THE WIND
The great trade winds, the violent hurricanes and monsoons of Asia, are all active and constant aids to plant dispersal. The fruits of many plants are provided with various devices to insure buoyancy in the air, such as maple, ash, and most seeds of pines and their relatives, birches, poplars, and willows, all of which may be carried distances of a few hundred feet in ordinary winds. In the buttonball tree, the milkweeds, fireweed, and many other plants there are feathery attachments or plumes that insure their seeds or fruits being carried very long distances indeed in regular or violent winds. But in the daisy family or Compositæ we find nearly all the eleven thousand species provided with a plumelike attachment of their light fruits, familiar enough in the dandelion, which may explain their being more widely distributed than any other plants. Some of them, however, are carried by animals, as they produce, in certain genera, barbed or hooked achenes.
The whole seed of some plants is so small that it can be lifted bodily by the wind, for instance Rhododendrons and many other heaths, one of the hanging pitcher plants of tropical Asia, many orchid seeds, and of hundreds of other plants. Many of these are less than one ten-thousandth of a gram in weight and, with dust and the spores of nearly all cryptogams, may be transported thousands of miles. When it is recalled that the dust from a volcanic eruption at Krakatoa in the Pacific was picked up on London window sills, and that some seeds and nearly all spores are as light as most dust, the wind as a plant dispersal agent becomes significant. In the western part of the United States over 800 million tons of dust are carried over 1,400 inches in a single year, according to estimates by J. W. Evans in his article on “The Wearing Down of Rocks.” Some effects of wind dispersal of seeds and spores furnish interesting data to the plant geographer.
In the Bahamas the natives speak of “hurricane grass” as a plant that was unknown on Great Bahama Island before August 13, 1890, when there was a great hurricane. Soon after this the sedge, which had been blown over from another island, began to be common on Great Bahama, where it is now thoroughly established. On Krakatoa near Java a violent eruption in 1883 completely destroyed the vegetation, covering the island with volcanic material. In thirteen years over sixty species of plants had arrived on the island, of which about twenty had been blown there as seeds or spores. Birds had carried about 7 per cent and the remainder had come by other means. No plants with sticky burs or other devices for catching in the coats of animals were found.
It is among the Compositæ or daisy family that the wind is seen to work most effectively as distributing agents of its light-plumed fruits. In the Falkland Islands, St. Helena, and Prince’s Island, all from 300 to 1,500 miles from the nearest land, species of Senecio or groundsel have been found that may well have been wind-driven onto these remote islands from the mainland. In fact the whole genus Senecio, consisting of over two thousand species, and of world-wide distribution, has in all probability been spread largely by its wind-borne achenes. They are not edible, nor are they suited to carriage by the ocean currents, but some of them are known to stick in the plumage of birds.
But it is hardly necessary to cite these far-flung examples of the wind’s action in distributing seeds, for there are many interesting cases much closer home. The tumbleweeds, such as the false indigo and certain grasses, are familiar sights scudding before the wind over prairies in the West and open places in the East. By a kind of foresight the winged seeds of the pines are so weighted, due to lack of symmetry, that instead of sailing quickly to the ground, they tumble and flutter about, thus prolonging the time of flight. And in the linden the curious winglike attachment from which the stalk of the fruit arises is admirably fitted to slide over the snow and ice upon which, through their often tardy falling, they are deposited. Scores of such adaptations of structure to function are known, where there seems direct response to conditions and there is the temptation to say that such adaptations are caused by the wind or other agency. Nothing could be farther from the truth, as plants do not produce winged seeds or luscious fruits to insure seed dispersal, but the seeds of plants having such devices, from whatever cause, are naturally favored in the ceaseless struggle to occupy new land, which is quite another thing.
WATER
The scattering of seeds along streams is too common a process to need more than a mere mention here, for it is to be seen along any fresh water stream at harvest time. But seeds that may be carried by ocean currents have a much greater influence on the distribution of plants, such as the coconut palm, already mentioned among the food plants, and now common throughout the tropical world.
The number of seeds that will float in sea water and still keep the power of germinating is not very great. Most seeds sink at once; many will float for months, but are useless when they reach land; but those that will both float and grow afterward have worked some curious changes in the floras of different islands. Sometimes the great ocean currents, like the Gulf Stream, appear particularly futile in the fruits they carry such long distances, as the pods of a tropical vine from the West Indies are not infrequently found on the coast of Norway and even of Nova Zembla, of course uselessly. But many ocean currents, particularly in the Pacific, have carried fruits and seeds thousands of miles, they have even carried so-called floating islands of vegetation bodily.
Perhaps the most remarkable case is that of Entada scandens, a tropical vine of the pea family, bearing large pods, sometimes several feet long. The plant is not typically a seaside plant, and there is evidence that fruits matured in the shade of its usual forest home will not float. Those that grow nearer the coast and more in the open develop, through partial drying, a small air chamber inside upon which the seed depends for buoyancy. Its original home is apparently somewhere in Central America, from the west coast of which it spread over the Pacific to the shores of the Indian Ocean. Over the Atlantic it has reached the shores of tropical Africa, and in fact wherever ocean currents cast up their refuse on lonely beaches parts of the giant pod or individual seeds of Entada scandens are found. Partly fossilized remains of them have been taken from peat bogs along the coast of Norway, of course dating since glacial times, but showing by their presence there how long this water-borne seed of the pea family has been attempting to populate the earth. Of course all that do not reach tropic shores are lost, but nearly throughout the Pacific Islands, with some exceptions, the vine is now established. The extraordinary feature of it all is that scarcely half the seeds of the plant will float at all, nearly all inland forms sinking at once in the fresh water rivers into which they may chance to fall. Only those that grow near the sea, in mangrove swamps and the like, or at any rate near brackish water, will float. These, however, apparently float indefinitely without loss of germinating power. There is abundant evidence that many plants of oceanic islands have similar characteristics so far as their inland and seaside forms are concerned.