The interspecific relationships of Juncus effusus and J. conglomeratus in Britain
Juncus effusus and J. conglomeratus appear to be specifically distinct in the British Isles; J. conglomeratus is the rarer species. Their principal diagnostic features are given. Evidence is presented to show that the two species hybridize at higher altitudes in Britain, probably because the overlap of their flowering periods is longer in these regions than at low altitudes. Evidence is also found for gene flow within J. effusus populations from J. conglomeratus and the reasons for the continuing specific distinctness of these two species is discussed.