Abstract
Peyssonnelia immersa sp. nov. is described from the British Isles and France. The species has been found intertidally on bedrock and stones and on mobile substrata in the sublittoral. It differs from all previously described species by possessing a unique combination of vegetative and reproductive characters; of particular note is the development of reproductive organs in extensive immersed sori. Terminal cruciate tetrasporangia, occasionally borne on thickened filaments, are surrounded by a complex stratified wall. Carpogonial branches, auxiliary cell branches and spermatangia are accessory in origin and develop among pigmented sterile filaments in the same sori. One or more gonimoblasts arise from the connecting filament after fusion with a series of auxiliary cells and the mature carposporophyte consists of simple or branched chains of 2–5 large carposporangia. In laboratory culture, tetraspores obtained from field material of P. immersa gave rise to crusts similar to field plants. Gametangia and immat...
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