What causes the the volva form?

The volva is a cup-like structure at the base of certain mushrooms, particularly in the family Amanitaceae. It forms as a result of the development of the mushroom's fruiting body from a spherical underground structure called a mycelial bulb or universal veil. As the mushroom matures, the universal veil ruptures, leaving the volva behind at the base of the stem. Environmental factors, genetics, and the specific life cycle of the fungus also play a role in its formation.