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.
While there is thus very little or in fact almost no struggle for water in the rain forest, the struggle for light is intense. In the deepest and most luxuriant of them the gloom of the forest floor is notorious and it was by no means a figure of speech for Stanley to describe his trip through equatorial Africa as “Through the Dark Continent.” So dark are most rain forests, and, as we have seen in a previous chapter, so inexorable are the plants’ demands for light, that the various devices to insure it are perhaps the one great difference between these forests and those of temperate regions. One effect of the struggle for light is the enormous production of vines often running hundreds of feet through the tree tops. In India the Calamus, or rattan palms, with stems no thicker than a walking-stick, will completely interlace the foliage of the canopy. Thousands of slender whiplike roots and stems of such plants descend from the topmost heights of the forest canopy, where the plants to which they are attached make such an inextricable tangle among the tree tops that orchid collectors have been known to travel considerable distances over the matted vegetation, with, it must be confessed, considerable danger. These vines or lianes as they are called, are however, often as thick as a man’s body and armed with great hooked prickles, an obvious aid in catching some support to reach that essential light for which all plants in such places are ever striving.
Besides the bewildering tangle caused by these lianes, the rain forest is further impeded by hosts of epiphytes or plants that are mechanically attached to tree trunks, branches, or anything else that will raise them to the light. Of all the plants of such regions the epiphytes are the most light-demanding. They must not be mistaken for parasites, as they have roots of their own through which they absorb nourishment, mostly as water vapor, but also as liquid water held in the bark and refuse in which they grow. Thousands of orchids are epiphytes, also ferns, and, only in the American tropics, thousands of different relatives of the pineapple. Many of the latter are among the most gorgeously colored of all plants, their superb foliage being much sought after and the specimens largely grown in our greenhouses. In most rain forests every available inch of space is covered by these epiphytes, so that no bark, scarcely any branches, are to be seen but those clothed in this motley array of plants that use the support to get the utmost possible light. Many of these epiphytes have rosettes of leaves arranged for holding water, and after a sharp thunder shower followed by fresh wind the writer has seen the ground strewn with thousands of relatives of the pineapple which, with the added supply of water, were unable to stand the strain and were consequently wrenched from their lofty perch. So enormous is the combined weight of these epiphytes, together with the lianes, that many trees crash down under the strain long before their time. Perhaps no sight of the rain forest so convinces one of the struggle for light as to see one of these forest monarchs come crashing down loaded with thousands of plants that have been using it for support, and to escape which it has pushed its canopy to the utmost limits of its growth. Such contests are common in a forest of which only the barest outlines can be conveyed to those who have never seen it. To those who have had that good fortune any description palls beside the wonderful actuality.
It is scarcely to be wondered at that these steaming rain forests with their gloom, and, as they were once described, “all hung about with fever trees,” should be dreaded by many, and the subject of fabulous tales to the credulous. The almost incredible difficulty of getting through them, not to mention the savage animals that inhabit many of them, have not lessened the tendency to exaggerate about these great forests. But the truth about them is so far beyond belief, the strange plants that intrepid explorers have brought out of them so almost incredible, that it only excited a temporary wonderment when the largest flower in the world was discovered in such a forest in the Malayan Archipelago.
Sir Stanford Raffles and Dr. Arnold, while exploring in Sumatra during the year 1818, discovered what was called “the greatest prodigy of the vegetable world,” and no flower since found equals its size. The plant, without stem or leaves, consists wholly of one gigantic flower about nine feet in circumference, and was subsequently called Rafflesia Arnoldii. It aroused a sensation in England which was not abated by knowledge of the fact that the flower is a parasite on the stems of certain tropical plants related to the grape. That such a huge flower should be the product of a parasitic mode of life is one more illustration of how this and related irregularities occur in widely separated families of plants, and under varying conditions. Relatives of it have since been found in India, some parasitic on roots, others, as in Rafflesia Arnoldii, on the stems of vines. The sticky seeds are in all probability carried from place to place on the hoofs of elephants, to which they have been known to cling. Only if they are deposited on a bruised or otherwise exposed tissue of their future host can they grow. These curious plants have been actually cultivated in the greatest tropical botanical garden in the world, at Buitenzorg, Java.
