Tribulus cistoides.—Hillebrand observes that this plant is found along the sea-shore and on the lower plains. I found it most frequently on the beaches and on the old lava-flows near the sea.
Vitex trifolia, var. unifoliolata.—It is confined, as Hillebrand remarks, to the beaches. Neither in Oahu nor in Hawaii did I ever find it straying inland, which is the more remarkable since this variety, or one closely similar to it, is one of the most characteristic inland plants of the Fijian strand-flora.
Vigna lutea.—This plant was found by me growing on the beaches and in their vicinity. Hillebrand merely speaks of it as “growing at short distances from the shore.”
Some of the trees, usually littoral in their station in the tropical Pacific, which are regarded as having been introduced in early times into the Hawaiian group by the Aborigines (see [Chapter VII.]), behave, nevertheless, quite like indigenous plants in the inland regions and in the lower levels. This is true, for instance, of Hibiscus tiliaceus and Pandanus odoratissimus, the last-named forming forests at the sea-board extending in places far up the mountain slopes. The same, however, may be said of other plants known to have been introduced since the discovery of the islands, as in the cases of Cactus Tuna and of Ricinus communis; and it also applies to Aleurites moluccana, the Candle-nut Tree, which, although it could only have been introduced by the Aborigines, now forms forests on the lower slopes of the mountains.
NOTE 32 (pages [19], [112], [165])
The Fijian Species of Premna
I was much interested in the small trees and shrubs of this genus in Fiji, more especially on account of the relation between the shore and inland species. This is an Old World genus containing some eighty species mainly characteristic of tropical Asia and Malaya, and represented in the South Pacific archipelagoes by two species, one Premna taitensis or tahitensis, spread over the region and very near P. integrifolia, an Asiatic species; the other Premna serratifolia, an Asiatic plant found in Fiji, the Marquesas, and other groups. Without endeavouring to give a precise value to the Fijian plants, I will merely describe the prevailing forms, which are, however, connected by intermediate varieties. These trees, I may add, are known by the same name in the various Pacific groups, “Avaro” or “Avalo” in Tahiti, “Alo-alo” in Samoa, “Yaro” and “Yaro-yaro” in Fiji.
The Fijian plants may be thus described.... (a) Premna serratifolia, an inland tree, growing in open woods and on the outskirts of the forest, 25 to 30 feet high, more or less hairy, leaves coarsely serrated with long tapering points, putamen prominently tuberculated and thick-walled.
(b) Premna taitensis or P. integrifolia, a low straggling coast tree or shrub of the beaches, the coral islets, the swampy borders of the estuaries, and the inland talasinga plains, its usual height being eight to ten feet, except in the inland plains, where it is dwarfed, and three to five feet high. It is more or less glabrous, the leaves being typically entire with obtuse or retuse and mucronate apices. The putamen is thin-walled and relatively smooth. (c) Intermediate forms found generally in the inland plains or talasinga regions.
On the Modes of Dispersal.—Speaking generally, the small drupes of both species float at first, but the soft parts are soon removed by decay, and the stone is freed. In the case of the coast species, P. taitensis, the stones float indefinitely and are often found afloat in rivers. In the case of the inland tree, P. serratifolia, most of the stones sink at once, whilst the others sink in a few days. It is probable that currents are one of the effective agencies in distributing the coast species, but this could not apply to the inland tree. The fruits of both the inland and the coast species would attract birds, and the stones would resist injury in their crops. This is the agency advocated by Prof. Schimper for the shore species, P. integrifolia, of Indo-Malaya; and fruits referred with a query to this genus were found in the collection of seeds and fruits obtained by me from the crops of pigeons in the Solomon Islands (Bot. Chall. Exped., Introd. p. 46, part IV. p. 312).
On the Cause of the Buoyancy of the Stone or Putamen of the Coast Species.—This is primarily connected with the empty seed-cavities, the four-celled stone usually developing only one seed, the other cavities being empty. This inference was established by the dissection of a large number of stones, but it will be seen from the table below that one-seeded stones are also frequent in the case of the inland tree (P. serratifolia), where they as a rule sink. With either species the substance of the stone has no floating power, but with the shore species, on account of the thin-walled stone, the empty seed-cavities cause it to be specifically lighter than water whilst with the inland species the walls of the stone are so thick that the empty spaces of the unfilled seed-cavities do not effect the same result. It may be remarked that when the coast species grows in the inland plains the buoyancy of the stone is preserved.