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_.

From Mr. Hemsley’s conclusion that the Leguminosæ are wanting in a large number of islands where there is no truly littoral flora, the presumptions arise that _when inland species exist that possess no capacity for dispersal by currents they are to be regarded as derivatives from the littoral flora, and that they owe their origin to a strand-plant possessing buoyant seeds originally brought by the currents_. It has been shown in the case of Afzelia bijuga and of Cæsalpinia that when Leguminous shore-plants extend inland the seeds often lose their buoyancy, and it is probable that divergence in other characters may occur, leading, as in the mountains of Fiji, to the development of a new species of Cæsalpinia. It is urged that by a continuation of the same process the inland species, Erythrina monosperma, has been developed in Tahiti and Hawaii, and the inland species, Canavalia galeata and Sophora chrysophylla, have been produced in the last-named group. All these species have non-buoyant seeds, and in all three cases there is no littoral species in Hawaii, it being assumed that the parent strand-plant has been driven inland from the beach. _It is not necessary that the littoral species should be now represented in the flora._

It is remarkable that _in almost all cases the cause of buoyancy is of the non-adaptive or mechanical kind, due either to cavities formed by the shrinking of the seed-nucleus during the setting of the seed or to the light specific weight of the kernel_. There is but little to show that the buoyancy of the seeds of Leguminosæ is anything but an adventitious character of the seed, as far as its relation to dispersal by currents is concerned. Although this capacity has been the great factor in the wide distribution of the species, yet it is evident that Nature here takes advantage of a quality that could never by its aid become a specific distinction. The upshot of the selecting process would be the dispersal by the currents of nearly empty seeds or seeds that have lost their germinating capacity.

The distribution of the Leguminosæ in the Pacific islands, and indeed of tropical islands generally, is often full of inconsistencies. This is the only order that sets at nought most of the principles established for the other plants of the sea-coast, and that defies the application of the laws of plant-dispersal now most in evidence. Take, for instance, the inexplicable affinity of Acacia koa, the well-known Koa tree of the Hawaiian forests, to Acacia heterophylla, a tree restricted to the Mascarene islands of Mauritius and Bourbon. Mr. Bentham, who placed them in the same group with three or four Australian species, even doubted whether the difference between the Hawaiian and Mascarene species amounted to specific rank. These two closely related Acacia trees of far-separated islands of the Indian and Pacific Oceans represent outliers of the great formation of phyllodineous Acacias that have their home in Australia (_Introd. Chall. Bot._ p. 26). As far as I can gather Acacia seeds have no known means of dispersal. Not even when the tree has a littoral station, as in the case of Acacia laurifolia in Fiji, have the seeds or pods any capacity worth speaking of for dispersal by currents. We must appeal to the birds; but to what birds we may ask, unless it be to the extinct Columbæ and their kin, or to the Megapodes. Some of the other Hawaiian difficulties connected with the inland Leguminosæ are repeated in the Mascarene Islands. Thus, Bourbon, like Hawaii, has its inland species of Sophora of the section Edwardsia.

In their irregular distribution the Leguminosæ of the Pacific islands are often a source of perplexity to the student of plant-dispersal. Take, for example, the inland Erythrina, E. monosperma, of Hawaii, Tahiti, and perhaps New Caledonia. Then look at the singular distribution of the Sophoras of the Edwardsia section in Chile and Peru, Hawaii, New Zealand, Further India, and Bourbon. The botanist, again, finds a climber like Strongylodon in the forests of Fiji, Tahiti, and Hawaii, and he picks up the seeds on the beaches of those islands and notices that they float unharmed for many months in the sea, yet when he pays heed to the distribution of the genus he finds that it only comprises four or five species, and that it occurs outside the Pacific only in the Philippines, Ceylon, and Madagascar. The extraordinary distribution of Entada scandens in the Pacific islands has been before alluded to in these pages. Here we have a plant, the seeds of which are known to be transported unharmed by currents all round the tropics. Yet it is absent from Hawaii and from almost all of the islands of Eastern Polynesia. In many cases an endeavour has been made in this work to explain these difficulties. But the order in the Pacific teems with such difficulties. We may ask with astonishment why it is that the genera, and sometimes even the separate species, of the Leguminosæ seem so often to follow in each case a principle of their own.