The original collectors of Rafflesia Arnoldii could scarcely credit their senses when they saw for the first time this extraordinary plant, whose whole life is spent in producing this great flower and fruit. As one of them says: “Had I been alone, and had there been no witnesses, I think I should have been fearful of mentioning the dimensions of this flower, so much does it exceed every flower I have ever seen or heard of.” The odor of the flower is repulsive, and, with its great size and curious mode of growth in the dark rain forest, it is surely one of the strangest productions of the vegetable world.
But as the utmost development of the plant world, and producing the greatest profusion and richness of plant life, these rain forests are, beyond the sporadic occurrence of such wonders as Rafflesia and some others, places of extraordinary interest. With every inch of space occupied by plants, the very epiphytes often having on their leaves still smaller plants, we see here what nature will produce when the maximum conditions for plant growth are so nearly perfect. Theodore Roosevelt in his book, “Through the Brazilian Wilderness,” gives a vivid picture of the rain forest there, and it may well end our account of those most interesting of all plant societies:
“In one grove the fig trees were killing the palms, just as in Africa they kill the sandalwood trees. In the gloom of this grove there were no flowers, no bushes; the air was heavy; the ground was brown with moldering leaves. Almost every palm was serving as a prop for a fig tree. The fig trees were in every stage of growth. The youngest ones merely ran up the palms as vines. In the next state the vine had thickened and was sending out shoots, wrapping the palm stem in a deadly hold.
“Some of the shoots were thrown round the stem like the tentacles of an immense cuttlefish. Others looked like claws that were hooked into every crevice, and round every projection. In the stage beyond this the palm had been killed, and its dead carcass appeared between the big, winding vine trunks; and later the palm had disappeared and the vines had united in a great fig tree. Water stood in black pools at the foot of the murdered trees, and of the trees that had murdered them. There was something sinister and evil in the dark stillness of the grove; it seemed as if sentient beings had writhed themselves round and were strangling other sentient beings.”
Many other forests in the tropics, where the rainfall is less, or less regularly distributed, are not unlike our own, having rather regular periods of leaf-fall that come with the dry season rather than with the autumn. The trees are of course never the same as ours, but the general aspect is not very different from that of temperate forests.
TEMPERATE FORESTS IN AMERICA
The transition from the tropical rain forest to our own woodlands is one of the most interesting, as it is certainly the most gradual in nature. Lack of space prevents our stopping to note those strategic points along this pathway from a hot, steaming forest to the cool shade of our open woods, where traveling, in at least a virgin forest, may be done easily on horseback. As we come northward, and if we could travel continuously through the forest, we should lose first the epiphytes, then most of the lianes, and finally all the condition of vegetation crowding into every inch of space suitable for it. While trees in our virgin forests are as thick as they can be, the forest floor is open and on it grow only a few herbs that will stand lack of sunshine.
But the really great difference is the long, unfavorable season in temperate regions where the forest must drop all its leaves, after, in at least our own Eastern States, the most gorgeous foliage coloring of any forest in the world. The winter months when the woody vegetation is practically fully exposed to the elements, are particularly severe in their effects. Leaf-fall, which is such a common sight as to arouse scarcely any interest, is the only device by which the great bulk of our forest trees survive, and only in the southern part of the region are there found such woody plants as the mountain laurel, rhododendron, American holly, and a few others which are evergreen but not cone-bearing ones, and are the only reminders of the truly evergreen forests of the tropics. The winter winds farther north and in the central treeless part of the United States prove too much for many kinds of trees, for instance, all the oaks and sassafras, none of which go very far north.
There is sufficient rainfall to produce forests much farther north than they are found, but lower temperatures prevent trees from growing just as too little water stops their growth altogether. Toward the northern limits, or upon high mountains, the upper limits of the forest, we get the best idea of how persistent woody vegetation is in the general forest area of the eastern United States. Stunted, wind-swept and weather-beaten trees are often found only a couple of feet high and over sixty years old. Sometimes they will be flattened out on the ground or on bare rock, making great patches of bushy growth quite unlike their lofty relations in the lowlands. The growth rate for such plants is so slow that their annual rings are all but obscured. With such persistence in the production of these elfin forests, high up on mountains under the most unfavorable conditions, it is little wonder that below this are trackless woods, and that the northeastern United States has one of the finest developments of the temperate or summer forest in the world.
