The extremely impoverished nature of the vascular flora in Antarctic regions has long been recognized and contrasted with the richness of comparable latitudes in the northern hemisphere. Rudmose Brown (1906), for example, reported finding no vascular plants on the South Orkney Islands (lat. 61° S), whereas over 100 species were then known from Spitzbergen (lat. 79° N). Even today only two native species of flowering plant are known from these islands and the neighbouring area of the Antarctic Peninsula, while no island even in the Sub-Antarctic zone, in latitudes as low as 50° S, rivals Spitzbergen in the richness of its flora (Greene & Greene 1963). Further south still the ice capped interior of the Antarctic is barren of flowering plants, its nunataks supporting only a meagre cryptogamic flora (Siple 1938; Greene 1964 a;Bowra, Holdgate & Tilbrook 1966). The vegetation now established in Antarctic regions may have resulted from the action of biogeographical as well as ecological factors. The oceanic barriers to the dispersal of land plants to the Antarctic are very great, and the floristic poverty of coastal areas may well result in part from their isolation combined with the relatively short time available for colonization since recession of the former more extensive ice sheets (Nicholls 1964) . This isolation is intensified by the additional barrier of the west wind belt, whose strong circumpolar airstreams and associated ocean currents must reduce the chance of airborne or waterborne propagules being carried south to the Antarctic. Even if the problems of dispersal are overcome, however, the environmental conditions within Antarctic regions are unfavourable for the establishment of many land plants, due to the low temperatures, the shortage of available water in many areas, and, locally, to intense competition from large and densely packed sea-bird colonies.
Strigolactone synthesis is ancestral in land plants, but canonical strigolactone signalling is a flowering plant innovation
