The Beach-Drift of Tropical Latitudes.

Tropical beaches, as a rule, present a much greater abundance and variety of stranded seeds and fruits than we find on beaches in temperate latitudes. Observers in different parts of the tropics have alluded to the enormous amount of vegetable drift floating in the sea off the coasts, particularly in the vicinity of estuaries. Though much of it is brought down by rivers, a good proportion is also derived from the luxuriant vegetation that lines the beaches. Gaudichaud speaks of the immeasurable quantity of drift (trees, branches, leaves, flowers, fruits, and seeds) floating amongst the islands of the Molucca Sea; and Hemsley, who quotes this author, gives other facts illustrating the same point. Moseley tells us that seventy miles off the coast of New Guinea, H.M.S. Challenger found the sea in places blocked with drift (Bot. Chall. Exped. iv. 279, 284). When the author of this book was in the Solomon Group, long lines of vegetable drift were frequently observed floating among the islands. The Rewa River in Fiji carries down a great amount of drift to the sea; and as described in [Chapter XXXII], the Guayaquil River in Ecuador bears seaward an enormous quantity of these materials.

When we come upon this floating drift out at sea off an estuary, we find, as Mr. Moseley pointed out, that the leaves have gone to the bottom, whilst the floating islets, composed of the matted vegetation lifted up from the shallows of a river channel, which form such a feature in the Guayaquil River, have been dispersed or sent to the bottom. However, a very large proportion of the seed-drift brought down by a river from the interior has no effective value for the purposes of dispersal. Many of the fruits and seeds brought down from inland owe their presence in river-drift entirely to the buoyancy acquired by the decay of the seeds. It is in its lower course when it traverses the mangrove belt that a river picks up most of the material that is of service in distributing the species; and this is mingled out at sea with the numerous buoyant seeds and fruits of littoral plants that are swept off the beaches by the currents.

A description is given in [Chapter XXXII] of the enormous amount of vegetable drift brought down by the Guayaquil River to the coast of Ecuador. Besides the huge tree-trunks and the floating Pistias, we observe large islets formed mainly of Pontederias and Polygonum, together with a host of seeds and seed-vessels, both large and small, including those of Anona paludosa, Entada scandens, Erythrina, Hibiscus tiliaceus, Ipomœa, Mucuna, Vigna, &c., accompanied by the empty seeds of Phytelephas macrocarpa and of many other strange plants from the slopes of the Chimborazo mountains. In addition, we notice the seedlings of Avicennia and of Rhizophora mangle together with the seeded joints of Salicornia peruviana and the germinating fruits of Laguncularia.

When in Fiji I made an especial study of the drift of the Rewa Estuary within tidal influence, the results of which are incorporated in various parts of this work. In the rainy season, when the drift is most abundant, the following would be its most characteristic components:

Amongst other seeds and fruits brought down by the Fijian rivers and stranded with a large amount of miscellaneous vegetable débris on the beaches in the vicinity of the estuaries are the seeds of Dioclea, Strongylodon lucidum, and Afzelia bijuga; the empty seeds of Musa Ensete (as identified with a query at Kew); the empty stones of the Sea tree, apparently a species of Spondias; the seeds of Colubrina asiatica; the fruits of an inedible indigenous Orange (Citrus vulgaris?) referred to in [Chapter XIII]; the cocci of Excæcaria Agallocha and Macaranga; and Coco-nuts.

The occurrence in Fijian beach-drift of the seeds of Musa Ensete, or of a wild banana much like it, is very remarkable. This species is found in the mountains of Abyssinia and on the slopes of Kilima-njaro in Equatorial Africa; but according to the monograph by Schumann on the Musaceæ (Engler’s Pflanzenreich, 1900) the species is confined to Africa, whilst all the other species of the subgenus are mostly restricted to the same continent with the exception of one or two in Further India. The empty seeds are frequent on the beach at Duniua at the mouth of the Ndreke-ni-wai in Savu-savu Bay, Vanua Levu, and are doubtless brought down by that river. Strangely enough the natives could give me no information about the parent plant which I never discovered. The seeds did not come under my notice in any other locality in Fiji. They answer to the description and to the figure given by Schumann for Musa Ensete; and their presence in the drift is one of the mysteries of the Pacific floras.