Nor are all our woods of this general type made up of the same species, for everyone knows about the endless spruce and fir forests of the north, exclusively evergreen, and in the summer nearly always moist. This spruce belt stretches practically across the continent, where, in the West, other and our most gigantic evergreens, replace the eastern spruce and fir. A little farther south is the region of the white pine now nearly unknown as a virgin forest type, as its great value led to early and ruthless cutting. The white pine region is generally the area from New England southward along the Alleghenies and westward to Illinois. But the most characteristic of the temperate forest types is our summer forest, so called from its general lack of evergreens and its beautiful green foliage of summer and its bare branches in winter. Beech and birch and maple, in different proportions according to local conditions, predominate in such woods. These hard-wooded trees, with many others that are scattered through them, have been among the most valuable of all the natural plant products of our country and their destruction has been upon such a scale that only in a few places may the virgin forest be seen at the present time. Where it does occur we find the forest floor often with nothing growing on it except a mass of spring flowers which are half matured before the leaves of the forest canopy close out nearly all the light and much water and put them to rest until another year. The great preponderance of spring-flowering herbs in Eastern North America is due to their early warming up before the foliage of the trees cuts off their light. And in some virgin forests of this sort, particularly where there is a large mixture of oak, the writer has seen hundreds of square rods without undergrowth or herbaceous vegetation of any kind. Such places, very rare indeed at the present time due to senseless and wicked cutting, are rather dark, perfectly open to view for hundreds of feet ahead, and dotted only with the huge trunks of the trees that characterize this climax type of the temperate or summer forest.
The absence of direct sunlight and interception of much rain under the forest canopy has other effects besides stopping the growth of herbs and shrubs which are common enough along the edges, or where openings are made by the fall of old trees. It prevents the germination and growth of nearly all the seeds falling from such trees, and in a really virgin forest of this sort, almost no seedlings will be found. Upon cleared or open land thousands of saplings will cover much of the ground, but nearly all these will die off due to crowding, and leave as the climax only enough trees to close over the forest canopy.
Forests may be found in all stages of succession from those just beginning the process to those final forest monarchs which, having won out in the race, are, until one of them falls, often slow to perpetuate the type. For, as often as not, a new growth will spring up once a very large tree falls, and a very different kind of growth from the climax forest. At once a host of species, that one might almost say had been waiting for the tragedy of the monarch’s fall, will rush in and convert the opening into a nondescript brush patch, out of which will rise another tree that means business. As it grows to maturity, it kills off these smaller triflers one by one, until, when the canopy is finally closed, all of them will have disappeared, or, as often happens, retreated to other parts of the forest, where they wait for another chance. This succession of different kinds of growth in a temperate forest is so well known that in England they have for centuries practiced it, for commercial or pleasure purposes. In their oak-hazel copses they cut the trees enough to partly open the canopy, which permits a dense growth of hazel bushes and other plants. Every twelve or fifteen years the latter are cut down for various purposes, and will gradually spring up again to renew the dense growth. The spacing of the trees is sufficient for them to branch freely and yet not close the canopy enough to kill off the hazel. The trees are cut off a few at a time, not oftener than one or two hundred years in any one spot. By this procedure the owner gets a regular crop of hazel once in twelve or fifteen years, occasional big trees, and on many places a cover for pheasants. Under the hazel there is a regular progression of herbs, very plentiful just after the bushes are cut, and decreasing almost to nothing when the end of the growth period of the bushes is near.
The English oak-hazel copse, now much less grown than formerly, and the general lack of undergrowth in our own virgin forests, are both responses of the forest to light and other factors that are related to an open or closed canopy. As we stated a page or two back, it is not impossible, it is even frequent in some parts of the country, for a forest to produce, by its own growth, conditions inimical to its perpetuation. Where the casual falling of a forest giant is the only opportunity which that forest offers to perpetuate its type, it may well be said to be a climax forest, incapable of further development. But in some such woods a curious provision of nature insures an invasion of the gloomy forests by trees less light-demanding than the dominant ones. And often these trees that can get along with less light will capture considerable parts of forests that light-demanding trees could never conquer.
If the soil in which these temperate forests happen to grow is sandy or otherwise poor in plant food, the broad-leaved trees that make our woods such a delight in summer are replaced almost universally by pines. Along the sandy stretches of the coastal plain from Long Island, New York, to the Gulf, there are immense tracts of these pine forests, different species often being locally dominant, such as the pitch pine in the pine barrens of New Jersey and the long-leaf pine farther south. Almost throughout the world there is this monopolization of the poorer and drier forest sites by pines, which maintain the forest plant society in regions where the broad-leaved rapidly transpiring trees could not grow.