Plants of this order in the Pacific conform to no one rule of dispersal or distribution, whether we regard a species, a genus, or the whole order. Take, for instance, the presence in Hawaii of Canavalia galeata, a plant that, as we know it now, could not possibly have reached there through the agency of the currents, and the absence from the same group of Entada scandens that could have been readily transported there by the currents from America. Or, if we take the whole order and look at the structures connected with the buoyancy of the seeds, we find two types of structure and the elements of a third. Then, again, whilst most littoral plants with buoyant seeds retain the buoyancy of their seeds when they extend inland, Leguminous shore-plants, like Afzelia bijuga and Cæsalpinia bonducella, when they extend inland in Fiji and Hawaii, lose in great part or entirely the floating power of their seeds.

Furthermore, most strand-plants, being typically xerophilous in character, when they extend inland shun the forests and prefer the dry soil and sparsely vegetated surface of the open plain; but the Leguminous genera and species (Mucuna, Afzelia, Entada, &c.) when they leave the coast take to the forests, growing usually as stout lianes, but sometimes as tall trees. Here again the Leguminosæ seem to follow a principle of their own. As far as I know, this is the only order in the Pacific possessing forest-trees which, as in the case of Afzelia bijuga in Fiji, are equally at home in the woods of the interior and of the coast.

Indeed, judging from Professor Schimper’s observations, the littoral Leguminosæ of the tropics often display a physiological constitution that seems in some respects out of touch with their surroundings. They may, as in Sophora tomentosa and in Canavalia, present the xerophytic character of strand-plants, but frequently they are not halophilous or “salt-loving,” like other plants associated with them on the same shore-station. They are often shy of salt in their tissues, though able to thrive in salt-rich localities. That capacity which strand-plants usually possess of storing up chlorides in their tissues, and especially in their leaves, without injury to themselves, is but slightly possessed by such characteristic shore-plants as Canavalia, Pongamia glabra, and Sophora tomentosa. This capacity, which, as Professor Schimper indicates, goes to determine whether or not plants are capable of living in salt-rich localities, has often no determining influence with the Leguminosæ. (See Note 60.)

Though the plants of this order form such a large element in the strand-flora of the Pacific islands and of the tropics generally, they seem in other respects, besides those just referred to, to act as if they were strangers to the station. Look, for instance, at the readiness of the floating beans of Mucuna, Strongylodon, &c., to germinate, as shown in Chapter IX, in the tepid waters of the warmer areas of the tropical oceans. This is a great deal more than a disturbing factor of distribution. It is significant also of the plants being out of touch with their dispersing agencies.

One may notice in conclusion the fact brought out in Chapter VIII that nearly all the littoral plants dispersed by the currents that are common to the Old and the New Worlds belong to the Leguminosæ. This is held to indicate that their home is in America, since that continent distributes but does not receive tropical littoral plants dispersed by currents.

_Summary._

The Leguminosæ are far more characteristic of the littoral flora than of the inland flora of the Pacific islands; and since the greater number of them have buoyant seeds, it follows that this order mainly owes its presence in this region to the currents.

As it has been shown that in a large number of islands where there is no littoral flora the Leguminosæ are wanting, the presumption arises that when, as in Hawaii, inland species occur which at present have no capacity for dispersal by currents, they have been derived from strand-plants originally brought by the currents, even though such shore species no longer belong to the flora.

As far as its relation to dispersal by currents is concerned, the buoyancy of the seeds of Leguminosæ is merely an adventitious character, and the structure connected with it has no specific value.

Plants of this order in the Pacific are a source of much perplexity and conform to no one rule of dispersal, whether as regards their disconnected distribution, their means of dispersal, the structural cause of buoyancy, the loss of buoyancy of inland species, and in other particulars. Even in their physiological constitution they are often at variance with the bulk of littoral plants when they grow on the sea-shore, since typical beach-plants of the order, though thriving in salt-rich localities, are shy of salt in their tissues.

It is probable that whilst the Pacific islands have derived most of their littoral plants that are dispersed by currents from the tropics of the Old World, they have received most of their strand Leguminosæ from America.