interspersed between the basidia are sterile hyphae, the paraphyses or cystidia.
15. The basidium mother cell or protobasidium (F) has two nuclei which function as gametes and fuse. The fusion of the two nuclei (karyogamy) in the protobasidium ends the dikaryophase in the life cycle.
16. The probasidium containing the diploid nucleus or synkaryon is called the young basidium. It represents the short-lived diplophase in the life cycle.
17. As the young basidium grows in size the synkaryon within undergoes meiosis during which segregation of strains takes place. Of the four haploid nuclei two are of plus strain and the other two of minus strain.
18. In the mature basidium the haploid nuclei migrate into the basidiospores through their respective sterigmata (G).
19. With the completion of spore production the basidiocarp collapses and dies.
20. The liberated basidiospores which are true meiospores (H), germinate to give rise to the haploid (primary) mycelia (A).