Pandanus pistillaris in the Caroline Islands: An Example of Long-Range Oceanic Dispersal

IN CONJUNCTION with the current revision of the genus Pandanus (St. John, 1960; St. John and Stone, in sched.) it seems appropriate to mention the factors of dispersal operative in the genus. Since a number of species of Pandanus are littoral in habitat, and because at least one (if not several) species is found on nearly every tropical atoll in the Pacific, it has rightly been assumed that oceanic dispersal of the buoyant fruits is responsible for the distribution. There is no question that throughout vast island groups in Micronesia, Melanesia, and Polynesia Pandamts has long been part of the indigenous vegetation. There are, in fact, some indications that oceanic dispersal accounts for the farthest limits of the genus, in West Africa at one extreme, Polynesia at the other, the Bonin Islands in the north, and Australia in the south. The actual number of species involved is not yet known, but species of section Pandanus (section Ke.ura of earlier authors; see St. John, 1960) play a prominent role, especially in the Pacific region. Certain species of other sections of the genus also are primarily ocean-distributed (for example, Pandanus dubius Spreng., and Pandanus tetrodon (Gaud.) Balf.£.). In general, fruits of these species which are buoyant in seawater exhibit anatomical structures which are presumably adaptations to permit, or prolong, flotation. Brown (1931) has discussed the tissues present in fruits of certain Polynesian species, and uses the apt term aerenchyma to designate the light, pithy, upper mesocarp of such fruits. In addition, the seeds are encased in a highly