It is not at first sight easy to account for the absence from Tahiti of the mangrove-formation and of so many of the plants that grow at the borders of a mangrove-swamp in Fiji. Their absence can scarcely be due to the want of suitable stations, as is indicated by the common occurrence in the Tahitian coast-marshes of Chrysodium aureum, the Great Swamp-fern, that not only abounds in the mangrove belts of Fiji, Tonga, and Samoa, but is associated with mangrove-swamps over much of the tropical zone. Nor can it be said that the currents are ineffective, or that the seeds or fruits of the missing plants possess, as a rule, insufficient floating powers. Most of the plants of the Tahitian beaches hail, like those of Fiji, from Malaya, and have been brought through the agency of the currents; and many of the absent littoral plants that have the same home, such as Heritiera littoralis and Clerodendron inerme, have fruits or seeds just as capable of floating unharmed over the same extent of ocean. It is not any defect in floating-power that has prevented the establishment of two such plants in the Tahitian area. Entada scandens, which in some parts of the world is a typical climber of the mangrove-formation, and in other places thrives well in the absence of mangrove-swamps, has only been recorded from Rarotonga in this region by botanists, but I believe Wyatt Gill refers to its occurrence in Mangaia in one of his books.
On the other hand, it is likely that the floating seedlings of Rhizophora and Bruguiera, which represent the only means of dispersal by the currents at the service of these mangroves, would not arrive at Tahiti in a condition favourable for the establishment of the plants. My observations, which are described in [Chapter XXX.], go to show that, though the seedlings will float uninjured in still sea-water for months, they will not withstand prolonged sea-buffeting. These two genera of mangroves, it is most important to remember, supply the pioneers and the principal components of a mangrove-swamp in the Western Pacific. Where they fail to establish themselves, the requisite conditions for the large number of plants and animals that find their home in and around a mangrove-swamp would not be provided. We thus perceive that the absence from the Tahitian coast flora of several plants that are associated in Fiji with the mangrove-swamps depends on a law of association, which has already been referred to in the preceding chapter, and is not concerned with incapacity for dispersal by currents (see [Note 26]).
Whilst the Tahitian coast flora does not, therefore, possess the plants of the mangrove-swamp and its vicinity, it includes most of the typical beach-trees of the coral islands and reef-fronted coasts of other parts of the South Pacific. Thus we find here on the sandy beaches Barringtonia speciosa, Calophyllum Inophyllum, Cerbera Odollam, Hernandia peltata, Guettarda speciosa, and numerous other plants that are indicated by the letter T in the list of Fijian littoral plants given in [Note 2]. The total number of Tahitian shore-plants is thus considerably less than that of Fiji (there are about 55 in Tahiti and about 80 in Fiji); but in its turn, as will subsequently be shown, it is much larger than that of Hawaii, where the number is about 30.
Quite three-fourths of the strand-flora of this region have buoyant seeds or seedvessels capable of floating for long periods; and there is no difficulty in assigning by far the greater share in the stocking of the beaches with their plants to the agency of the currents. The currents in their operations have indeed carried the fruits or seeds of many of these plants across the South Pacific as far as the islands extend, namely, to Ducie Island and to Easter Island. There are few more significant proofs of the efficacy of the currents in distributing plants over the Pacific than the discovery, by Mr. Arundel, of Barringtonia speciosa in Ducie Island in association with Tournefortia argentea (Challenger, Botany, III. 116).
The residue of the Tahitian coast flora possessing fruits or seeds that are unsuited for dispersal by currents includes such plants as Heliotropium anomalum, Triumfetta procumbens, Tephrosia piscatoria, Wikstrœmia fœtida, &c. The small nucules of the first-named are perhaps dispersed by granivorous birds; the fruits of Triumfetta are probably transported in birds’ plumage; those of Wikstrœmia are distributed by frugivorous birds; and the seeds of Tephrosia may be dispersed like those of Heliotropium.
The recruits or intruders from the inland flora do not appear to be numerous or to give any special character to the shore flora. (See [Note 27].)
From not having a personal acquaintance with this region it is not possible for me to discuss the extension of the shore-plants inland except in a general way. From the pages of the work of Drake del Castillo we can, however, infer that several plants such as Cassytha filiformis, Cerbera Odollam, Colubrina asiatica, Hernandia peltata, Morinda citrifolia, and Pandanus odoratissimus have extended inland to the mouths of the Tahitian valleys, and have ascended the lower slopes of the hills that lie near the coast. Others, like Cæsalpinia Bonduc, Gyrocarpus Jacquini, and Ochrosia parviflora, have climbed far up the mountain-sides to elevations of from 2,000 to 2,400 feet above the sea. It is also evident from Mr. Cheeseman’s memoir on the Rarotongan flora that coast plants also stray inland in that island. In an island like Rarotonga, where a sorry substitute for a mangrove-swamp exists in the form of a few coastal muddy places occupied by Vitex trifolia and Sesuvium Portulacastrum, Entada scandens takes to the hills; and thus it is that in this island it is most abundant in the interior, climbing to the tops of the highest trees and “covering acres of the forest with a dense canopy of green.”
Summary of the Chapter.
(1) The Tahitian region possesses most of the plants that frequent the sandy beaches of the Pacific islands.
(2) But it lacks the mangroves and the associated plants of the mangrove-swamp.