To enumerate the seeds and fruits found stranded on beaches in Fiji would be to give a list of all the littoral plants with buoyant seeds or fruits that are included in the list given in [Note 2]. I may here allude to the fact that the Coco-nut, whether brought down by a river or transported by a current, is able to germinate and establish itself when washed up on the Fijian beaches. I have found these fruits germinating amongst the drift stranded on the beaches near the mouths of rivers, some just beginning to germinate and others already striking into the sand and showing the first leaves. White residents living for years in one locality were quite convinced that this frequently happens. One of them pointed out to me some newly formed land at a river’s mouth, not over two years old, on which were growing young plants three or four feet high of Barringtonia speciosa, Calophyllum Inophyllum, and several other plants including young Coco-nut palms, all growing from fruits washed up by the waves and therefore self-sown.

Like the littoral flora the beach-drift proper to the Hawaiian Islands is very scanty. This is due to the scarcity of rivers, to the absence of the mangrove-formation from which much of the drift is derived in other tropical regions, and to the paucity of shore-plants with buoyant seeds or fruits. As is observed in [Note 30], where the composition of the beach drift is described, the presence of a large amount of timber and of other materials brought by the currents from the north-west coast of America masks much of the local drift.

Remarks on the beach-drift of the Panama Isthmus, and of the Ecuadorian, Peruvian, and Chilian coasts of South America will be found in [Chapter XXXII.] I have examined beach drift in other tropical regions, as in the Solomon group, on Keeling Atoll, and on the south coast of West Java; whilst there are at my disposal the data supplied by Schimper and Penzig for the Malayan region including Krakatoa, and by Hemsley for tropical regions generally. It will, I think, be best, if instead of describing in detail the composition of the drift for each locality, I refer briefly to the features that distinguish the tropical beach-drift of the Old World from that of the New World.

The beach-drift reflects the characters of the coast flora; and since tropical littoral floras belong to two great regions, the Asiatic including Polynesia and the African East Coast, and the American including the African West Coast, the seeds and fruits stranded on the beaches may be similarly referred to the same two regions.

All over tropical Asia, as well as in the tropical islands of the Indian and Pacific Oceans, the drift stranded on the beach presents the same general character, and as a rule possesses seeds and fruits of the same species that range over the whole or the greater part of this region. Almost everywhere we find seeds or fruits of the same plants of the beach formation, such as Barringtonia speciosa, Cæsalpinia Bonducella, Calophyllum Inophyllum, Canavalia obtusifolia, Cerbera Odollam, Cordia subcordata, Entada scandens, Guettarda speciosa, Hernandia peltata, Hibiscus tiliaceus, Ipomœa pes capræ, Mucuna, Scævola Kœnigii, Sophora tomentosa, Terminalia Katappa, and Tournefortia argentea. In those localities where mangrove-swamps occur we find generally diffused in the stranded drift of this region the seedlings of Bruguiera and Rhizophora, the seeds of Carapa moluccensis, the fruits of Heritiera littoralis and Lumnitzera coccinea, and the pods of Derris uliginosa. Amongst sundries found over much of this region may be mentioned, the drupes of Pandanus, the seeds of Erythrina, Vigna lutea, and Hibiscus tiliaceus, and the “nuts” of Aleurites moluccana. With the exception of the last-named all the fruits and seeds here enumerated are effectively dispersed by currents over great areas. The sound nuts of Aleurites have no buoyancy; and the nuts only acquire their floating power through the decay of the kernel (see p. [419]).

