Achene Morphology of British and Irish Mayweeds and Chamomiles: implications for taxonomy and identification

Whilst determining the mayweed and chamomile specimens in the Natural History Museum’s British & Irish Herbarium we decided to produce a photographic guide and key to assist in the determination of these plants using achene morphology. Because of our observations, and in line with North American treatments, we propose recognising Matricaria discoidea subsp. discoidea and M. discoidea subsp. occidentalis as separate species. We also discuss possible introgression from Tripleurospermum maritimum subsp. phaeocephalum in northern parts of Birtain and Ireland, the implications for the taxonomy of Anthemis/Cota and Chamaemelum and the usefulness of achene glands in determination of specimens. Finally we consider the problems that may arise when using immature achenes and herbarium specimens for determination and we discuss the usefulness of being aware of changes in colour and form that occur during development, particularly as many collected specimens lacked ripe capitula.