Today’s entry was composed by Bryant. He writes:
I would like to thank Wouter Bleeker for the photographs of Ruta graveolens (common rue) and for spurring some interesting discussion regarding floral morphology. The recent post on Mitella diphylla and its variable inflorescence intrigued Wouter. That same day he came across this Ruta graveolens in Ottawa, which displayed both pentamerous (5-parted) and tetramerous (4-parted) flowers on the same plant, with the pentamerous flower usually located in the centre of the cyme. Wouter then queried if this Ruta was in the process of reducing its merosity over time, and whether or not there was a general trend of reduction among angiosperms.
These questions were re-directed to Dr. Quentin Cronk, an expert on floral morphology at the University of British Columbia (and former UBC BG director). His response: “This is an interesting example of the ‘terminal flower effect’ in which the terminal flower of a cyme has a different form or symmetry to the rest. Briefly, this is because symmetry is oriented by relation to an axis. The central flower cannot develop a symmetry relation to the axis because it is the central axis–therefore it develops as a radial flower. This phenomenon can be seen in mutants of foxglove and Campanula in which there is a terminal flower (terminal flowers do not normally form in these). Similar effects also occur in species that naturally have inflorescences with a terminal flower (look at the distinctive central flower of a carrot inflorescence!)”.
Ruta graveolens is a member of the Sapindales, which happens to include several other members that demonstrate this phenomenon. Kirkia wilmsii was observed displaying “the co-occurrence of tetramerous and pentamerous flowers on the same individual” in Bachelier JB and PK Endress. 2008.Floral structure of Kirkia (Kirkiaceae) and its position in Sapindales. Annals of Botany 102: 539-550 doi:10.1093/aob/mcn139. For additional information and photographs on this subject, see: Rudall, PJ and RM Bateman. 2003. Evolutionary change in flowers and inflorescences: evidence from naturally occurring terata. Trends in Plant Science. 8(2):76-82. doi:10.1016/S1360-1385(02)00026-2.
Ruta graveolens is native to parts of eastern and southeastern Europe, but has been widely distributed throughout the temperate and tropical regions of the world. It has a rich history of medicinal and culinary use in almost every area it has naturalized. It was used as an abortifacient in ancient Europe and Latin America, while in Italy it was/is sometimes used to flavour grappa. It is also known for causing phytophotodermatitis on those who come in contact with the plant in sunny weather.
Thanks again to Wouter Bleeker for making such a keen observation and sharing your curiosity with us!