The part of a flower that was once a bud is the calyx.

Here's why:

* Bud: A bud is the undeveloped flower. It's protected by specialized leaves called sepals.

* Calyx: The calyx is the collective term for all the sepals. They are often green and enclose the bud.

* Flower Development: As the bud matures, the sepals typically spread out, sometimes falling off entirely, revealing the flower's petals and reproductive parts.

So, while the bud itself transforms into the whole flower, the calyx is the part that directly originated from the bud's protective covering.