Dianella (Liliaceæ).—This is a genus of herbs, possessing often pretty blue berries, that extends over tropical Africa, tropical Asia, the Mascarene Islands, Malaya, Australia, and New Zealand, and is found in all the larger Pacific archipelagoes. Of the twelve species named in the Index Kewensis only two belong to America, occurring respectively in Cuba and Venezuela. There are two species in the islands of the tropical Pacific: (a) Dianella ensifolia, found in Hawaii and ranging over the Mascarene Islands, India, China, Malaya, and tropical Australia; and (b) D. intermedia, recorded from most of the groups of the South Pacific (Fiji, Tonga, Rarotonga, Tahiti), and occurring also in Norfolk Island and New Zealand. These two plants occur in similar stations all over Polynesia, sometimes growing in the grassy plains on the dry side of an island, at other times extending up the thinly wooded mountain slopes and reaching the hill-crests some 2,000 or 3,000 feet above the sea. Their berries would readily attract birds; and their seeds, about one-fifth of an inch (5 mm.) in size in the case of D. ensifolia, could be carried uninjured in the stomach and intestines of a bird.
Summary.
(1) A later period in the era of the general dispersal of Malayan plants over the Pacific is indicated by the genera that contain species found outside each group as well as species restricted to it.
(2) In this period the extremely variable or polymorphous species plays a conspicuous part, as represented in such genera as Alphitonia, Dodonæa, Metrosideros, Pisonia, and Wikstrœmia.
(3) The first stage is displayed by a solitary widely-ranging species found over most of the Polynesian archipelagoes, and varying independently in every group.
(4) The next stage is shown where the polymorphous species, having done its work of distributing the genus, ceases to wander and settles down and “differentiates” in all the groups; and the genus thus includes both peculiar and widely-ranging species in each group. Most of the genera possessing polymorphous species are in this stage.
(5) The following stage is displayed by those genera like Elæocarpus, Eugenia, and Peperomia, where peculiar species are especially developed in particular groups, and we get subcentres of distribution for the genus, that is to say, small gatherings of peculiar species. A few species, however, still keep up a connection with neighbouring island-groups. Should this be severed we get the type of genus belonging to the earlier period of the Malayan era as described in the preceding chapter, a genus possessing only peculiar species and destined, after ages of further isolation through the failure of the dispersing agencies, to give rise to a new generic type or types.
(6) Frugivorous birds were chiefly active in dispersing these genera over the Pacific. Some of the genera possess seeds or “stones” of such a size that at first sight their transport by frugivorous birds to Hawaii seems improbable; but, as in the case of Elæocarpus, it is shown that this difficulty does not apply to all species of a genus, some of them having much smaller seeds or stones.
(7) The close of the era of the general dispersal of Malayan plants over the Polynesian Islands is indicated by those genera that are represented more or less entirely by widely ranging species. Though such species may vary among the different groups, they rarely take the rank of polymorphous species, the agencies of dispersal being sufficiently active to check marked variations.
(8) Several of the genera of this concluding stage, like Rhus, Viscum, and Plectronia, are known to be dispersed by frugivorous birds, whilst others, like Osteomeles and Dianella, are equally well suited for this mode of dispersal.