Cytology of the fern flora of Tristan da Cunha

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Abstract
Twenty species out of a total of twenty-six leptosporangiate ferns known to occur on Tristan da Cunha have been examined cytologically. Four species have been independently examined from Gough with concordant results. Hymenophyllum tunbrigense, included among the Tristan species, is a new record for the island. Other points of special interest about individual taxa include demonstration of an extremely high chromosome number (n = 174) in the supposedly endemic species Eriosorus cheilanthoides, demonstration of the existence of two different cytotypes (one only present on Tristan) within Blechnum penna-marina in the southern hemisphere and of complex polyploid relationships affecting several other taxa which are discussed individually. In addition the cytological composition of the fern flora as a whole is discussed in relation to other temperate and tropical floras in or near the Atlantic basin.