What is "petrobium"

Petrobium is a genus in the sunflower tribe within the daisy family.

The only known species is Petrobium arboreum, called Saint Helena Whitewood. It is found in the tree-fern thicket at the top of the central ridge of island of Saint Helena in the South Atlantic Ocean. The plants are either female or hermaphrodite, i.e. the species is gynodioecious.