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.