The beach drift of the American region, a region which comprises both the Pacific and Atlantic coasts of tropical America as well as the African West Coast, has some features in common with the Asiatic beach-drift and other features peculiar to itself. The plants, however, that are represented in the drift of both regions are comparatively few, and none of the large fruits of the Asiatic region are here to be noticed. We observe, however, that the drift of the two regions possess in common the seeds of Cæsalpinia Bonducella, Canavalia obtusifolia, Entada scandens, Erythrina, Mucuna, Sophora tomentosa, and Vigna lutea, all belonging to the Leguminosæ; and to these we must add the seeds of Hibiscus tiliaceus and of Ipomœa pes capræ, and the seedlings of Rhizophora and Avicennia. (Avicennia occurs in tropical Asia, but not in Polynesia.) The distinctive characters of the beach-drift of both coasts of America and of the west coast of Africa would be shown in the presence of seeds of Anona paludosa, the fruits of Laguncularia racemosa, Conocarpus erectus, Spondias lutea, and other plants. But the beach-drift of the American region is much more scanty. Of the shore plants generally dispersed in this region there could not be more than a couple of dozen that are indebted for their wide dispersal to the currents, and these alone figure in the effective beach drift. In the Asiatic region these plants would number at least seventy or eighty.

Summary.

(1) Effective dispersal by currents is mainly restricted to warm latitudes, as is indicated by the scanty character of the seed-drift stranded on the beaches of the south of England, Scandinavia, the Mediterranean, and Southern Chile.

(2) The present distribution in temperate latitudes of littoral plants possessing buoyant seeds or seed-vessels is to be attributed more to the influence of geographical and climatic conditions than to the agency of currents. With some of them, such as those that occur on both sides of North America, it is evident that their distribution antedates the present climatic conditions within the Arctic Circle.

(3) Time has gathered on an English beach current-dispersed plants that could tell us strange stories of many latitudes.

(4) The seed-drift that is often found in such abundance in tropical seas is partly brought down by rivers and partly swept off the coast. Very little of the seed-drift brought down by the rivers from the interior is of any service for plant-dispersal, nearly all the floating seed-drift found at sea which has any effective value being derived from the plants of the beach and of the mangrove belt.

(5) The tropical beach drift of the Old and New Worlds reflects the characters of the littoral floras of those regions, more especially with regard to the plants provided with buoyant seeds or seed-vessels. The plants represented in the beach drift common to both these regions belong mostly to the Leguminosæ. The large fruits so characteristic of Old World beach-drift are not found in the New World. The number of shore plants with buoyant seeds or seed-vessels that are widely dispersed in the American region are only one-quarter or one-third of those in the Old World region; and this difference is reflected in the scanty character of tropical American beach-drift.

CHAPTER XXX
THE VIVIPAROUS MANGROVES OF FIJI
Rhizophora and Bruguiera

Rhizophora.—Represented by Rhizophora mucronata, Rhizophora mangle, and the Selala, a seedless intermediate form.—Their mode of association and characters.—The relation of the Selala.—Polyembryony.—The history of the plant between the fertilisation of the ovule and the detachment of the seedling.—Absence of a rest period.—Mode of detachment of the seedling. Capacity for dispersal by the currents.—Bruguiera.—The mode of dispersal.—Peculiar method of fertilisation.—Length of period between fertilisation and the detachment of the seedling.—Mode of detachment of the seedling.—Summary.

Between 1897 and 1899 I made numerous observations on the Fijian species of Rhizophora and Bruguiera (mostly around the coasts of Vanua Levu and in the Rewa delta); and these were supplemented in the early part of 1904 by observations on the first-named genus in Ecuador. I did not make any collections in Fiji until Prof. Schimper asked me to obtain specimens; and a fair-sized collection containing specimens dried, and preserved in spirit, was sent to him. His illness and death shortly followed, and I lost the advantage of his great experience in these matters. In a letter written to me in 1898 he expressed the hope that I would publish my notes on the mangroves of Fiji. Years have since passed by, and as I read again his words of encouragement I take up once more the interrupted task.