No account of forests, however brief, can omit some mention of the greatest agency for their protection in North America, the United States Forest Service. With corporation and individual cutting and attendant fire hazard upon a scale almost beyond belief in its ruthless disregard of our chief natural plant product, the Government soon found that Federal ownership or control of forests was the only policy that would maintain even a partially adequate timber supply. National forests, set aside either for pleasure or profit, now total more than the area of France or than all the New England and most of the Middle Atlantic States. These huge tracts, in every part of the country where forests are found, are well managed, properly planted, and most important of all, constantly guarded against fire. Forest fires not only destroyed over $25,000,000 worth of timber annually, but leaving devastation behind them, depleted the water supply in many parts of the country. Nothing but forests will hold the rainfall, to release it slowly through a thousand rivulets and springs that are the source of countless rivers. With the forest cut or burned off these streams are dry most of the summer and raging torrents for a few weeks in the spring, washing out all the priceless accumulation of the ages which the forest has conserved for its own and our benefit. While the reservation of these great national forests has worked individual hardship, experience for many years back in India and Germany shows Federal ownership or control the only wise policy.
Forest covering, whether temperate or tropical, depends for its occurrence all over the world upon an adequate rainfall. As we have seen in the tropics, this may be so great that coupled with the heat it produces a wealth of vegetation beyond the powers of description. Where it is less and the country cooler, the forests are of a different type, but even there the forest covering is, without interference, practically complete. Where, as in parts of Chile, southeastern Australia, and of Japan, there is a heavy rainfall but cool climate, there is a so-called temperate rain forest. Such forests are cold, drab, wet woods of peculiar aspect and extreme interest. For in them grow trees sometimes related to our own, but, due to the special conditions, producing a forest landscape quite unlike anything in America. It would seem as if we might almost plot the distribution of forests in our own country with a weather map showing rainfall, and such is actually the case. When the rainfall becomes less than will maintain forest growth it stops, often very abruptly. Generally speaking, the region west of the Mississippi, and some just east of it, westward to the mountains, is entirely devoid of forest, except in the river valleys. The forests give place to an entirely different type of vegetation—the prairie or grassland.
THE PRAIRIE OR GRASSLANDS
In nearly every region in the world there is an absence of forests and a replacement of them by grasslands, where the rainfall is less than about twenty-five inches a year, and where the winter winds, often far below the freezing point, are hostile to trees. The distribution of the rain mostly through the growing season also makes a condition peculiarly unfavorable for trees during the winter. Someone has said that the nations have fought since the days of the Romans for the belt of grassland which these climatic conditions have produced all round the world, and it is certainly true that these naturally grass-covered areas have produced the cereals of the world, all of which, except rice, grow to best advantage in such regions.
With of course different species of grass and quite different associated herbs, these grasslands are now found in our own prairies, the steppes of Russia, the plains of Hungary, the pampas of southern South America, the grasslands of Australia, the veld in Natal and in many other but mostly less extensive developments. Some of the grassland regions are warm, but without more rainfall than characterizes such areas the greater heat does not produce a forest. Usually, but not always, these grasslands are not found near the coast, where, as in America, the rainfall is double that of the plains and produces the forests that clothe the Atlantic and Pacific sections of the country. From the east westward there is a gradual decrease in the rainfall, until from about the Mississippi to the mountains it falls below the point where trees can compete with the prairie.
Another characteristic of prairies that once they have started tends to keep out trees is their almost annual firing. Tree seedlings cannot survive this, and we know that the Indians fired huge tracts of prairie every year, not to mention fires started by lightning which may set fire to grasslands and actually does set fire to forests every year.
The prairies in the United States—perhaps the most extensive in the world—are characterized chiefly by several grasses, buffalo grass (Buchloë dactyloides), gama grass (Bouletoua oligostachya), and several prairie grasses, such as Sporobolus asperifolius, Koeleria cristata, and some others. Among these, depending on the soil, are hundreds of prairie flowers which, during different parts of a single season, give quite different aspects to the region. Both the grasses and their associated herbs are well protected against too violent transpiration which their exposure to nearly continuous sunshine, high summer heat, and very considerable winds makes particularly active. In many places where the country is rolling, the lower and moister sites, besides developing more luxuriant growth of prairie plants, permits low shrubs, and in river bottoms even trees to flourish. But climatic conditions of small rainfall, high winds, and bitter winters make anything like a forest development out of the question.
