From The Collaborative International Dictionary of English v.0.48 [gcide]:
Volva \Vol"va\, n. [L. volva, vulva, covering.] (Bot.)
A saclike envelope of certain fungi, which bursts open as the
plant develops.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
volva
n 1: cuplike structure around the base of the stalk of certain
fungi