This genus presents us with the same puzzling question put to us by several Fijian genera, such as Myristica and Podocarpus, that occur in both Asia and America; and until we answer that query it seems almost futile to study modes of dispersal.
Elatostema (Urticaceæ)
This genus of annual and perennial herbs belongs to the tropical regions of the Old World. It is represented in Samoa by fifteen known species and by at least four or five in Fiji, whilst with the exception of a solitary Tahitian species it is not recorded from East Polynesia. Reference is here made to it particularly on account of its great development in Samoa. We have here a genus that, like Psychotria in Fiji and Cyrtandra in Fiji, Samoa, and Hawaii, runs riot in respect to the production of species (see p. [317]). Dr. Reinecke describes fifteen Samoan species, of which, with the exception of two found in Malaya, all seem to be described for the first time. So sensitive, he remarks, is the genus to external conditions that station-forms abound; and he points out that if we were to follow the dividing lines usually recognised between species, we should account every station-form a new species. It is, of course, obvious that the polymorphism of the Samoan Elatostemas depends primarily not on the varying influence of station but on their sensitiveness to external conditions. One might put the question to the Samoan Elatostemas that Hillebrand put to the Hawaiian Cyrtandras, and ask why nature in this particular genus in this particular locality thus luxuriates in formative energy. Almost every Pacific group in respect of some of its plants presents the problem so well stated by Dr. Reinecke for this genus in Samoa. It is noteworthy that Schimper, in his work on Plant-Geography (English edition, pp. 291, 297, 299), especially singles out Elatostema and Cyrtandra as growing socially in the tropical rain-forests of Java and of the Asiatic mainland.
Scirpodendron costatum (Cyperaceæ)
As far as I can gather, this giant-sedge has not been previously recorded from Fiji; but it is included in the Samoan flora, and has also been found at Penang and Singapore, as well as in Borneo, Java, and Queensland. In Samoa, as we learn from Reinecke, it grows both in the coast swamps and on dry ground. In Fiji it is very common in the mangrove-swamps at the mouths of rivers, especially in the Lower Rewa; but in Vanua Levu it is also frequent in the marshy localities of inland plateaux, 700 to 800 feet above the sea, as well as by the side of streams in swampy districts on the lower hill slopes. This double station in the salt-water swamp of the coast and in the fresh-water marsh of the interior seems to be repeated in Java, where the plant was first discovered by Zippelius on the banks of torrents in mountainous regions and in swampy places.
The genus comprises, according to the Index Kewensis, only this species, though variations are to be observed in plants from different localities. The species was described by Kurz in the Journal of the Asiatic Society of Bengal (vol. 38, 1869) and by Bentham in his Flora Australiensis; and an illustration is given by Miquel in his Illustrations de la Flore de l’Archipel Indien (1871). The plant is so common in Fiji that one can only suppose that its resemblance to a stemless Pandanus, from which, as Kurz observes, it is with difficulty distinguished except when in flower or fruit, led to its being overlooked by both Seemann and Horne. Its leaves, from 9 to 12 feet in length, are commonly used for making mats and for thatching, both in Fiji and Samoa. The plant usually attains a height of 3 to 5 feet.
The fruits occur abundantly in the floating and stranded river and sea drift in Fiji, a circumstance that led to my discovery of the parent plant in the swamps. The fruit, which is about half an inch (12 mm.) long, consists of a hard, stony nut invested by a thick ribbed cork-like covering, to which it owes its buoyancy, since the nut sinks. The detached fruit is perforated at the base through both coverings, and only a little soft tissue closes the aperture in the inner shell, the protection against the entry of sea-water in the case of floating fruits being quite inadequate. This explains also why the stranded fruits were so frequently found by me germinating on the beach, where, as my observations showed, they never established the plant. This early germination would prove to be an advantage in the case of fruits stranded in a suitable locality.
But though the perforation in the fruit favours its early germination, it lessens its ability to withstand a long sea-passage without injury to the embryo. I found in different experiments on fruits of plants growing in the mangrove swamps, that when placed in sea-water 40 per cent. sank during the first fortnight, whilst 15 per cent. floated after five or six weeks, but all were at the bottom in two months. On the other hand, fruits from plants of the swamps of the inland plateaux displayed much feebler floating power, in some cases sinking at once, in others floating for a few days, and in others again floating for a week or two. In this case the outer cork-like covering proved to have lost most of its floating power.
From the number of empty seed-vessels found, both in the floating and stranded drift, it appeared evident that the seed had often rotted away during the flotation. It is apparent from these observations and experiments that Scirpodendron costatum is not suited for dispersal by currents over wide tracts of ocean. The fruits might be able to float unharmed for a few weeks, but they would be unable to accomplish much more than the 500 or 600 miles intervening between Fiji and the nearest groups to the west.