Mushrooms include edible species of the genera Agaricus (button, portobello, and cremini mushrooms), Pleurotus (oyster mushrooms), and Volvariella, (straw mushrooms). Mushrooms are highly variable in appearance, depending on their stage of development and variety. Agaricus have cap and stem, the cap is a pale grey or brown in color and is round immature, but flattens when mature. Pleurotus mushrooms may not have a stem and may be attached laterally to a growing substrate, such as the bark of a tree. Volvariella mushrooms are small, with pink gills and a characteristic covering (volva) at the base of the stem.