Just as societies or races of men have often obscure beginnings, reach a climax, and afterward die, so these plant societies may be considered as exhibiting a similar progression. What these plant societies are, at least the more important of them, will be considered in the next section of this chapter. It should never be forgotten that the species of plants making up the dominant plant societies in different parts of the world are dictated by quite other conditions than those that result in the dominance of the society itself. Perhaps as good an illustration as any is the aristocratic type of mankind, recognizable throughout the world by the possession of finer qualities than the common run, but differing in individuals as much as the best type of Americans, the British peerage, and the samurai of Japan differ one from the other.
Not the least interesting feature of these plant societies is that we must view them as associations of plants, often of widely differing origin due to the vicissitudes of plant distribution, but all taking their part in the society to which they belong and often, as individuals, losing their life that the society may live. Upon such a conception a wood or prairie, or river bank, or salt marsh or alpine garden upon a mountain summit are, with many other plant societies, places of intense conflict. More cruel than any human society, these plant communities exist under conditions where only the individually strong survive, and only those societies are destined to reach their climax which can take advantage of every aid, quite without regard for severe losses or even death to the individual members of it. It is as if we poured into a crucible molten metals from many different sources, and after the incredible and relentless forces of manufacture had worked their magic upon them there resulted a product, purified and cleared of all dross. So the inexorable and relentless processes of nature work over the materials found in these plant communities, the results of which are the dominant types of vegetation in the world to-day. With this understanding of the part they play in plant distribution we may now consider a few of the most widely recognized plant societies and see how they have affected the vegetation, sometimes even the history of the regions in which they are found.
4. Some of the Chief Plant Societies
Forests
No one who has ever seen both our temperate forests and those in the tropics can fail to be impressed with the difference between them. Not only for the different plants in them, but for their wholly different aspect, tropical and temperate forests stand far apart as an expression of the forest covering the earth. Not all of us realize, however, that the heat of the tropics is not the deciding factor in the luxuriance of those dim jungles, and that a rainfall far above anything occurring in the United States is even more important. Upon the distribution of rainfall depends the occurrence not only of the two forest types that will be mentioned here, but of most of the other chief plant societies.
TROPICAL RAIN FOREST
A small section along the lower side of the Gulf of Mexico, the northeastern edges of Cuba and Santo Domingo, nearly all of the region drained by the Orinoco and Amazon Rivers; in the Congo, Zanzibar, and Madagascar in Africa; all of southeastern Asia, including the East Indies and part of the north coast of Australia—these comprise the regions of the tropical rain forest. All of them have, besides continuous heat, a nearly continuous or in some places a periodic rainfall, averaging over, and usually much over, eighty-five inches a year, as compared with about half that near New York. There are, of course, other places in the world where these rain forests, so called from their abundant moisture and some of the effects of it, are found. But in the regions mentioned they are at once the most wonderful and to most white men the most awesome manifestations of the plant world.
Such forests seem, and actually are, pulsating with life, as instruments stationed in them have many times proved. With some kinds of bamboo growing over two feet a day, and a eucalyptus tree in Java forty-five feet in three years—and these are not isolated cases—the tremendous annual increase in the amount of vegetation can be glimpsed. Of course not all the plants in them grow at any such rate, but the great heat and abundant moisture does make tropical rain forests irresistible in their power. Plantation owners, and railways that have been run through such forests, wage constant warfare against the recapture by the teeming forest of man’s intrusion of it. The writer once saw in Santo Domingo a railway cut through such a jungle and abandoned only two or three years before. Not a trace of the roadbed could be found, ties, rails, and switches all covered with a dense vegetation, and overhead the canopy of the forest had closed over the opening and was already sending down hundreds of adventitious roots that would complete the obliteration of man’s handiwork. Everywhere there is the evidence of vegetable life run riot, ever crowding and pushing to close up openings made by the crashing down of old trees or the artificial clearings of man. Those living on the edges of such forests speak and think of them as dim, mysterious places where strange creatures and the ever-present fevers join forces with the vegetation to keep out humankind. That they are places of actual danger everyone knows who recalls that Stanley’s trip through equatorial Africa cost one hundred and seventy lives, many of which were sacrificed to disease and strangely enough nine were lost through starvation. While the tropics supply much of the food used there, these jungles produce almost none of it and because of the scarcity of edible fruits, the extraordinary difficulty of getting about and collecting what does grow, starvation faces anyone who goes into them without adequate supplies.
In the Amazon grows the largest water lily in the world, Victoria regia, with giant leaves upon which a moderate-sized man may stand in safety. It produces a flower over a foot in diameter, and it is surrounded by a forest the like of which it is difficult to describe. H. H. Rusby, who spent two years in this region hunting for medicinal plants, has described the country a few hundred miles below where Victoria was discovered. He writes: “Passing down the river Madeira to the lower Amazon, we come into a region of such grandeur in its vegetation that it is difficult of comprehension, even by one who is an eyewitness. Everything is in such proportion that one is apt in its size to miss the gigantic. Many of the trees of this region are undoubtedly many centuries old and appear to be good for many centuries more. Most of them have enormous buttresses at the base, and these buttresses often begin as high above the ground as are the tops of ordinary forest trees in our land. All are bound together with an impenetrable mass of tough vines. Running through these swamps are the most beautiful little bayous or canals. Nothing can exceed in interest and delight a day’s canoeing among these narrow waterways, although there is great danger that the inexperienced boatman will hopelessly lose his way. In the rainy season this river rises sixty feet or more above its low water mark and the boatman travels among the tree tops which a few weeks before were high above his head.”
The abundant water supply in the rain forests results in an atmosphere saturated with water vapor and in some of them it is a common sight in the morning to see the forest rising out of an unbroken blanket of mist. As this dries up under the heat of the day, or if there occurs one of the torrential downpours to which such regions are frequently subjected, there rises from the forest in plainly visible waves a vast quantity of water vapor. It is this that has so often made them be described as steaming forests. The water requirements of the plants are more than supplied, nay, there is such a surfeit of available water in all these forests, that there are numberless devices to get rid of the excess. Dripping points to the leaves, already described in an earlier chapter, are common. But in addition many plants have wonderfully colored leaves such as Begonias, some relatives of our jack-in-the-pulpit belonging to the Arum family, many orchids, and other plants. The colored leaves in the predominantly dark green and gloomy rain forest, because of their greater absorption of light and consequently higher transpiration rate, are of decided advantage.