Botany picture #32: Prunella 'Summer Daze'

The interesting thing about the small genus Prunella (Lamiaceae) is that all the species can be crossed, and their crosses are even fairly fertile. This cultivar, here seen on our balcony when we were in Switzerland in 2009 and before I had to give all my plants away (sigh), is the result of a plant breeder's work on a hybrid swarm. As can be seem from the dissected leaves, genes from P. laciniata feature strongly in the mixture. The cultivar is very popular because of its robustness and the profusion of pink flowers it produces.

An obvious question arising from the interfertility of the various species is how they are able to maintain their identity and distinctness. A possible explanation can be found in this interesting study published in 2000. The authors examined neighboring populations of P. vulgaris and P. grandifolia and found that hybrids are maladapted to the two types of habitats inhabited by the two parental species; they can't grab either of the two available chairs and are left standing, so to say. That means that while they don't show any prezygotic isolation, the two species may be ecologically isolated through selection against hybrid plants.