In some regions, both in America and central Europe, a rainfall that is high enough to permit trees to exist and low enough to favor at the same time a grassland formation has resulted in the parklike landscape that creates the most beautiful scenic features of the regions where it occurs. In such places there are irregular patches of forest and grassland, and the struggle for supremacy as between the different types depends, not upon the general climatic conditions to which they both respond, but to local conditions of available water supply and soil conditions and often upon fires. Naturally such regions are places of intense strife for dominance, and in them some remarkable collections of plants have been found.
One of the most interesting of these struggles between grassland and woody vegetation in a region climatically able to produce both, is in Natal. Large sections of that country are grasslands or veld, as the people there call it. Scattered through the veld are various species of acacia trees, locally called thorn, with feathery compound leaves. These do not shut out enough light to prevent the development of grasses directly under their shade, yet the annual firing of the veld prevents, except accidentally, the production of the acacias. But the seeds of this tree, whether from long usage to this burning or not, are actually hastened in their germination by the firing, and it is a common practice in that country to roast or partly boil the seeds of the tree to hasten germination. The presence of the climatically favorable environment for both trees and grassland results in the latter being the dominant type of vegetation over large tracts of the country, largely because fires destroy tree seedlings, and yet the tree seeds, by a quite extraordinary fitness for their peculiar environment, offer a measure of insurance against the total destruction of woody vegetation by the grassland.
The pampas of the Argentine have been vividly described by P. G. Lorentz, who, in writing of the drier parts of it, says: “Viewed from a distance, these grasses seem to form a close grassy covering, and the pampa presents the appearance of extensive grassy tracts whose coloring varies with the seasons: coal black in the spring, when the old grass has been burned; bright green, the color of the mature grass; finally—at flowering time—when the silvery white spikes overtop the grass, over wide tracts it seems like a rolling, waving sea of liquid silver.
“After the Gramineæ (grasses), the family of plants that is represented in the pampas by the greatest number of individuals is that of Compositæ (daisy family); usually twiggy undershrubs with inconspicuous flowers, a bright yellow Solidago (golden-rod) alone gleams out from among the others.”
Here, as in the other grasslands of the world, if a local water supply above the general requirements of the grass exists, there is always a small element of woody plants, low, thick-leaved shrubs usually, but where water is more plentiful, trees, as in the region in Natal, already mentioned, and in many other parts of the world, notably parts of Australia, China, Brazil, and many sections of the western part of the United States.
We see in grasslands a plant response to rainfall and other climatic conditions which, with a little more rainfall, or in locally wet places, always produces woody growth, either as tongues of woods in river bottoms or the parklike landscape, already mentioned. If, however, the rainfall is too low to produce even the grasses and their associated herbs, an entirely different type of vegetation usurps these still more dry regions, resulting in some of those strange plants of the deserts, among which water storage is practically universal, as is the ability to live for long periods without rainfall.
DESERTS
Of absolute deserts there are none in North America, for no part of it is so dry that plants of some sort do not grow, and in fact in hardly any part of the world are there regions of any considerable extent where plant life of any kind is lacking. There is a small section of northern Chile and adjacent regions on the western slopes of the Andes where nothing grows, and the traveler is met with a cheerless landscape of bare ground and sandy or stony soil. There is no record of it ever having rained in such places, and if there be only a single rain consisting of a fraction of an inch a year, a few plants, usually scraggly low herbs with thick leaves or else quick-flowering annuals will be found.
But there are hundreds of thousands of square miles of the earth’s surface where the rainfall is sufficient to produce plant life, but where it hardly exceeds five or at most six inches in a year, and in some regions is less than two inches. From several different stations in the desert area of our own Southwest the average annual rainfall is about five and a half inches. Such regions produce a desert type of vegetation and are popularly, if not properly, called deserts. The largest is of course the Sahara in Africa, but there are huge tracts in Arabia, China, Thibet, Australia, South Africa, and in some other countries where the conditions for plant life are so unfavorable that only in the better sites are plants found at all. In all deserts there are very large areas entirely without plants, due to shifting sand or other local conditions, which, added to the generally unfavorable climate, make plant life impossible.
With rainfall so low and in most of the regions the temperature so high, the vegetation must be insured against too rapid transpiration. Perhaps the best illustration of how high transpiration both in plants and in man may be in such regions is gained by a statement of D. T. MacDougal, who writes of man traveling in the desert that “the amount (of water) thrown off the skin is correspondingly great, and if the loss is not made good, thirst ensues and ten hours’ lack of water may thicken the tongue so that speech is impossible.”
