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.