The history of the genus Spartina in Britain is traced from herbarium records and publications. This historical information and a morphological study of living plants strengthen the view that sterile S. x townsendii arose in c. 1870 in Southampton Water as a hybrid between native S. maritimu and introduced American S. alterngora. The fertile Amphidiploid arose in 1890 and spread at the expense of the species populations. It is shown that species and hybrids of the S. x townsendii complex can be separated by a combination of only a few well-defined morphological characters, and by stomata and pollen grain details.

A polyhaploid seedling of low vigour among Amphidiploid progeny is described and its existence and resemblance to S. x townsendii F1 emphasizea the relationship between this PI hybrid and the Amphidiploid. Putative backcross hybrids of S. x townsendii Amphidi- ploid with X. alternijfora in Southampton Water provide conclusive evidence of S. x