Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 2 / Plant-Dispersal

by H. B. Guppy

Pittosporum (Pittosporeæ).

This genus, which contains nearly a hundred species, usually of small trees, is widely spread in the warmer regions of Africa, Asia, Australia, and New Zealand. It is also especially a genus of oceanic islands, occurring not only in those of the Pacific but also in Madeira and Teneriffe in the Atlantic.

Though found in most of the larger Pacific groups, it has apparently never been recorded from Samoa. From Hawaii ten species are known, all peculiar to that group. About half a dozen have been described from Fiji, of which three at least have been observed outside the group in the neighbouring Tongan Islands. Rarotonga possesses a peculiar species which, however, is so near to two other Fijian and Tongan species that, according to Cheeseman’s memoir, they may have to be subsequently united. Tahiti is credited by Drake del Castillo with a solitary species widely distributed in the Old World, whilst in the Index Kewensis a peculiar species is assigned to it. They form small trees of the wooded mountain-slopes of Fiji; whilst in Hawaii, beside occurring in the lower forests, they may extend to altitudes of between 5,000 and 7,000 feet. In the connection that more or less exists between the species of the South Pacific archipelagoes, and in the endemic character of all the Hawaiian species, we see the principle exemplified that there are two regions of distribution in the islands of the tropical Pacific—the Hawaiian region and the South Pacific region.

Before their dehiscence, the wrinkled, woody capsules would seem very unlikely to attract birds; but the observer on handling an opening fruit, with its orange or brightly coloured lining and displaying black or dark-purple seeds immersed in a semi-liquid pulp, would form a different idea of the plant’s capacity for this mode of dispersal. The mature dehiscing fruits are very conspicuous on the tree; and the seeds covered with the “sticky” material of the pulp might possibly adhere to birds pecking at the fruit. But this would only aid in local dispersion, since the weight and size of the seeds, 5 to 8 millimetres (²⁄₁₀ to ³⁄₁₀ inch), would unfit them for this mode of transport across an ocean. They are, however, sufficiently protected by their hard tests to be able to pass unharmed through a bird’s intestinal canal.

Yet the distribution of the species of Pittosporum in the Pacific would show that their dispersal is more a matter of the past than of the present. Out of the ten peculiar Hawaiian species, Hillebrand designates none as generally distributed over the group. But it is evident that, though it is on the point of breaking off, some sort of connection still exists in the South Pacific between the Tongan and Fijian species, and until recently between the species of those two groups and of Rarotonga.

Reynoldsia (Araliaceæ).

The Polynesian genus of Reynoldsia, originally established by Gray, is merged by Hooker and Bentham into the Malayan genus Trevesia, a step that brings the Pacific plants into line with many other of the plants hailing originally from the Old World. The significant fact in the distribution of this genus of small trees in the Pacific is that its dispersal over the ocean has ceased long ago, since the three species here occurring are restricted each to a particular group, namely, to Hawaii, Tahiti, and Samoa. Yet the inter-island dispersal still continues in the Hawaiian Group, the species characteristic of that archipelago being found in all the islands.

Reynoldsia sandwicensis came frequently under my notice in Hawaii, and the fairly fleshy drupes, about one-third of an inch, or 8 millimetres, in size, with their crustaceous pyrenes appeared to me well fitted for assisting the dispersal of the plant by frugivorous birds. Yet here the same question arises that presents itself with so many other Hawaiian plants, and that is, How has it happened that the birds have continued to disperse the species over the scattered islands of this group long after they ceased to transport fresh seeds from the outside world? The answer is an obvious one. The birds that originally brought the seeds of the parent species from some distant region came at last to remain permanently in the Hawaiian Group, and not only the plant but probably also the bird has since undergone specific differentiation. This link between bird and plant in the floral history of a group of Pacific islands is the common theme of the story of most of the endemic species of plants in this region of the globe.

Gardenia (Rubiaceæ).

This genus, comprising about a hundred known species, is spread over tropical Africa, Asia, and America, and over all the groups of the tropical Pacific. On account of their handsome, white, scented flowers these shrubs are much appreciated by the Pacific islanders, who employ the flowers for personal decoration. Some ten species have been described from the groups of the open Pacific, all of which, with the exception of Gardenia tahitensis, which ranges the South Pacific from Fiji to the Marquesas and Tahiti, are seemingly peculiar to the different archipelagoes. Thus there are some six species endemic to Fiji, one to Samoa, and two to Hawaii.