A taxonomic study of Phascum cuspidatum Scheb. ex Hedw. (Pottiaceae) and its allies
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The moss genus Phascum is common in the Northern Hemisphere. Its species may be frequently encountered by botanists studying the moss flora of bare earth in fallow fields and gardens. Species within this genus have been recognised by taxonomists since at least 1741. The present form of the generic name dates from 1753. Yet Phascum still appears to be poorly understood. Even with available modern treatments, in floras such as Nyholm (1960) and Lawton (1971), bryologists find it difficult to recognise taxa and seem hesitant to name species. Why are these taxa difficult to identify? Is it because they are a complex of "good" but difficult to recognise species and varieties or a loose assemblage of intergrading forms and ecotypes? The present work attempts to answer the following questions: - 1. Is Phascum divisible into recognisable entities in the Northern Hemisphere? And, if it is divisible, what should the subunits be correctly called? 2. If the species Phascum cuspidatum Schreb. ex Hedw. is proved to be a valid taxon, is it possible to subdivide it into infraspecific units? And, if it is divisible, at what level should the subunits be recognised and what should they be correctly called? The problem was tackled in three phases. First, in order to cover the nomenclatural questions, using the Index Muscorum edited by Wijk (1967) as a starting point, all the relevant publications which contained citations of putative taxa or new combinations within the genus were assembled. From the information provided in these works a chronological survey of the taxonomy of the genus from 1770 to the present day was compiled (Chapter I). Hopefully this synopsis, of what turned out to be an extremely complex literature, will provide a reference point not only for this study but for any further researches in the genus in Europe and North America. Alterations and amendments to the information provided in the Index Yuscorun, together with other notes and comments, are incorporated in the survey where appropriate. Secondly, from these taxonomic publications, a list of the definitive characters used in the descriptions of the many putative taxa was compiled. This information was used to provide direction in a study of the taxonomic usefulness of these characters and thus the validity of the proposed taxa. This work, which included some growth experiments, was carried out within the eight fields of study suggested by Turrill (1942). Attention was paid to both traditional and more modern taxonomic methods, including computational techniques. An attempt was also made to fill the gaps in areas not covered by this work by reference to the specialised published literature within some of these eight fields (Chapter II). Thirdly, the genus Phascum is discussed in terms of both historical treatment and current practice concerning generic and specific concepts. And, finally, from the results of the studies carried out and with due reference to the literature and specimens seen the genus Phascum is redefined in terms of its component, taxa. These are thereafter typified and named, as far as possible at the present state of knowledge, in accordance with the International Code of Botanical nomenclature (I.C.B.N.) of 1972. An indication is provided of work which still remains to be carried out (Chapter III).
Thesis, PhD Doctor of Philosophy
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