The body of Agaricus consists of two parts. Mycelium and fruiting body or sporophore.
1. Mycelium : The body of Agaricus is composed of slender, cylindrical, branched and multicellular colorless hyphae. Hyphae join together to form mycelium. It is a perennial fungus. The mycelium of Agaricus is acinocytic. That is, it is divided into many cells by a transverse wall. Each cell contains granular cytoplasm, multiple nuclei, lysosomes, ribosomes, endoplasmic reticulum, small cell cavities and oil droplets as stored food. Cell wall is made of chitin. It does not contain photosynthetic pigments. The mycelium intertwines to form a rope-like structure. It is called rhizomorph. A mushroom can produce a kilometer long hyphae in a day. Basidiocarps produced from the mycelium are located in rings above the soil. This type of ring is called Fairy ring.
2. Footbody: The aerial hyphae like an umbrella formed from the mycelium is called footbody. The process of fruit body formation is called fructification. The fruit body of Agaricus is also called basidiocarp. It consists of two parts.
(a) Stipe : The unbranched stem-like part arising from the mycelium is called stipe. It is 5-8 cm long. It bears pileus. The circular portion at the head of the stipe is called the annulus. It forms the connection between the stipe and the pileus.
(b) Pileus : The umbrella-like swelling at the tip of the stipe is called the pileus. When young the pileus is covered with a membranous covering called velum. Its diameter is 5.0-12.5 cm. Its surface is smooth but its digits are uneven. There are several leaf-like parts at the base of the digit. It is called Gill. Each pileus contains 300-500 gills. Immature gills are pink in color and mature gills are red-yellow or red-pink in color. Each swallow has three parts. These are-
(i) Trama : The sterile region in the middle of the gill is called trama. Cells in this region are long, dense and multinucleate (dikaryotic).
(ii) Sub-hymenium : The region on either side of the trama is called sub-hymenium. The cells in this region are small, round and have 2-3 nuclei. Such arrangement of cells is called progenchyma. Mace-shaped basidia are formed from this region.
(iii) Hymenium : The outermost region of gill is called hymenium. This region contains paraphyses and basidia cells. Paraphyses are slender and sterile cells or filaments. Basidia are thick and fertile cells. Each basidium has 4 narrow sterigmata. Each sterigma contains 1 round or oval basidiospore. Basidiospores germinate to form new mycelium.