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.