CHAPTER XXIX

BEACH AND RIVER DRIFT

In the south of England.—On the coast of Scandinavia.—In the Mediterranean.—Southern Chile.—Very little effective dispersal by currents in temperate latitudes.—Cakile maritima.—In tropical regions.—River drift.—River and beach drift of Fiji.—Musa Ensete.—The coco-nut.—River and beach drift of Hawaii.—Comparison of the beach drift of the Old and New Worlds.—Summary.

THE BEACH DRIFT OF TEMPERATE LATITUDES

DISPERSAL by currents seems to be mainly restricted to warm latitudes. Whilst in the tropics seed-drift is abundant on the beaches, in the cooler regions of the globe it is usually very scanty and often masked by other vegetable _débris_.

Let us take, for instance, a beach in the south of England. We can find by careful searching amongst the stranded drift the seeds and seed-vessels of various littoral plants of the buoyant group, such as Arenaria (Honckeneya) peploides, Cakile maritima, Crithmum maritimum, Convolvulus soldanella, Euphorbia paralias, &c., and such sundries as bits of stems of Salsola kali bearing fruits; but their amount is scanty; and they are often difficult to find on account of the great amount of rubbish with which they are associated, such as empty stones of cherries, plums, and peaches; empty seeds of grapes; hazel-nuts, beech-nuts, chestnuts, acorns, all either empty or with decaying seed; the spiral pods of Medicago; besides quantities of leaves, sticks, and bark. Although the occasional shell of a Spirula, or the horny skeleton of a Velella, or a genuine pumice pebble (see Note 76), may tell us of long wanderings in mid-ocean, we find little that is not English or derived from neighbouring coasts on a beach in the south of England. I have examined numerous beaches on the coasts of Devon and Cornwall, and have never come upon any indubitable tropical seed-drift.

On one occasion I examined many of the beaches between Ilfracombe and Padstow with the object of finding tropical seeds, but to no purpose. Portions of bark, generally 2 to 4 inches across and much water-worn, together with a quantity of steamer-slag or cinders, often largely composed the stranded drift. No doubt this bark is stripped off by the waves from floating trees, which are generally stranded in a bare condition after a long ocean voyage. This is the case with the timber brought in the Oregon drift to Hawaii; and Sernander (p. 117) remarks that bark seldom occurs on the trees washed ashore with the Atlantic drift on the coasts of Scandinavia. Modern marine deposits ought to contain much bark _débris_.

On the beaches in the vicinity of estuaries we find a certain amount of river drift, and amongst it fruits or seeds of Sparganium ramosum, Iris pseudacorus, Alnus glutinosa, Rumex, and many other river-side plants, such as I have mentioned in my paper on the Thames drift (_Journ. Linn. Soc. Bot._, xxix). Most of them are capable of reproducing the plant, but not on the sandy beach where the waves have stranded them; and we thus see here one of the limits of the efficacy of currents as seed-dispersers.

From the labours of Lindman and Norman, the results of which are summed up by Sernander (p. 116) we can learn what are the components of the “Atlantic Drift” on the Scandinavian coast; and a strange assortment we here find, in which it is difficult to detect much indication of effective seed dispersal. Besides the seeds of Cæsalpinia Bonducella, Entada scandens, and Mucuna urens, familiar to us as occurring in the drift of tropical beaches, there is a quantity of vegetable drift hailing sometimes from North America, sometimes from the Canary Islands, and sometimes from the West Indies, mingled with much local drift in which the larch and steamer-slag or cinders predominate. The seed-drift derived from the proper beach-plants of the coast plays a subordinate part, though it is stated by Norman and others that seeds and seed-vessels, as the case may be, of Arenaria peploides, Cakile maritima, Convolvulus soldanella, and Lathyrus maritimus, with those of other plants, are also to be found.

