Interestingly, the upper rim of the hypanthium appears to develop a black band after the flowers drop off the developing fruit capsules.

Earlier, I had speculated that the specific epithet marginata might refer to this black band. But it turns out the type-form for W. marginata had white-margined leaves...which presumably gave rise to the specific epithet. However, it appears that not all 'forms' of this (quite variable) species have white-margined leaves or black-rimmed fruit capsules...the current circumscription includes many different forms originally given different names but later synonymized under W. marginata. For what it's worth, this plant here seems to have closest affinities to the form referred to by the name W. agrestis in Lothian(1947).

For more details on this plant, see remarks under first photo in this series.