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

by H. B. Guppy

Entada scandens (Benth.).

The story of Entada scandens, a plant familiar to many of my readers under the name of the Queensland Bean, is a story of three continents, Africa, Asia, and America. From the point of view of its dispersal two features at once attract attention in the case of this giant-climber; in the first place its wide distribution over the tropics of the Old and New Worlds, and in the second place the great capacity of its large seeds, often two inches across, for dispersal by the currents. But before discussing these matters it will be necessary to glance at the distribution of the genus, since much light will thereby be thrown on some of the numerous difficult points affecting this extremely interesting tropical plant. Of the thirteen species enumerated in the Index Kewensis, seven are African, three are American, one is Burmese, one hails from Madagascar, and, lastly, there is the world-ranging Entada scandens, concerning whose home botanists are not agreed. Most of the species would seem to be inland plants, whilst Entada scandens thrives both inland and at the coast. Africa would thus appear to be, as with Afzelia, the principal home of the genus, but with America as a subsidiary centre.

In many points Entada scandens presents a parallel to Cæsalpinia bonducella, another Leguminous tropical plant which occurs also at the coast and inland. But since they both owe their wide distribution to their littoral station, it will be as coast plants that they will be most properly considered in this and the following chapter. Yet if the student were to regard the distribution of these two plants in a continental region as in India, where they extend inland to the Himalayas, he might fail to discern their true station. To accurately gauge the matter of their station, it is necessary for him to look at the plants as they occur in the islands of the Pacific. There he will first see the stranding of the seeds on a shore by the currents, then their germination and their development into giant-climbers over the littoral trees or into straggling bushes on the beach; and afterwards he will observe the plants of both species extending inland, and in these three stages he will learn their history in the Pacific; but a history, it may be observed, that in this region represents their efforts to return to an inland station, such as they once possessed in their birthplace in some distant region of the globe.

Dealing first with the station of Entada scandens, it may be remarked, as Dr. Seemann points out, that in Fiji it is most characteristic of the mangrove-formation. But it also occurs amongst the trees at the back of the mangrove swamp, on the beaches, on the banks of the estuaries, and at the edge of the inland forests where they border on the plains. Sometimes in the company of Derris uliginosa it grows not as a climber, but as a prostrate plant on the sandy beaches; and here, not being able to assume its normal habit of a climber, it does not seed. It is to be found at times far inland in open-wooded districts. Thus in Vanua Levu I found it growing in the Mbua district four miles inland, and 1,400 feet above the sea. Reinecke speaks of it in Samoa only in connection with the “urwald,” or primeval forest. Cheeseman describes it as most abundant in the interior of Rarotonga, covering the trees with a wide-spreading canopy of green. In the Malayan region Schimper refers to it as a plant of the beach-tree formation. In Ecuador and on the Panama Isthmus it grows not only at the coast, but also on the hill-slopes in the rear of the mangrove-belt.

With reference to the distribution of the plant, it may be remarked that, although it is found all round the tropics and possesses great capacity for dispersal by currents, there are certain difficulties in explaining its wide area and in accounting for its very peculiar distribution in the Pacific islands. It was doubtless in allusion to some of these difficulties that Mr. Darwin, in a letter to Sir Joseph Hooker, remarked: “Entada is a beast” (More Letters, &c., i, 93). There is at first the question of the identity of the species in the Old and New Worlds. It is here assumed that it is the same in both hemispheres; but it must not be forgotten that the identity is “not beyond doubt” (Bot. Chall. Exped. iv, 147).

Then there is the difficulty connected with its occurrence on both coasts of tropical America. In this respect it is at one with some other littoral plants, like Ipomœa pes capræ, as well as with the plants of the mangrove formation, as is pointed out in [Chapter VIII.] Whilst with the mangroves it is necessary to assume that they antedate the land connection between North and South America, this is not requisite in the case of Entada scandens, since it grows in the interior of the Panama Isthmus, and rivers on the north and south sides now carry its seeds seaward from the same “divide” to the Atlantic and Pacific Oceans, as described in [Chapter XXXII.]

But, as I have also shown in [Chapter VIII], America forms with the West Coast of Africa a region characterised by the same tropical littoral flora. This region, on account of the arrangement of the currents, stands in a very peculiar relation with the Asiatic region, which comprises the rest of the tropics, and to a great extent possesses its own peculiar strand-flora. There are a few littoral plants, like Entada scandens, Canavalia obtusifolia, Sophora tomentosa, and Ipomœa pes capræ that occur in both areas; but the large majority are confined to one or other of them, either to the American region, including the African West Coast, or to the Old World region, which includes the African East Coast. The American region gives to the Old World, but it can receive nothing in return. For this reason, it is argued, we are compelled to regard most, if not all, of the cosmopolitan tropical shore plants that are dispersed by the currents, such as those above named, as having their home in the American region. Entada scandens would, therefore, from this standpoint have its home in America.

