Hypanthium

In angiosperms, a hypanthium or floral cup[1][2][3] is a structure where basal portions of the calyx, the corolla, and the stamens form a cup-shaped tube.

Some geometric forms are obconic shapes as in toyon, whereas some are saucer-shaped as in Mitella caulescens.

It provides weather protection and a medium to sustain the lost pollen, increasing the probability of fertility and cross-pollination.

[6] The retained pollen can then attach to pollinators such as birds, bees, moths, beetles, bats, butterflies and other animals.

It helps the stem fuse with the flower, in turn strengthening the bond and overall stability and integrity.

Ovary superior to hypanthium
In Spiraea , the hypanthium supports a nectar-producing "disk" which is ring-shaped and may have lobes as it does here. The stamens arise between the petals and the disk.
Hypanthium in Rosa
Narcissus pseudonarcissus , showing from the upper bend to the tip of the flower: spathe , ovary, hypanthium, tepals , corona