Under such conditions it is not surprising that desert plants are among the most curious and weird members of the plant world. Every device both to retard transpiration and to store up water to last over a completely rainless period, may be found. In America, to which all the hundreds of different species of cactus are practically confined, we find giant forms, often covered with spines and prickles which prevent their destruction by cattle, and many others that hug the dry sandy soil with curious tortuous branches. None of them have leaves such as plants of the forest or grassland possess, for desert plants cannot afford the luxury of foliage that, because of too rapid water loss, would destroy their chance to survive. Cacti do produce tiny leaves at the ends of their joints, but as if recognizing the inhospitable world into which they are born, practically all of them drop off, so that for the great bulk of the life of most cacti only the bare branches are evident. In most kinds these branches are green, assume the functions of leaves, such as transpiration and the manufacture of food by photosynthesis.
While in cacti and in the giant cactuslike spurges of South Africa the ability to store water is tremendously developed—our giant cereus or saguaro often holding 125 gallons—most desert plants rely upon retarding transpiration for their existence. Leafless shrubs and trees whose often spiny branches are green and perform, on a much-reduced scale, the function of leaves, are among the most common characteristics of desert plants. Some, as Parkinsonia microphylla or paloverde, have tiny leaves which they put out during the spring showers, but quickly lose them as it gets hotter and drier toward midsummer. Many of the plants that do produce leaves regularly have the surface of them so shiny as to appear varnished, or so thickly coated with hairs as to simulate cotton or wool, both of which reduce transpiration. There are many plants, some of which do not even live in a desert but in a locally dry habitat, that also have the utmost development of structure to prevent transpiration. One of the most extraordinary is the vegetable sheep in New Zealand. An inhabitant of dry rocky places, its water supply, although rainfall is fairly abundant, is precarious due to drainage and the failure of the rock to prevent run-off. The different species of Raoulia, of which R. eximia is one of the best known, are admirably adapted to exist under such conditions. L. Cockayne, an authority on the flora of that island, writes of these strange plants: “Perhaps the most striking denizens of rocks are the various kinds of vegetable sheep (species of Raoulia), which form hard cushions, mostly white, but occasionally green—and of enormous size. The raoulia cushions are all constructed on the same plan. Above, the stems branch again and again, and toward their extremities are covered with small woolly leaves, packed as tightly as possible. Finally stems, leaves, and all are pressed into a dense hard convex mass, making, in the case of Raoulia eximia, an excellent and appropriate seat for a tired botanist. Within the plant is a peat made of rotting leaves and branches, which holds water like a sponge, and into which the final branchlets send roots. Thus the plant lives in great measure on its own decay, and the woody main root serves chiefly as an anchor. The vegetable sheep are not inaptly named, for at a distance a shepherd might be misled.” The genus Raoulia belongs to the daisy family, and furnishes another illustration of the remarkable diversity of this largest and probably most recent of the families of flowering plants, which appears to have originated in the Andes and now covers the world.
In Damaraland, Southwest Africa, the most remarkable desert plant was discovered years ago, growing in a sandy and stony plain where the rainfall for fourteen years has not averaged more than two and a quarter inches annually. There are sea fogs, however, upon the condensation of which upon sheets of glass the discoverers of Welwitschia mirabilis relied for some of their scanty water supply. The plant, whose woody stem is deep buried in the ground with only the top appearing above the surface, looks not unlike “the burnt crust of a loaf of bread.” To this only two large leaves are attached. These are many feet long and split into several sections which undulate over the ground very like the tentacles of an octopus. With such strange products of the desert scattered over the plains it is little wonder that Welwitschia caused a sensation only equaled by the discovery of Rafflesia in the rain forest of Sumatra.
While deserts seem to be the most unfavorable places in which plants can exist, and their very existence in many deserts is often a precarious affair, it should be kept in mind that the soil of such places is often by no means sterile. As we found in the section of this book on “How Plants Get Their Food,” water is absolutely necessary for the absorption of food through the root hairs. Where, as in an oasis in the desert, water is locally plentiful a luxuriant vegetation springs up, and one of the most fertile parts of our Southwest was transformed from a desert by irrigation. Then, too, in many deserts there is a pronounced rainy season during which there is a marvelous development of showy flowering herbs that die down as rain ceases or becomes too slight, to wait for another opportunity to make the desert blossom into often gorgeous coloring.