Then, again, there is the difficulty connected with the distribution of this plant on both sides of tropical Africa. Though Oliver in his Flora of Tropical Africa mentions this species only in connection with the West Coast, he says it is probably widely spread in that continent, and he refers to a pod in the Kew Museum indistinctly labelled “Lake Ngami.” I have not come upon any reference to its being a littoral plant on the East Coast, but since numerous littoral plants of tropical Asia are found on that coast its occurrence there or in the East African islands would be expected. However, as the genus has a centre in America, and as this species is regarded as of American birth, we are not called upon to employ the argument used in assigning to a non-American genus like Afzelia an African home. Since the African West Coast belongs to the American region of tropical shore plants dispersed by the currents, the presence of Entada scandens on that coast of Africa can be readily explained, whilst if it has reached the Malayan Archipelago from America by way of the Pacific, it would, by extending like many other Malayan coast-plants along the shores of the Indian Ocean, almost complete its circuit of the globe. It is in this fashion, I believe, that the other littoral plants,, like Cæsalpinia bonducella, Canavalia obtusifolia, and Ipomœa pes capræ, that are found all round the tropics, have performed the circuit of the globe with America as their home.

One may remark in passing that the double home of the genus in America and the Old World, though offering a serious difficulty in plant geography, has no immediate bearing on the present mode of distribution of Entada scandens. Questions relating to the distribution of tropical shore-plants that are dispersed by the currents at first resolve themselves into considerations of the arrangement of the currents. Entada is not alone amongst the genera containing littoral species in having a home both in the Old and in the New World. Carapa is another instance, and additional cases might be cited.

The next peculiarity in the geographical range of this species is concerned with its irregular distribution in the archipelagoes of the tropical Pacific. Notwithstanding its great capacity for dispersal by the currents, although it occurs in all the groups of the Western Pacific as well as in the Cook Islands, it has not been recorded from the Society Islands, the Paumotus, the Marquesas, and Hawaii. Since, however, its seeds have been gathered by Mr. Arundel on the beaches of Flint Island, lying about six degrees north of Tahiti (Bot. Chall. iv, 302), it is not unlikely that it will be found growing in other parts of Eastern Polynesia south of the equator. One might have looked for an explanation of its rarity in Eastern Polynesia to the absence of mangrove swamps, in which, as in Fiji, it is sometimes most at home; but this is negatived by its abundance in Rarotonga, where mangrove swamps do not exist.

The dispersal of Entada scandens by the currents.—This plant offers one of the most conspicuous examples of the transport of seeds across oceans through the agency of the currents. In the pages of many botanical works, from the close of the 17th century onward, reference is made to the transport of its beans (often in association with those of Mucuna urens and Cæsalpinia bonducella) by the Gulf Stream or other currents across the Atlantic to St. Helena, the Azores, the west coast of Ireland, the Hebrides, the Orkney Islands, the coasts of Scandinavia, and even as far north as Nova Zembla (see Hemsley’s Bot. Chall. Exped.; Sernander’s Skand. Veg. Spridningsbiologi, &c.). That the seeds of Entada scandens retain their germinating capacity after this ocean-transport has been demonstrated not only by the germination of stranded seeds on the shores of St. Helena, but also by the germination when sown at Kew of seeds drifted to the Azores, as well as by the results obtained by Lindman, who procured the germination of the seeds of this plant and of Mucuna urens that had been washed up on the Scandinavian beaches (see Sernander, pp. 7, 390).

One of the most interesting references to the conveyance by currents of these seeds to the coasts of Europe is to be found in Dr. Sernander’s recent work on the modes of dispersal of the Scandinavian flora, where he sums up the results of Lindman’s investigations respecting the Gulf Stream drift. The stranded seeds of Entada scandens, it appears, have been found all along the Norwegian coast, but occur most frequently north of the Söndmöre district. They have even been found in a sub-fossil condition in the peat-bogs of Tjörn on the Bohuslän coast in Sweden, having been originally stranded on a beach in that locality at some distant, but post-glacial, epoch. Few phenomena in plant-distribution are more suggestive than this ineffectual transport through the ages of these large tropical beans to coasts within the Arctic Circle. The seed, no longer under the care of the mother-plant, becomes a waif, exposed to the pitiless laws of the physical world which here prevail. It was not thus that the plant was reared, but it is in this haphazard fashion that its seeds are spread. The philosopher could unravel most of the tangled problems connected with present and past plant-dispersal, if he could follow the clue supplied by this stranded tropical seed on a Scandinavian beach.

It is a far jump from the North Cape to the coral islands of the Pacific and Indian Oceans; yet it is within the area covered by the drifting Entada bean. The stranded seeds occur commonly on the Fijian beaches and on other islands of the South Pacific; but I never found them in Hawaii. They were gathered by me on the shores of Keeling Atoll in the Indian Ocean, and on the south coast of Java. Penzig found on the Krakatoa beaches, in 1897, not only the stranded seed but the established plant. They came under my notice in numbers on the beaches of Ecuador and on the Pacific and Atlantic coasts of the Panama Isthmus; and, as I learned, they are equally common on the other parts of the coasts of Central America. Not uncommonly these stranded seeds in various parts of the world are to be found incrusted with polyzoa and tubicular annelids, which afford proof of prolonged flotation in the sea. These seeds are also to be frequently noticed floating in the drift of the tropical estuaries. Thus they came under my observation afloat in numbers in the Fijian estuaries, in the Guayaquil river, in the estuary of the Chagres at Colon, and in the mouth of a river on the Panama side of the isthmus.

