(9) The nature of the influence of “station” on the seed-buoyancy is obscure; but it is evidently not connected with the usual differences between coast and inland localities, such as those concerned with exposure or shade, dryness of soil, relative humidity, and similar contrasts.
(10) The buoyancy of the seed is developed during the final shrinking process associated with its maturation, a large cavity between the cotyledons being usually produced.
Note.—Since most of the principal conclusions of this work are involved in my especial study of the littoral species of Afzelia, Cæsalpinia, and Entada, the reader is advised, if he wishes to form an opinion of the author’s method of investigation, to read this chapter carefully through. With most other shore-plants, though in not a few cases studied with the same detail, the exigencies of space have often limited me to the employment of the general results in the appropriate chapters without entering into details. Should he desire to test any view of his own relating to plant-dispersal, he could not do better than begin with the materials here provided.
CHAPTER XVIII
THE ENIGMAS OF THE LEGUMINOSÆ OF THE PACIFIC ISLANDS
Leguminosæ predominate in tropical littoral floras.—The anomalies of their distribution in the Pacific islands.—They conform to no one rule of dispersal or of distribution.—Strangers to their stations.—The American home of most of the Leguminous littoral plants.—Summary.
It is my intention here to gather up some of the “ends” of the great tangle presented by the Leguminosæ in the Pacific. When we look at the indigenous phanerogamic floras of Fiji, Samoa, Tahiti, and Hawaii we find that the Leguminosæ form 5 or 6 per cent. of the total in each of the three first-named groups, and only about 2·5 per cent. in Hawaii. The paucity of Leguminosæ in oceanic floras was long ago pointed out by Sir Joseph Hooker, whose work forms the foundation of much of our knowledge of insular plant-life. This is emphasised by Mr. Hemsley in his volume on the Botany of the “Challenger” Expedition (Introd. p. 25), where he makes the very significant remark that the Leguminosæ are wanting in a large number of oceanic islands where there is no truly littoral flora. The islands, however, here more especially referred to, are those of the southern Atlantic and Indian oceans, such as St. Helena, Tristan da Cunha, and Amsterdam. It is especially true of New Zealand, where the Leguminosæ barely make 2 per cent. of the total. Of the Polynesian islands, as he points out, it is not so correct; and, in fact, the proportion found in the Fijian, Samoan, and Tahitian floras, respectively, is much the same as that which characterises the British flora, namely, 5 to 6 per cent.
When we come to explain the paucity of the Leguminosæ in the Hawaiian flora we bring to light the singular principle that Leguminosæ are far more characteristic of the littoral flora than of the inland flora of a Pacific island. About half of the Leguminosæ of Fiji and Tahiti are coast plants; and about 30 per cent. of the littoral plants of the islands of the tropical Pacific belong to this order. Since, therefore, Hawaii possesses much fewer shore-plants (30) than does Tahiti (55) or Fiji (80), the paucity of its Leguminous plants is readily accounted for.
We have next to notice a principle, which is, in fact, deducible from the first, namely, that buoyant seeds are much more characteristic of the Pacific Leguminosæ than of any other order. Three-fourths of the species have buoyant seeds, and, in fact, about a third of the littoral Polynesian plants with buoyant seeds or fruits belong to this order.
It may, therefore, be inferred that the Leguminosæ owe their presence in the islands of the tropical Pacific mainly to the currents.