THE five main eras of plant life were not of equal duration. The age of seaweeds (from the first beginnings of life to the appearance of land-plants) was perhaps the longest, and the succeeding era the shortest, though these two overlapped, at any rate, in the Silurian epoch. The central era, which is much better known, extends from the Upper Devonian to the end of the Permian. Its close, therefore, coincides with one of the great geological divisions—that between the Palaeozoic and the Mesozoic, and it is most characteristically developed in the Carboniferous period. That is to say, it is the age of the great coal forests of the northern hemisphere, with an introductory period covered by the Upper Devonian and Lower Carboniferous and a period of decline in the Permian.
A WORLD WITHOUT FLOWERS
WHAT types of plants were dominant in this important period? We have called it the age of the-ferns— those familiar coal plants with fern-like foliage which are now known to have borne instead of spores—and though these perhaps best typify the era, it was also the age ^f the tree horse-tails and the giant club-mosses. True ferns were present, though, like the mosses and liverworts, they were never a dominant feature in the flora, and there were other of less important fern-allies, such as the slender Sphenophylls.
The higher-plants, including forms allied to the conifers which are grouped together as Gymnosperms, were an important element, especially towards the end of the era, but there were no true . The spore-producing fern-allies and the fern-like seed-bearing Pteridosperms roughly held the balance between them in the vegetation. The primitive land-plants had disappeared; the conifers were still unimportant; the flowering plants had not yet arrived. It was a green world, but the forests may not have been entirely without other colours. The great yellow cones of the giant club-mosses, the feathery, branched pollen-
producing organs and the huskedof other plants may well have diversified the dense mass of foliage much as do in a forest to-day. But although insects were there—cockroaches and giant dragonflies—the plants seem to have been mainly if not entirely wind-pollinated, and there was no occasion for gay colours and elaborate to attract insect visitors.
PLANT LUXURIANCE TURNS TO MINERAL WEALTH : THE COAL BEDS FROM the debris of the luxuriant vegetation of this age, coal was produced. Coal may be described as the compact residue of plant remains which have undergone varying degrees of maceration, decomposition and chemical change, resulting in a hard brittle substance rich in carbon. In some cases the plants grew in swamp forests or peat bogs and accumulated on the spot; in others the vegetable matter drifted into lakes, estuaries, or even out to sea. Some coals are formed of a mixture of many different kinds of plants; others are mainly the product of a few or of only one kind. Some are built up mainly of wood; others contain abundant, twigs, bark, cones and spores; some consist almost entirely of spores. Nearly all coals, even the much-altered anthracites, consisting of almost pure carbon, retain traces of the plant tissues from which they were produced, as can be demonstrated by examining thin sections, or highly polished and etched surfaces, with a microscope. The original vegetable matter is always much compressed and reduced in bulk in the course of conversion into coal; it has been estimated that a coal seam one foot thick represents some fifteen to twenty feet of plant debris.
Although the familiar coals of this country are all of Carboniferous age, coal deposits are known in various parts of the world in all geological periods from the Upper Devonian to the late Tertiary. The later coals were, of course, derived from different types of plants but were probably formed in similar ways.
PLANTS THAT BECAME STONE INSTEAD OF COAL IN many coal-fields the seams of coal rest on an under-day which is full of stumps and ramifyingevidently belonging to the plants now forming the seam. Clearly in such cases the plants must to a large extent have accumulated on
the spot where they grew, though some may also have drifted there through inundations. In some seams stony nodules, called coal-balls, are found; these do not consist of coal, but are calcareous bodies containing petrified plant remains. They are in short, only partly compressed masses of the same debris as that of which the coal is composed, but petrified by solutions containing carbonate of lime before the rest of the deposit was converted into coal.
Under certain conditions the solutions started centres of concretionary action, and sometimes a number of such concretions grew or were cemented into one solid block, rarely as much as five feet long and four feet thick. It is fortunate for mankind in general that most of the plant accumulations were not thus petrified, but were turned into a combustible mineral; it is, however, fortunate for the botanist that some relics of the ancient vegetation were almost perfectly preserved for his inspection.
Our knowledge of the anatomy of Coal Measure plants is very largely derived from these coal-balls. Thin sections ground down on glass until they are translucent can be examined under the microscope, and reveal the structure of the tissues almost as if they were sections of living plants. This is because in the course of petrifaction mineral matter has been laid down along the cell-walls and in the cell-cavities, completely preserving a skeleton, as it were, of the tissues. The cell-contents have disappeared or been replaced by mineral, except in certain rare cases; resins, for example, are sometimes preserved in petrified woods. In many coal-balls much of the vegetation had decayed before petrifaction, but in others the preservation is astonishingly perfect. An ingenious method of making microscope preparations from coal-balls depends on the fact that the organic matter of the cell-walls is not mineralised. By etching a polished surface with hydrochloric acid the mineral matter is removed, and the thin layer of plant tissues which remains upstanding is then embedded in a solution of cellulose or gelatine. This dries to a thin film which can be readily peeled off.
Petrified plants of all geological ages can thus be prepared by one method or another for microscopical study. The commonest petrifying agents are silica (as in the case of the Rhynie plants mentioned above) and carbonate of lime (as in the coal-balls). In very few geological horizons, however, were the conditions favourable for the preservation of plants
on such a scale as in the coal measures, and in most deposits one only meets with isolated examples of petrified wood and cones. Sedimentary deposits from estuaries and lakes often contain abundant impressions of plant remains, especially of leaves, and much of our knowledge of fossil plants must be derived from these. They supplement what can be learnt from petrified fossils, but since both types are usually fragmentary the task of piecing together the evidence in order to get an idea of the plant as a whole is a long and difficult one. Even in the coal measures, which have such an abundant flora, and which have been intensively studied for a hundred years, we scarcely know one single species of plant completely in all its parts. Anyone, therefore, who lives near a coal-mine may add materially to our knowledge by carefully collecting and studying the plant impressions to be found in the shales of the tip-heaps.