Cytology of the fern flora of Tristan da Cunha

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.