It refers to the bowl shaped calyx tubes.

It consists of 4 small nuts which often remain locked inside the calyx tube.

Pistil: Two to three carpels inserted in the bottom of the calyx tube and united into an inferior ovary.

Its petals are obvate, with white or red stamens found in pairs on the rim of the calyx tube.

The base of which is sunk into a six sided calyx tube.

The actual flowers have five lobes and form a calyx tube.

The sepals form a calyx tube around the greenish yellow petals, which turn maroon as they age.

Tapiscia and Huertea have a calyx tube and compound, rather than simple leaves.

Botanically, the leaves are usually in pairs (opposite), and the flower petals emerge from the rim of the calyx tube.

The calyx tube is five-toothed with ten stamens.