3. Carboniferous Plants and the Formation of Coal
The carboniferous time, or the period when the earth was covered with huge forests of strange shrubs and trees, most of which were unlike their modern successors, apparently had a climate so nearly uniform and seasonless that fossil remains of these plants have been found throughout the world. Even in the Arctic the rock strata show the flourishing of forests that must have needed a climate very different from the frigid condition there to-day, and furnishing indisputable evidence of a warm, most probably frostless, climate practically throughout the world.
A modern horsetail or scouring rush, common in the north temperate zone. Ancestors of these formed huge forests at the time that coal was being formed.
The giant club mosses have already been mentioned, with their persistence to the present day in much reduced number, and vastly reduced sizes. No one can picture the grandeur of those ancient forests, peopled with queer animals long since extinct and with dragon flies known to have a wing-spread of two feet or more. But with the club mosses were giant horsetails, which in somewhat changed form have also come down to our times, but in much reduced stature and frequently are familiar enough as weedy plants along railway embankments, and sometimes in more natural environments. Most of our modern representatives of the genus Equisetum ([Figure 108]), or horsetails, are low herbs, but one South American kind still retains the ancient habit of growing to considerable heights, as specimens up to twenty feet high are known. Related to the ancient treelike horsetails were queer vines with slender twining stems, which, judging from their fossil remains, must have been very common. Both the ancestors of our club mosses and the horsetails must have occupied vast swampy areas, as their stem structure indicates a fondness for water, to which, as we have already seen, their still more ancient ancestors were always confined.
Vigorously competing with these plants for occupancy of those great swamps were vast quantities of plants that have been called cycadlike ferns from their likeness to ferns on the one hand and plants like the so-called sago palm on the other. The sago palm, or Cycas revoluta, is a modern representative of these ancient forms, and retains the remarkable characteristic of having its male fertilizing cell capable of movement as we know to be the case in nearly all cryptogamous plants. Yet Cycas, with its related genera, which are found in nearly all the warmer parts of the earth to-day, are true flowering plants which bear cones. We see, therefore, in these old cycadlike ferns one of the first, almost experimental, evidences of the seed habit, and consequently the breaking away from the spore habit which overwhelmingly characterized the reproductive processes of its ancient associates.
The inhabitants of higher parts of that dim, mysterious world, of which we know only that part revealed in the fossil record, were largely ancestors of our modern cone-bearing evergreens. They are known as Cordaitales and have long since disappeared. Forming forests of huge size and making long, slender trunks with a crown of leaves at the top not unlike some modern conifers to which they are, of course, related, these progenitors of our pines and spruces must have been striking objects of that strange landscape. Rooted stumps of these ancient trees have been uncovered, and their narrow leaves, often three feet long, are common as fossils. By some our present conifers and the Cordaitales are both thought to be descendants from a still more ancient group, of which the existence is only conjectured.
We can, perhaps, best summarize our sketch of the plant life existing at the close of this period by saying that all the forms show unmistakable evidence of being crytogamous so far as their reproductive processes are concerned, or else, as in the progenitors of our conifers and cycads, the beginnings of a definite seed habit are indicated. Most of the lowland representatives of this flora were cryptogamous in their characters and ancestry, while some of them, and nearly all the flora of drier sites, appear to have shown the beginnings of flower production. Some of these flowers, which are always cones, are unmistakable as such, and pollen in tremendous quantities has been found among their buried remains. These cones belong to trees that are actual gymnosperms or obvious ancestors of them, for no herbs are known as yet. Nor are angiospermous flowering plants known from this period, nor have any ever been found in strata millions of years younger than the fossil-bearing strata of this age of the ancestors of our modern ferns, conifers, or cycads.
Nor must we picture the development of these different plant inhabitants of that time as passing from one to another in orderly sequence, for that would give us the impression of a regular progression from simple to complex, which may or may not be the truth. There appears to be such a sequence, and the internal structure of the remains of many of these ancient types of plant life have greatly aided our ability to understand their relationships. But with the possibility of various reproductive processes or other structures appearing in quite unrelated forms, and with the comparative paucity of the fossil record in mind, no one can say for certain what are the true lines of descent. The necessity for water in the reproductive act of nearly all the crytogams, the origin of the vascular structure, and the consequent ability to live upon the land, and finally the production of a conelike flower structure with pollen, and all that that implies, are all found during this period.
To the vegetation inhabiting the swamps during this period, man owes a debt perhaps as great as to our modern food plants, for it is upon this, and some later plant remains, that we rely for coal. This period has been well called the Carboniferous, for its chief claim to attention, outside the realms of botanical research, was the deposition of those great collections of plant remains, which, as coal, contain as high as 90 per cent of carbon and furnish the fuel of the world. This is scarcely the place or time to go into the composition of different kinds of coal, but some mention of the conditions under which these ancient swampy forests were transformed into that valuable substance may well conclude the account of a vegetation period the history of which has in large part been found written in the very strata from which coal itself is derived.
