Botany: The Science of Plant Life

by Norman Taylor

While Entada scandens has spread in spite of unfavorable adaptation for seed floating, there are some plants whose seeds always float, and in spite of the sea water retain their power of germination. H. B. Guppy, from whom much of the above data are taken and who has experimented for years on the buoyancy and germinating power of sea-borne seeds, reckons about two hundred species that may have spread by ocean currents. Many of these are nearly world-wide in their distribution within their climatic requirements, and most are confined to the tropics.

2. Invasion, Migration, and Relics

It is perhaps a natural enough question why such elaborate and effective methods of seed dispersal are necessary and why plants, once they grow in any particular locality are not satisfied to stay there. The answer to this is that individual and racial competition is so great that without means of dispersal, which may be looked on as equipment for seeking a more favorable site, species would often be crowded out.

There is no better place to see this than at the edge of a forest and grassland. The presence of the forest tells us at once that what may be described as the forest type of climate must have existed in the past to have produced the woods. In clearing off parts of this forest the openings will usually be grassland at first, but never in the end if nature is allowed to work out the solution. Along the edge of such a forest will be found a host of pioneers pushing out among the grasses, making ready, by conditions of shade, protection from drying winds and other influences, for the seedlings of the forest trees that creep slowly but resistlessly out to capture areas that by right of previous occupancy belong to them. In practically all parts of the world, with a few local exceptions, wherever the forest was the original type there is this ceaseless struggle to reclaim the open places, often or usually peopled by grass. It may be set down as almost a rule that if the open places produce herbs with broader leaves than grasses, the forest will capture the area many years sooner than if grass alone is the temporary tenant. Grass by its exclusive growth, its complete monopolization, so far as low seedlings are concerned, of much light and nearly all available surface water, is singularly well able to take care of itself once it is thoroughly established. But plotted and marked areas of this contact between forest and artificial clearings in it, show that in the end the forest will win, often at the rate of five hundred feet in a hundred years, sometimes much quicker than this.

If, on the other hand, what may be called the prairie type of climate has resulted in the formation of grassland, which has happened in our own West, in the steppes of Russia and less extensively in many other places in the world, forests can hardly ever get a foothold. Where, as in river bottoms, they sometimes flourish, the line between forest and grassland is sharp and apparently an impassable barrier for trees.

This invasion of immediately adjoining territory is going on constantly not only by different types of vegetation but by the units of it. The frequency of different plants in different years, their final ascendancy or extinction, all point to the struggle for expansion which in a score of ways the plant world is constantly waging. In many cases we are not yet able to see the struggle, but only its results, while in some places the bitterness of it may be gauged by the dead and dying that strew these silent battle fields.

The dominance of certain species of plants, such as grasses on a prairie, the fir and spruce in the coniferous forests of our North, the blue-gum trees in parts of Australia and the giant dipterocarp forests in the Philippines, are all based on the ability of individuals to spread from their point of origin. All species of plants must have one day been of very local distribution and confined to the region where they were born, but from their often very modest beginnings some of them at any rate have gone to the ends of the earth. The common bracken fern is found in nearly every country in the world from far northward through the tropics to the antipodes, and yet no one knows where its original home might have been. Somewhere up in the northern Andes it is supposed that the first ancestor of the huge daisy family had its origin. From there its many descendants, now ramified into hundreds of genera and over eleven thousand species, have spread to the very limits of plant growth. And the daisy family is one of the most recent of all the families of plants.

The distribution of plants is of many kinds depending on local conditions of climate and soil, on individual and racial competition, on methods of fertilization or other means of propagation or seed dispersal, and particularly upon the distribution of the plant or its ancestors in past ages. While it is often difficult or impossible to determine upon which of these factors, or upon what combination of them the distribution of any particular species is based, certain facts of plant dispersal appear to be indicated by a study of existing floras. A few instances must suffice here to illustrate the principles by which many plants are scattered over the earth, or else restricted to localized regions, and which, without knowledge of the factors involved, seem merely the wayward caprice of nature.

In the flora of eastern North America there are many genera that appear to be endemic there (found nowhere else), but are actually duplicated in eastern Asia, if not as to species, at any rate by plants so closely allied as to be of obviously common origin. These plants are unknown in Europe, or on our own western coast. The skunk cabbage, sassafras, twinleaf, May apple, Canada moonseed, spice-bush, ginseng, sour gum, trailing arbutus, fringe tree, lopseed and many others, all fairly common in eastern North America, are unknown between this and eastern Asia where, if not the identical species, which often happens, closely related forms are duplicated. The explanation of such discontinuous present distribution appears to be that at some time in the distant past there was a land connection between Asia and the western coast of America, the remnants of which form the Aleutian Islands, and over which there was a constant migration of plants and animals. With subsequently changing climatic conditions on our own western coast, due to warm ocean currents, most of those Asiatic migrants, or it may have been a migration in the opposite direction, were crowded out by later types which now dominate the Pacific coast. There is small chance that these plants were spread by birds, as an east-west bird migration is hardly likely. Nor is there any record that the seed of these plants, even where they might float, would survive ocean transport.

Sometimes the ancestors of now widely separated species or genera once covered all or nearly all the intervening area, and again sometimes only a minute fraction of the ancient distribution is left at the present time. Our own Big Trees of California were once known to grow in England, Iceland, all through central Europe and eastern Asia, Australia, New Zealand, southern Chile, and from Texas to Alaska. In the face of such widespread occurrence in the past their present distribution over an area of a few square miles is merely a pitiful relic of ancient grandeur. Scores of cases are known where, instead of a single outpost as in the Big Trees, there are only a few widely scattered survivors from a probably much more continuous distribution in the past. In northeastern North America there grows along pond sides and fresh-water beaches the shore-weed, a relative of our common weedy plantain. The only other species, and a close relative, comes from the southern tip of South America.