Annulus (mycology)

In mycology, the annulus, also known as the ring, is a skirt-like or collar-like structure found on the stipe (stem) of some mushrooms. It is a remnant of the partial veil, a protective membrane that covers and protects the developing gills or spore-bearing surface of the immature mushroom.

As the mushroom matures and the cap expands, the partial veil tears away from the edge of the cap, leaving the annulus attached to the stipe. The characteristics of the annulus, such as its shape, texture, and whether it is fixed, movable, single, or double, are important features used in mushroom identification.

Annuli can vary greatly in appearance. Some may be membranous and delicate, easily damaged, while others are thick and substantial. Some annuli are attached firmly to the stipe and immovable, while others are loosely attached and can be slid up and down the stipe. The presence or absence of an annulus is also a key diagnostic feature for differentiating between various mushroom species, and its appearance should be noted carefully when attempting to identify a mushroom. It should be noted that some mushrooms may have an absent or ephemeral annulus, due to weathering or simply its fragile nature. The term "annulus" refers specifically to the remnant of the partial veil; the remnants of the universal veil, called a volva, are found at the base of the stipe instead.