The Mediterranean beach drift, as illustrated by the results of my examination of numerous beaches in Sicily as well as in the islands of Stromboli and Lipari, and of the beach at Cumæ, is of a scanty nature. If we eliminate the various evidences of cultivation which seem to occur over much of the temperate regions of the globe, very little remains of an interesting character. As in the south of England and in other regions, the empty stones of the cherry, plum, and peach, the empty nuts of the oak, hazel, &c., together with the spiral pods of Medicago figure largely in the drift; and here and there we come upon the seeds of littoral plants, such as Convolvulus soldanella and Euphorbia paralias.

I have found Medicago fruits in all these localities on the beaches. They often contain seeds, which, it may be added, have no buoyancy, the seeded pods themselves floating from two to five days. The pods of several kinds of Medicago form the great feature of Sicilian drift and are often indications in other places of the vicinity of cultivated districts. A small hairy species thrives on Letojanni beach near Taormina, and I observed its seeds together with those of Euphorbia paralias germinating in the drift stranded on the same beach. Arcangeli, in his _Flora Italiana_, enumerates as many as thirty-three species of Medicago. Many of the species grow in maritime districts, and their fruits must often get into the beach drift independently of cultivation. I noticed the pods amongst the drift brought down by the Alcantara, a river near Taormina, a fact which goes to explain their presence in beach drift.... On the beach of Trogilus Bay, near Syracuse, I gathered several fruits of a Vitex, apparently V. agnus castus. After being kept afloat for six weeks in sea-water some were placed in soil, when they soon germinated and reproduced the plant.

The beach drift of temperate Chile is described in Chapter XXXII. There, as in other beaches of cool latitudes, it is not easy to find seeds amongst the rubbish; but amongst the scanty seed-drift may be recognised much of what we are familiar with in the Old World, such as the seeds of Convolvulus soldanella, bits of the fruiting stems of Salsola kali, as well as the rubbish indicating the white man’s presence, such as empty stones of cherry, plum, and peach, Medicago pods, &c. In addition, we find the seed-vessels of plants like Franseria and Nolana that are peculiar to American beaches; and now and then, the seeds of Sophora tetraptera, a tree of the immediately adjacent hill-slopes, come under our notice.

Before quitting this subject of the beach seed-drift of temperate latitudes, it may be observed that when at San Francisco I visited the beach running south from the Golden Gate. With the exception of the fruits of Cakile maritima, a plant growing on the beach, few other seeds or fruits were observed in the drift.

The inference that there is very little effective dispersal by currents in temperate regions is of some importance, and Sernander arrived at a similar conclusion when discussing the origin of the Scandinavian flora. The few plants with buoyant seeds and fruits, such as Arenaria peploides, Cakile maritima, Crithmum maritimum, Convolvulus soldanella, Euphorbia paralias, and Lathyrus maritimus, are no doubt thus dispersed, and Norman is quite right in attaching some value to the distribution by currents of certain plants within the region of the Arctic flora; but after all it amounts to little, and geographical and climatic conditions have often had a predominant influence in determining the distribution in the temperate latitudes of littoral plants possessing buoyant seeds or fruits.

Nowhere is this shown more plainly than with the littoral plants with buoyant seeds or seed-vessels that are found on our English beaches. Some have evidently acquired their present distribution before ice and snow reigned supreme in the extreme north. Though it may be possible, it seems highly improbable, that either Arenaria peploides or Lathyrus maritimus, both of which occur on beaches in high northern latitudes in the Atlantic and Pacific Oceans (as in Arctic Norway, Spitzbergen, and Behring’s Straits), could possess in our own day any means of communication between their areas of distribution on the borders of these two ocean-basins.

