We have been growing these annual vinca plants for several years by bringing their pots inside at the onset of winter. But early yesterday morning, while the outside temperature was still below freezing, the pots were taken out to allow for a house renovation project to commence. When they were brought back in several hours later, it appeared that several of the plants in the largest pot had succumbed to the elements. Those are the ones with drooping leaves in the photo.
But there were survivors. So, now the question is why some plants died and some survived.
The distribution of the survivors suggests that location within the pot was irrelevant. So we can assume that air and soil temperatures were more or less uniform. Presumably, the plants were not genetically identical; some may have descended from one plant that was the original occupant of the pot and others from other original plants that occupied different, but nearby, pots (incidentally, they do seed well). It is, therefore, possible that we witnessed a selection event: the survivors had the right phenotypes coming from the right genotypes that enabled them to survive the freezing temperatures.
We should have flowers soon.