Rain forests, temperate forests, grassland, and deserts—all are immense developments of plant societies depending upon climatic differences for their occurrence. There are some other plant communities which also depend for their development on still other differences of climate. Two such are the vegetation of mountain tops in the tropics, and that strange tundra vegetation near the poles which lives all its life on the ice, only the roots and soil in which it grows thawing out during the brief summer. Temperature rather than rainfall is the cause of these and some other plant societies of more local occurrence.
But what of such well-known plant societies as bogs, in which peat is formed, or the plants growing along the sea beaches all over the world? These, and scores of other plant communities play their part in the distribution of plants, but nearly all of them depend not upon climate, but upon usually purely local conditions of soil. Sandy, actually nearly sterile soil, the acidity of cranberry bogs, the alkaline regions in our own West, the salt lakes and inland seas, regions below sea level, the serpentine outcrops, all the hundred and one differences which local conditions exhibit—all these have a very direct bearing upon plant distribution. It is impossible here to go into the details of the different sorts of plant societies which inhabit such specialized places, nor into the truly wonderful adaptations of certain species to peculiar conditions. But in looking at the vegetation of regions through which one travels it must never be forgotten that its general type, such as forest, or grassland, or desert, or what not, is the result of usually widely operating climatic forces, while many, often quite extensive, plant societies in the region are the result of the local environment. There is often an active struggle as to the dominance of the type dictated by the climate of the place, and the local conditions of soil that tend to nullify general response to it. On Long Island, New York, for instance, there are areas which climatically should produce dense woods of the summer forest type so general all through the Northeastern States. Actually the water-worn sands and gravels that covered the south side of the island in glacial times, are so poor in plant food, that many square miles of this region are now covered only by low scrub oaks and other plants suited to poor soils.
A final word of caution is necessary to those who see in the foregoing brief account of some of the chief causes of plant distribution an answer to questions that many of us ask about why plants or vegetation are of such and such a kind in a particular locality. It has been convenient—nay, it has been necessary—to consider these various factors one by one, but the distribution of almost no individual, and certainly of no widely spread plant community, is the result of any one of these factors operating singly.
The geological history of the region, the links with the past of the species composing the vegetation, the climate, the cooperation of various outside agencies in seed dispersal, the conflict of different species, and of different vegetation types, these and scores of other factors, operating to-day, or having operated in the past—it is all these that are reflected in the plant covering of the earth. The variety and beauty of that covering are too well understood to need further mention here. The extraordinary efforts that the plant world makes to keep all but a minute fraction of the earth clothed with some sort of vegetation we have seen in the pages just turned. No other phase of the study of plant life is so replete with interest as plant distribution. Rightly understood, it is a study, “the cultural, esthetic and practical value of which may well outweigh any other.”
GENERAL CONCLUSION
We have now traced, all too briefly and with the many omissions that such a general account as this makes necessary, the broad outlines of plant life. From the architecture of their outer characteristics, which takes up the first chapter, we have gone step by step into the story of what goes on within the plant, and how it reproduces its kind. These actions or behavior of plants have resulted in many things of great practical importance as well as being of absorbing interest in themselves. What some of these results have been, we see reflected in the uses of plants to man, in the history of their development, and, most of all, in the way they are distributed over the earth to-day.
If plants are still “just plants” to most readers, this book has been written in vain. Those who have gleaned from its pages some conception of what a fundamental thing plant life is, will doubtless want more information than could be included here.
If any considerable proportion of the readers of this volume feel that they have already outgrown it, and that they have many questions about the plant world for which they will have to go to more specialized works for the answer, then this book has more than fulfilled its mission.
FOOTNOTES:
[1] After living probably as long as the Big Trees of California, the most famous dragon tree in the world was destroyed by a great storm. It has been replaced by seedlings.
[2] A process recently discovered in England for extracting the fiber of this rush by a chemical bath has greatly increased the fiber possibilities of this common rush. Heretofore it has been used only for coarse weaving of rugs and mattings in Japan. By the new process a fine fiber capable of spinning is extracted that may eventually compete with jute.
[3] Copyright, 1912. Doubleday, Page & Company.
[4] The term “indifferent” in this connection is used to signify that the plant will adapt itself to average conditions.
[5] Plants marked thus belong to the heath family and require special conditions as indicated in text.