So again with Cakile maritima, the occurrence of this or of two closely allied species on both sides of North America cannot be attributed to any present working of the currents for two reasons. In the first place, as is remarked in Note 18, the results of two independent experiments made by me show that the fruits will not float more than a week or ten days in the sea, a capacity that will not admit of their transportation by the currents over tracts of ocean more than one or two hundred miles across. In the second place, this species is not an Arctic plant like Arenaria peploides and Lathyrus maritimus; and the possibility of inter-communication between the Atlantic and Pacific Oceans having any effective value from the standpoint of dispersal, shadowy as it is with the two Arctic species, is still more so in the case of Cakile maritima. Norman’s observations on the coast of Norway, as quoted by Sernander (page 123), indirectly indicate how hopeless it would be for this plant to attempt to traverse the Arctic region. Just as I have noticed on the north coast of Devonshire, the fruits occur plentifully in the beach drift and germinate freely in the upcast wrack as far north as Senjen in latitude 69°. Further north the plant has been recorded from only eight localities, and since it is there sterile and but a summer annual, the seed-vessels, it is argued, must have been brought by the currents from the south.

The reference to Cakile maritima as a summer annual on the north coast of Norway is of interest; but I may point out that it displays a similar behaviour in England on the north coast of Devonshire. Here, during the latter half of July, 1903, I found the fruits common in the stranded drift, and often in a germinating condition, whilst numerous seedlings one to two inches high with the fruit-shell still attached were growing out of the sand. From this arises the curious reflection that an annual which germinates in the end of July could scarcely be expected to mature its fruit before the winter. It would seem that this beach plant hampers its own dispersal by its misdirected efforts; and the idea suggests itself that we have here the explanation of its sterility in the north of Norway. Had it been a perennial like Arenaria peploides and Lathyrus maritimus it might have had a similar distribution within the Arctic Circle.

Quite other considerations seem to be suggested by the perennials Crithmum maritimum and Euphorbia paralias. In these cases, although the seeds or fruits, as the case may be, will float for months in sea-water without apparently sustaining any injury, the species are confined to the warmer parts of the European region.

From Convolvulus soldanella we obtain another story. Its occurrence in the temperate regions of both the northern and southern hemispheres, great as the floating powers of the seeds may be, is concerned with something more than with questions relating to modes of dispersal. The circumstance that in its distribution in the temperate regions it is practically coterminous with Ipomœa pes capræ in the tropics is very significant (see Note 49).

Each one of the English beach plants with buoyant seeds and fruits has its own story of the past to tell. Time has indeed gathered on our beaches current-dispersed plants, which, if they could speak, would tell us strange stories of many latitudes, stories of change within the Arctic Circle, and stories of great events within the temperate regions, and, as in the case of Convolvulus soldanella, stories of a past within the tropical zone. It cannot be said that investigators lack clues leading to lines of inquiry into the age that immediately precedes our own.

Yet valuable as our British plants would be for this purpose, they do not afford any indication that currents have played an important part in plant distribution in temperate and arctic latitudes. Ekstam strikes the true note for these regions when discounting the agency of currents in the instance of the Spitzbergen flora, he regards the wind as the greatest factor in seed-dispersal and after that the bird. The several interesting points raised by this botanist are discussed in Chapter XXXIII.

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:

Seedlings of Bruguiera and Rhizophora. Fruits of Barringtonia racemosa and B. speciosa, the first-named most abundant and often germinating. Seeds of Carapa obovata, most of them far advanced in germination. Fruits of Lumnitzera coccinea. Fruits of Cerbera odollam, abundant. Fruits of Inocarpus edulis, with the seed generally rotten. Fruits of Heritiera littoralis, Parinarium laurinum, and Pandanus. Empty seeds of Aleurites moluccana. Fruits of Scirpodendron costatum, abundant. Fruits of Clerodendron inerme and Smythea pacifica, both of them in some cases germinating. Pyrenes of Morinda citrifolia. Small fruits of Vitex trifolia and Premna taitensis, both sometimes abundant. Seeds of Entada scandens, Mucuna, and Vigna lutea. Pods of Dalbergia monosperma and Derris uliginosa, the last sometimes in a germinating condition. Seeds of Hibiscus tiliaceus and of different species of Ipomœa, such as I. peltata and I. pes capræ.

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.