Although, however, the currents have played a part in stocking the Hawaiian beaches with their plants, their share in the work has been unimportant, and the number of plants concerned is limited. If we take away the seven or eight littoral plants introduced by the aborigines, as well as the three endemic species as indicated in the list in [Note 28], and then remove from the residue the plants with small fruits or seeds possessing little or no buoyancy, there remain only the following eight species, the presence of which in Hawaii might be attributed to the currents, namely, Cæsalpinia Bonducella, Cassytha filiformis, Colubrina asiatica, Ipomœa glaberrima, Ipomœa pes capræ, Scævola Kœnigii, Vigna lutea, and Vitex trifolia. Of these plants, three species, those of Cassytha, Scævola, and Vitex, possess fruits that would be likely to attract frugivorous birds, and are in some cases known to be dispersed by them (see [Chapter XIII.]); so that we are not in these instances restricted to the agency of the currents. With the other five the currents offer the readiest explanation, but, as is indicated in the cases of Cæsalpinia Bonducella and Ipomœa glaberrima ([Chapter XVII.]), it is quite possible that birds have occasionally intervened. Altogether we may infer that in stocking the Hawaiian beaches with their littoral plants the currents have taken a subordinate part.
Coming to the Hawaiian littoral plants having seeds or fruits that have no floating power, we find that they present a motley group. It has been already remarked that this is the group of shore plants that derives most recruits from the inland flora, and that it is in this group that the differences between the shore-floras of tropical regions find their expression. Yet a very odd collection of plants is here exhibited. Sometimes the beach-flora is composed in great part of these plants; and a sorry spectacle is presented by a beach possessing such plants as Gossypium tomentosum, Heliotropium anomalum and H. curassavicum, Lipochæta integrifolia, Tephrosia piscatoria, Tribulus cistoides, &c. Yet to the student of plant-distribution such a motley collection would be full of suggestiveness. From the circumstance that species of Cuscuta, Jacquemontia, and Lipochæta, that are peculiar to the Hawaiian Islands, have made their homes on the beach, he would infer that since Nature has been compelled to borrow from the endemic inland flora, there has been some difficulty in stocking the beaches with their plants. The occurrence of endemic species amongst the strand-plants would be viewed by him as especially indicating incapacity on the part of the ocean currents.
Yet in the quantities of drift timber, showing evidence of many months and probably even of years of ocean-transport, to be seen stranded on the weather coasts of these islands, the observer discerns undoubted evidence of the efficacy of the ocean currents. But what he finds are huge stranded pine logs of “red-cedar” and “white-cedar” from the north-west coasts of America. He may search the drift for days together, as I have done, and discover no tropical fruits or seeds except such as could be supplied by the present Hawaiian flora. The subject of this drift is especially discussed in [Note 30]; and it need only be mentioned here that it is not improbable that, as shown in the next chapter, some drift may reach Hawaii from tropical America under exceptional conditions, and that its presence is masked by the Oregon drift.
The agency of the drifting log in carrying small seeds in its crevices would be effectual in the instance of plants from the temperate coasts of North America. For example, the nutlets of Heliotropium curassavicum, which have no buoyancy, might easily be washed, together with sand, into the cracks of a pine log stranded temporarily on the Oregon coast where this plant occurs. The modus operandi was brought home to me when examining the drift brought down by the Chancay River on the coast of Peru. Here I found this species of Heliotropium growing on the margin of a swamp near some stranded logs, that would probably be carried out to sea when the river was next in flood.
It is probable, I may add, that the seeds or fruits of some of the plants of the non-buoyant group of the Hawaiian littoral flora may be dispersed in birds’ plumage. For instance, the spiny fruits of Tribulus cistoides sink in sea-water; but they are well suited for entangling themselves in birds’ feathers.
It is possible that the hairy seeds of Gossypium tomentosum may have been thus distributed; but there is much that is enigmatical about this plant (see [Chapter XXVI]).
The Inland Extension of the Beach Plants of Hawaii.—When we regard the inland extension of littoral plants in Hawaii, we get fresh indications of the meagreness of the strand-flora. Several of the species, as Cæsalpinia Bonducella, Cassytha filiformis, Tephrosia piscatoria, &c., show themselves only occasionally on the sandy beaches, though they are common enough on the old scantily vegetated lava-flows near the coast and are often found miles inland. Indeed, Dr. Hillebrand not infrequently in describing the station only gives prominence to the situation of the plants away from the beaches, and places most of them on the old lava plains that extend inland from the coast. It is only by a detailed examination of extensive coast lines in these islands that I have succeeded in preserving to a small degree their reputation as beach plants. A few of them behave somewhat strangely in their inland station. Thus, the seeds of Cæsalpinia Bonducella obtained from various localities showed no buoyancy in my experiments; and had I not found a solitary buoyant seed in the stranded drift I should have inferred that this was a rule without exception.
It is to be remarked that whilst some plants like Scævola Koenigii occasionally stray a few hundred yards inland on the surface of the old lava-flows, others like Ipomœa pes capræ and Vitex trifolia, that are spread far and wide over the inland plains of Fiji, are confined in Hawaii to the beaches and their immediate vicinity. Some of the plants like Hibiscus tiliaceus, Morinda citrifolia, and Pandanus odoratissimus, that are regarded as having been introduced by the aborigines, behave exactly like indigenous plants in the inland plains; but this is not necessarily an indication of an indigenous plant in this group, since the Cactus (Opuntia Tuna) and the Castor-Oil Plant (Ricinus communis) have spread all over the drier lower regions of the islands, whilst Aleurites moluccana, the Candle-Nut Tree, which has no means of reaching these islands without man’s agency, now forms entire woods on the mountain slopes, usurping the place often of the original forests.... Further details relating to this subject are given in [Note 31].
The principal points in the foregoing discussion of the strand-flora of Hawaii may be thus summed up:—
(1) The indigenous, that is, the pre-aboriginal, strand-flora of this group lacks not only the mangroves and their associated plants, but also most of the characteristic beach-trees of the South Pacific, which are known to owe their wide distribution in tropical regions to the currents.