I find unconvincing the existing explanation of how several hydrophilic endemic plants became established at St Helena. This is one of the most isolated islands in the world, has an arid semi-desert coastline and moist and humid highlands where endemic hydrophilic plants grow. It has been suggested that plants originally arrived either as driftwood from the beaches or arrived inland because occasional birds accidentally blown to the island had seeds on their foliage or claws. However, not only would the brushwood of these hydrophilic plants be unlikely to propagate or survive the arid conditions of the coastal region, these plants normally propagate by root, not by seed.
Has the following third mechanism ever been suggested for the establishment of plants on an isolated island? I wonder whether brushwood fortuitously arrived at the point where streams were discharging into the sea. Despite the arid conditions at all other coastal regions, some plants could take root at these wet beach areas and then spread along the streams growing in the damp alluvial soil into the interior heights of the island. I know the migration of plants along the length of a stream or river (riparian zones) have been studied but I can find no reference to the idea that this can also explain the presence of hydrophilic plants in damp highland regions of an island with a surrounding arid coastal region.