The Southern Marsh Orchid, D. praetermissa, is the widespread marsh orchid across England and Wales but more likely in southern areas, and also in northern continental Europe - in the Low Countries. Its British distribution in fact is the counterpart to the Northern Marsh Orchid though there is a fair degree of this species’ northern range and across Wales that overlaps with D. purpurella. This may be due to recent expansion by either species. It is surmised that the polyploid event that lead to D. praetermissa occurred in southern Europe while the north was still covered by the ice sheets. 18,000 years ago would not be an bad estimate. It is likely that the continental Broad-leaved Marsh Orchid, D. majalis, also evolved in the same broad area and time period. Could the two orchids even be two sub-specie of the same species? I have been looking for papers that could shed some light at a molecular level on this question, but they seem to have used too small sample sizes for any real conclusions, at least those I have seen.

It is rather surprising, to me at least, that this species wasn’t actually recognised until 1914. In fact its Latin name means ‘overlooked’. But even in 1937 a comprehensive book, The Handbook of British Flora, put all marsh orchids as one species, Orchis latifolia, and made no distinctions as to sub-species or below. It merely said that the species appearance was highly variable. We now know better! The flowers are rather paler than D. purpurella with lip markings less noticeable and often a fine spotting. Whilst pure bred populations are unlikely due to past hybridisations and introgression, identification should be straightforward unless one is looking at a hybrid swarm.

More recently it has been discovered that the colonies labeled D. traunsteineroides from Norfolk and southern England (based on both appearance and habitat) are virtually indistinct genetically from D. praetermissa. Could it be that the genes that both allow a marsh orchid to colonise fens and give them a rather ‘skinny’ appearance have come from the same ancestral population which contributed to both species’ polyploid event (as well as the other similar continental species such as D. traunsteineri on the continent)? Could this be more of a genomics than genetics issue? Methylation of certain DNA bases could be the basis of this phenomenon, explaining how the skinny plants are able to pop-up and prosper where others can not.