The mode of liberation of the seeds is worthy of a passing remark. The huge pods, often several feet in length, ultimately break up into separate joints bearing the seeds. The joints may decay on the ground, and the seeds are thus freed; or not infrequently in a mangrove-swamp they fall at once into the water, and there they float, as may often be observed in Fijian rivers, until their decay sets free the seed.

The seeds of Entada scandens are often quoted, and justly so, as striking examples of the dispersal of seeds by currents. Yet in few plants could the nature or the structural cause of the buoyancy have so little claim to be considered as adaptive in its character. Quite half, and sometimes even the majority, of the seeds freshly liberated from the plant have no buoyancy at all. The mean specific weight of the seed is about that of sea-water, but markedly higher than that of fresh water; whilst the principal determining cause of the buoyancy is, as shown below, purely mechanical, and one that, whilst favouring the wide distribution of the species, could not be improved by or come within the scope of Natural Selection.

From experiments made in Fiji and Ecuador, it appears that at least 50 per cent., and often more than half, of the seeds when first liberated from the pod have no buoyancy in sea-water. Of those that float in sea-water, a proportion varying between one-third and one-half sink in fresh water, so that in the case of plants growing on the banks of a river only about one-fourth or one-third would be carried down to the sea. So fine is the adjustment of the specific weight of these seeds to the density of water, a subject discussed in its general bearings in [Chapter X], that if one gathers a number of drift seeds on a beach, let it be in Fiji or in Ecuador, although, of course, all will float in the sea, only one-half or two-thirds will float in the neighbouring fresh-water stream. Those that float appear to be able to float almost indefinitely. This is sufficiently established by the transport of the seeds in a sound condition by the currents across the Atlantic, and by such evidence as the stranding of seeds incrusted with polyzoa and serpulæ on the beaches of Keeling Atoll. It has been also proved by the following experiment. Several years since, I placed a seed in a vessel of sea-water, where it still floated buoyantly in a perfectly sound condition twelve months afterwards.

With regard to the cause of the buoyancy, investigation shows that neither the seed tests nor the seed contents have any floating power, the buoyancy arising from a large central cavity produced by the shrinking and bending outward of the cotyledons during the drying and hardening of the maturing seed (see figure in [Chapter XII]). With the seeds that sink, this cavity is, as a rule, reduced to small dimensions, and may be represented only by a narrow slit. In some cases, however, where the cotyledons are unusually thick and heavy, even a large central cavity will not give floating power to the seed. There is an indication in my experiments that seeds from inland plants that have matured their pods in the forests sink in a much greater proportion than seeds of coast plants, or of those growing on the banks of estuaries. This we might expect, since in the shade of the forests the drying process that accompanies the setting and final maturation of the seed would be less complete and the intercotyledonary cavity smaller than with the seeds matured in more exposed situations. This is a point, however, that requires further investigation.

It will thus be seen that in respect of buoyancy the seeds of Entada scandens are to be referred to the mechanical or non-adaptive group of buoyant seeds, described in [Chapter XII], which comprises several other Leguminous strand-plants, including Cæsalpinia bonducella. I especially studied the various stages in the development of the buoyancy of seeds in this mechanical group in the case of the species of Cæsalpinia just named, and the description of the process as given under that plant will apply to all.

Summary relating to Entada scandens

(1) This plant, which has been distributed by the currents over the tropics of the globe, has its station in the mangrove swamp, on the beach, by the estuary, and in the inland forest.

(2) It is regarded as an American plant that has reached the shores of the Indian Ocean by crossing the Pacific, and the coast of West Africa by crossing the Atlantic.

(3) Its occurrence on both coasts of America is attributed to its having a focus of dispersal in the forests of Central America, from which its seeds have been transported by the rivers to the shores of the Atlantic and Pacific Oceans.

(4) Its irregular distribution in the Pacific islands, to wit, its absence from Hawaii and its rarity in the Tahitian region, is not to be easily explained, but it is more than likely that it will be subsequently recorded from other localities in Eastern Polynesia.

(5) Although the seeds offer a striking example of dispersal by currents, since they are to be found stranded on beaches over much of the globe, from within the Arctic Circle to the Coral Sea, in few plants could the character of the buoyancy and the structure connected with it have so little claim to be considered as adaptive in their nature. At least 50 per cent. of the seeds sink in sea-water, and the cause of the buoyancy of the other seeds is only to be connected with the large size of a cavity produced by the shrinking of the embryo within the seed tests during maturation.