Abstract
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.
Highlights
IntroductionAchene morphology within the Anthemidae Plants of the Asteraceae are characterised by possessing one or more compound inflorescences, or capitula, in which each capitulum consists of one or more whorls of bracts (phyllaries) and a cluster of true flowers, or florets, grouped together on a receptacle
Achene morphology within the Anthemidae Plants of the Asteraceae are characterised by possessing one or more compound inflorescences, or capitula, in which each capitulum consists of one or more whorls of bracts and a cluster of true flowers, or florets, grouped together on a receptacle
Chamaemelum nobile (Chamomile) and Chamaemelum mixtum (Mediterranean Chamomile) The ray florets of Chamaemelum are female and sometimes fertile, those of C. nobile develop fully and are probably fertile, whereas those of C. mixtum developed to varying degrees suggesting variable fertility of the ray florets
Summary
Achene morphology within the Anthemidae Plants of the Asteraceae are characterised by possessing one or more compound inflorescences, or capitula, in which each capitulum consists of one or more whorls of bracts (phyllaries) and a cluster of true flowers, or florets, grouped together on a receptacle. The distal end (achene apex) of the mature achene bears a tissue outgrowth called the stylopodium (‘foot of the style’, Fig. 1) at the position the style was attached when in flower This term is variously defined as a platform of tissue (generally parenchyma derived from a meristem) supporting the style, or the style base plus a surrounding ring of nectiferous tissue. The distal end frequently bears a full or partial ring or pappus of hairs or scales, which in Mayweeds and Chamomiles is either absent or reduced to a paper corona or corolla or an apical rim (Fig. 1) This pappus or rim is noticeably asymmetric in Anthemis, being reduced or absent on the abaxial side and longer adaxially. The ribs may bear wart-like protuberances (tuberculae) as in Anthemis cotula
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