In the lowest and wettest parts of those forests there occurred, just as there may occur to-day, a large accumulation of fallen trees and other vegetable refuse. In the ordinary way these would simply rot, due to the work of insects and the fungi of decay, and in a few score years there would be nothing to show. “Dust to dust” would be, and is, the history of so many living things that it is only some machinery for arresting this process which will give us very different results. In the case of coal formation the original impetus appears to be certain microscopic organisms, probably saprophytic fungi, fossil remains of which have been identified, which work upon the fallen mass of vegetation and start its decay, but which can only do so while their prey is still within the influence of the air. The initial stages of decay must, therefore, have been going on while the water was low enough for these organisms to work. But in many parts of that ancient landscape the water level was a fluctuating quantity, due to local conditions or to changes in the earth’s crust. So that many times partially decomposed vegetation masses would become submerged, stopping the work of these organisms of decay that demand air, but providing the only conditions under which certain others could complete the transformation. These bacterial organisms that will work only when deprived of air continue the process, but in a different way. For one thing, the lack of air delays decomposition or almost stops it, as witness the resistance of logs under water, some of which are known to be hundreds of years old. And forest stumps off the coast of Cape May, in New Jersey, are in nearly as perfect a state as when first submerged, over 40,000 years ago. In the production of coal these anærobic (living without air) bacteria release oxygen and hydrogen from the partly decayed mass, leaving as a residue a substance known as peat, which is largely carbon. The transformation of peat into coal depends upon requisite pressure of the strata that may be laid down on top of the peat bed, and probably upon chemical changes that go on after such covering strata have been laid down.
The fact that coal is sometimes found only in thin veins, with layers of shale and other material between, tells us that its origin must often have been a precarious affair, where alternate emergence and submergence would permit first the vegetation to develop and then its transformation to peat, followed by the deposition of fine sands or silt covering the bed. Several such cycles occurred, sometimes separated by untold ages of time, or again by much briefer periods. Certain mines, however, contain over 200 feet of solid coal. The length of time necessary for such a vast accumulation, or how many generations of these ancient plants went into their making, is beyond calculation. With the mining of coal running into the hundreds of millions of tons yearly, we get some idea of how great were those Carboniferous forests, and how extensive they were is proved from the widely separated localities in which coal mines are found.
The Carboniferous age of fern, cycad, and conifer ancestors was by no means a quiet, orderly period, as from geological evidence it appears to have been much subject to alternate emergence and submergence of great tracts of land. Compared with what followed, it actually was a period of comparative quietness, however, and it must, in at least most parts of the world, have permitted the slow development of certain of its plant groups to a state of perfection never reached since. This is particularly true of the ancient relatives of our club mosses and horsetails.
Perhaps one of the most obvious questions to ask about these plants is how long ago they lived, and upon the answer to such a question depend many others. What, for instance, is the position of the Carboniferous as compared to what preceded it and came after? How old is the earth and when did life first appear on it? The evidence upon which such questions are answered comes from the estimates of physicists as to the age of the earth; from students of fossil animals and plants; from astronomers, from geologists and other students. A compromise of these different estimates, and one that has consequently been widely accepted, gives the age of the earth, dating from the time of its having a definite crust with land and water masses, as somewhere near a hundred million years. Such figures are beyond our comprehension and consequently mean almost nothing, but the proportion in time of the different stages of the development of plants may be stated with greater certainty. Taking the total age of the earth as 100 per cent, the period when there is no record of life of any sort may be set down as about 45 per cent of the total, the reign of algæ and development of land plants about 8 per cent, the carboniferous or coal-forming plants about 28 per cent, which leaves only 19 per cent from that distant time to the present. And many things happened in this comparatively brief fifth of the plant world’s history, among them the origin of some plants that have come straight down to us, without discoverable change.
Found in most fossil strata and in a practically unchanged condition from the upper part of the Carboniferous to the most recent fossil records. Now unknown as a wild tree and preserved for us through its cultivation in ancient temple gardens in eastern Asia.
We could hardly leave the Carboniferous time without at least brief mention of the ginkgo tree ([Figure 109]), or, as some call it, the maidenhair tree. From the upper strata of the Carboniferous it is common, as it is in practically all subsequent fossil accumulations down to the most recent. And yet the tree has never been found wild, although its frequency in temple gardens in China and Japan, always as a cultivated tree, suggests that its disappearance as a wild plant must have come since the priests began preserving it, which can be only a matter of a few thousand years at most. In other words, we have just missed seeing in the ginkgo what has so many times happened to these very ancient types of vegetation, namely, their final extinction. This must have occurred within historic times, and, judging by its frequent use as a temple tree in eastern Asia, that region was its last outpost after its long journey from the dim past. Thousands of other ancient plants have completely disappeared, and one cycad from New South Wales is at this moment putting up a losing fight against modern competitors, but in the ginkgo tree the actual twilight and extinction of its wild existence has missed observations by modern plant geographers by only a brief period. It is almost as though we had waited all our life to see some great event and then missed it by a few moments. Fortunately the tree is now common in cultivation, and not the least interesting feature of it is the fact that its male fertilizing cell retains its power of movement, which dates back to its early associates. Among modern flowering plants only the ginkgo and the relatives of the sago palm or cycads retain this relic of an overwhelmingly cryptogamous ancestry.
The end of the Carboniferous or coal-forming ages was marked by great changes in the earth’s surface, some of them cataclysmic in their effects. What they were in detail is described in the volume on geology and need not be repeated here. What happened to the development of the plant kingdom after this will be considered in the next section of this chapter.