Abstract
This article describes a study of Tripleurospermum maritimum (L.) W.D.J. Koch and T. inodorum (L.) Sch. Bip. (Asteraceae) in the Orkney Islands (v.c.111), the results of which suggest that intermediates between these taxa may be rather common, and that T. maritimum subsp. nigriceps and subsp. maritimum are both involved, the former more frequently. Obviously this results in a complex taxonomic situation, evidently not confined to Orkney in the far north. Key identifying characters of the taxa are systematically examined and guidance offered on determining hybrids using a population level approach. The taxonomic complexities of these northern populations are discussed, with regard in particular to the identity of T. inodorum occurring there and the place of T. maritimum subsp. nigrescens in the forms and subspecies of T. maritimum found in the north Atlantic region.
Highlights
The samples combined the characters of T. maritimum subsp. maritimum, T. maritimum subsp. nigriceps P.D
With regard to Tripleurospermum, the authors concluded on the basis of the herbarium material available to them that the vast majority of specimens could be determined clearly as either T. maritimum or T. inodorum but a small minority of specimens in which the distinctions were ‘blurred’ were putative hybrids
This minority had mixed and conflicting achene characters. Some of these putative hybrids were from Orkney and Shetland (v.c.112). Several specimens from these islands were unusual in other ways and the authors discussed probable reasons for this, including introgression between the two species, the presence of the northern T. maritimum subsp. nigriceps in the area and the possibility of introgression from the Arctic subspecies T. maritimum subsp. phaeocephalum (Ruprecht) Hämet-Ahti
Summary
Kay (1972) considered achenes from plants with fleshy leaves and other T. maritimum characters such as short little-branched stems, with more weakly developed ribs and less strongly elongated oil-glands as populations showing probable introgression from T. inodorum. He conducted breeding experiments, generating hybrids, but we have been unable to trace any of his original material, and detailed descriptions of hybrid characteristics and how these characters segregate were not published by Kay. In addition to achene morphology, vegetative characters distinguishing the species comprise fleshiness of the leaves, shape of the phyllaries and width of phyllary margins. They comprised: shape of oil sacs on achenes; separation of achene ribs; achene size; leaf, whether fleshy or not; phyllary shape; and width and colour of phyllary margin (Table 1)
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