Abstract
Asplenium chathamense sp. nov. is a tetraploid species, closely related to both A. flaccidum and A. terrestre, growing in scrub or on rock in coastal situations on the Chatham Islands of New Zealand. Its similarity to A. flaccidum subsp. haurakiense suggests that these two taxa may have evolved from coastal populations of A. flaccidum sens. lat. which were formerly more widespread in New Zealand and perhaps like the morphologically variable plants seen today on the Kermadec Islands. It is postulated that octoploid plants of A. terrestre arose by autopolyploidy from these tetraploid coastal populations and replaced them in areas other than those where A. flaccidum subsp. haurakiense and A. chathamense still persist. The latter taxon is shown to have hybridised occasionally with other coastal species on the Chatham Islands.