Why do some Eucalyptus flowers lose their calyx lobes as a cap?

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common myrtle — Myrtales Common myrtle (*Myrtus communis*).

In the order Myrtales, the calyx lobes (free sepals) are sometimes shed as a cap when the flower blooms. In the genus Eucalyptus, this cap—which is woody in nature—is shed to allow the flower to open.1