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

by H. B. Guppy

Canavalia.

This genus is represented in the tropical islands of the South Pacific from Fiji to Tahiti by three littoral species, none of which have been found in Hawaii, where only an endemic inland species exists. Reference will alone be made here to such facts as bear on the probable history of the mysterious Hawaiian species, additional particulars being given in [Note 54]. The littoral species, Canavalia obtusifolia (D.C.), C. sericea (Gray), and C. ensiformis (D.C.), have buoyant seeds and are dispersed by the currents; whilst the inland Hawaiian species, C. galeata (Gaud.), a forest climber peculiar to that group, has non-buoyant seeds. We thus have repeated the problem of Erythrina monosperma. The absence of the littoral species from Hawaii can scarcely be attributed to the failure of the currents, since Ipomœa pes capræ, which accompanies C. obtusifolia as a beach-creeper all round the tropical globe, is present on the Hawaiian beaches. Nor can it arise from lack of floating-power on the part of the seeds, since experiment indicates that the seeds of C. obtusifolia will float for months unharmed in sea-water. Nor can it be ascribed to climatic conditions, since this tropical shore species extends into cooler latitudes than those of the Hawaiian Islands, being found in the Kermadec Group and in the Bermudas, which are subtropical both in position and as regards much of their vegetation. The reason perhaps we may never learn from the plants themselves, though it may be possible to obtain some light on the problem from outside sources.

Canavalia galeata differs much in its habits, as well as in some of its characters, from the existing littoral species of regions outside the Hawaiian Group. It is a stout climber ascending the forest trees to a considerable height, though, as is indicated in [Note 54], the shore species sometimes display a tendency in the same direction. It is described by Hillebrand as occurring “on all islands, in forests up to 2,000 feet.” Like those of the inland species of Erythrina (E. monosperma), its seeds sink in sea-water even after being kept for four years, nor could the pods be utilised for dispersal by the currents, since they float, when unopened, only for four or five days. Here also, as with Erythrina, the seeds of the inland species no longer possess the buoyant kernels to which the floating capacity of the seeds of the coast species is due. Though we have to exclude the currents, we can scarcely in its case appeal to bird-agency when we wish to account for the transportal of the original seeds to Hawaii, as that would imply that birds can carry beans nearly an inch, or 2 to 2.5 centimetres, in length unharmed in their stomachs over a tract of ocean some 1,500 or 2,000 miles across. We should have to learn much that is unexpected of the modes of dispersal of the Leguminosæ before we could accept such an hypothesis.

Canavalia galeata indeed presents to the student of dispersal one of the enigmas of the Hawaiian flora; and it should be noted that the mystery of its distribution is concerned not only with the means of transportal of the seeds of the original species to the group, but also with its present dispersal among the islands. It is, however, suggestive that Dr. Hillebrand mentions two varieties, one of them found on Kauai, with somewhat smaller seeds; so that some inter-island differentiation is evidently in progress. No attempt is made here to connect this inland species directly with the absent beach-plants. That is a matter for the systematist; but we are not tied down to existing shore-plants in finding an ancestor, since the common parent of the littoral and inland species may have been a shore-plant dispersed by the currents.

Mezoneuron.

Another closely parallel instance, offering, from the standpoint of dispersal, the same difficulties presented by Canavalia galeata, is to be found in Mezoneuron kauaiensis (Hillebr.), a tall inland shrub also peculiar to the group and belonging alike to the Leguminosæ. The difficulties are so nearly identical that the same explanation will have to cover both; but it is significant that with Mezoneuron there is no littoral species to which we can appeal to extricate us from the difficulty. Yet the genus is related to Cæsalpinia, and the species was first described by Mann as C. kauaiensis, so that it may have once possessed a littoral species that has ceased to exist as such. When we come to discuss Cæsalpinia and Afzelia ([Chapter XVII.]) we shall obtain from those genera many suggestions as to the probable past of Canavalia galeata and Mezoneuron kauaiensis, two of the greatest riddles presented by the Leguminosæ of Hawaii.

The flat seeds of this species of Mezoneuron measure about an inch (2·5 cm.), and seem most unsuitable for dispersal by birds over a wide extent of ocean. Nor can we appeal to the currents, since my experiments in Hawaii show that the seeds have no buoyancy and that the pods only float for a week in sea-water. Dr. Hillebrand records this shrub from Kauai, Oahu, and Maui; I found it also on the lower slopes of Hualalai in Hawaii and therefore the same question of inter-island dispersal here presents itself that was connected with Canavalia galeata, since we have also to explain the transport of the seeds between islands 70 to 150 miles apart. The critical point in the history of these two enigmatic inland plants of the Hawaiian Islands was doubtless concerned with the loss of buoyancy of the seeds of the original littoral plant. It will subsequently be shown that this is what is now in actual operation with Cæsalpinia and Afzelia in different parts of the Pacific.

Sophora.

In this genus, as in Erythrina and Canavalia, we have a littoral species, Sophora tomentosa, that ranges over the tropical beaches of the globe, including most of the islands of the Pacific, but does not occur in Hawaii, where the genus is represented by an endemic inland species, S. chrysophylla. Here also we find the shore-species with seeds capable of floating for months on account of their buoyant kernels, and the inland species with seeds that sink even after years of drying (see [Note 56]). Unless other inland species of Sophora have recently been described from the tropical Pacific, the Hawaiian species is the only one of its kind known from this region.

But the problem wears a different aspect in the case of this genus, since the endemic inland species of Hawaii is a tree of the mountains where a temperate climate prevails, whilst Sophora tomentosa is a shrub of the tropical beach that only at times extends into subtropical latitudes. The Mamani tree, as the Hawaiians name S. chrysophylla, extends up to 9,000 or 10,000 feet above the sea, forming, with Myoporum sandwicense and one or two other trees and shrubs, the highest belt of the forest in the larger islands. It is in the open woodland between 6,000 and 7,000 feet that it is most at home, and here it attains a height of 20 to 30 feet. It descends in places to as low as 2,000 feet above sea-level; but here is living under uncongenial conditions, and, like Myoporum sandwicense, becomes dwarfed and shrubby. The climatic conditions under which S. chrysophylla thrives in the Hawaiian mountains are therefore those of the temperate zone. From the data given in [Chapter XIX.], the mean annual temperature at an elevation of 6,000 to 7,000 feet would probably be about 55°, the average temperature of New Zealand.