Coniferales.—The conifers are the most important group of living gymnosperms and there is evidence of their existence as early as the Palaeozoic. Walchia which appears in the Upper Carboniferous, though typically a Permian genus, had leaves and branches very like those of Araucaria excelsa. Casts of the pith cavity which show the course of the primary bundles show that in organisation the stem was also in substantial agreement with Araucaria. Cones have been found on the ends of the twigs of some species and R. Zeiller has shown that the seeds were in one species borne singly on the cone-scales, a distinctly Araucarian character. More is known about Voltzia a characteristic Permian and Triassic genus. The foliage shoots (fig. 22 A) of some species are much like those of TV alchia and Araucaria excelsa and show the same variability in size of leaf even on closely adjoining por tions of the same twig. The ovulate or seed-bearing cones (fig. 22, B) are built up of spirally arranged loosely packed scales. Each seed-bearing scale in the Permian species V. Liebeana (fig. 22, C, D) had three main lobes, each with a seed attached to its upper surface near its base, and two smaller lobes alternating with the three main lobes but set slightly behind them. In one specimen of a scale which has been investigated a thin pointed scale is present attached to the back of the lobed scale (fig. 22, D). The presence of this scale is of interest in considering the interrela tionship of the different groups into which the living conifers are classified and in estimating their relative antiquity. In the Abie tineae, the group of conifers to which the pine, cedar, larch, etc., belong, the cone is built up of two kinds of scales each with a vascular system of its own, sterile bract-scales and seed-scales which are situated in the upper angle between the bract scale and the axis of the cone and are partly coherent with the former. In the Japanese cedar (Crypto meria) the two scales are coher ent for a greater part of their length and only the tips are free.
In others again e.g., Sequoia (giant tree of California) there is no outward evidence that two scales are present but the appar ently single scale has a double set of vascular bundles which reveal its fundamentally double nature.
In the Araucarineae the scale shows practically no indication of being double. For this reason and because the Araucarineae differ in many other ways from the rest of the Coniferae it has been sug gested that they have had a sepa rate ancestry from the others. It would appear however from a consideration of the type of dou ble scale found in V oltzia which is otherwise very like an Arau caria that these two divisions of the Coniferae may have diverged from a common ancestral group which was represented in Permian and Triassic times by plants of the Walchia and Voltzia type.
The Distribution of Palaeozoic Floras.—We have seen that the Predevonian Floras consist of Thallophytes, either marine or fresh-water, and we have no evidence of a land vegetation until after the close of the Silurian Period. In the Devonian there is evidence of a rich vegetation from as far north as Ellesmere land and Spitsbergen and as far south as the Falkland islands; so that either the distribution of climatic zones fluctuated consider ably or else the climate of the earth was more uniform. The former seems to be the more likely hypothesis. In the Lower Devonian some large Thallophytes are still found, e.g., Nematophy ton, while Zosterophyllum, Psilophyton and Arthrostigma, also characteristic of the period, may represent transition types be tween Thallophytes and Pteridophytes, but are distinctly nearer in their affinities to the latter. In the Middle Devonian more complicated types appear and in the Upper Devonian several groups of a more modern aspect such as the Lycopods and Ferns become distinct. In the Lower Carboniferous the important groups are the Pteridosperms, Lycopods, Equisetales and ferns and with this constitution the flora persists right through the rest of the Carboniferous and Permian. In the Upper Carboniferous there is evidence of a change of a far reaching nature in the southern hemisphere for a different flora is found there from that in the north. This difference was probably connected with the glacial period which prevailed in the south in Carboniferous times and which left unmistakable traces in contemporary rocks in South Africa and Australia. This southern flora, characterized by the fern-like plant Glossopteris, extended into parts of Russia and India but the greater part of the northern hemisphere had an Upper Carboniferous flora of the west European type. An outlier of the northern type of flora has been found in Carbon iferous rocks in Sumatra, while in northern South America and in South Africa there is evidence in rocks of the same age of a flora of an intermediate type.
Introductory.—The middle period of geological history, which embraces a succession of ages extending over a few hundred mil lion years, is known as the Mesozoic era ; it is divided into three periods, the oldest or TRIASSIC period, followed by the JuRAssic and CRETACEOUS periods. The term RHAETIC (from the Rhaetian
Alps) is used for the stage of earth-history between the Triassic and Jurassic periods. Before considering the march of plant-life through the Mesozoic era, it is desirable to obtain a general idea of the relation of the world's vegetation as it was at the beginning of the middle period and of the broad features of the vegetation which has been reconstructed from the remains preserved in the rocks of the post-Mesozoic or Cainozoic era. We shall then be in a better position to appreciate the relation of the floras with which we are now concerned to those which preceded and followed them. In the course of the Palaeozoic era a large portion of the earth's surface had become colonized by many different kinds of plants; some of them comparatively simple, others rivalling in the com plexity of form and structure forest trees of the present day. These ancient land-plants were in all probability the modified de scendants of inconceivably remote ancestors which lived in the primeval seas. A botanist familiar with the vegetation of to-day, if he could wander through the forest-covered swamps of the latter part of the Carboniferous period (the Coal age), when the Palaeozoic floras reached the zenith of their luxuriance, might at the first glance think himself in a world where tree ferns, giant horsetails (Equisetum) and club mosses (Lycopodium, etc.) played a dominant part, but on closer inspection he would find that the great majority of the plants were in many respects far removed from all modern types. If he travelled across what are now Europe and North America and extended his journey into the Arctic regions, he would discover comparatively few well marked differences between the western and eastern floras in this area. He would see certain plant-associations on the drier ground and others occupying the low-lying swamps, but the vegetation as a whole would not show any well defined contrasts. Were he to travel into central China and to the Malay region he would be struck by the general resemblance of the dominant plants to those in the northern hemisphere. We know that the forests of the Coal age included many extinct members of the class Pteridophyta (ferns, horsetails, club mosses, etc.) which in their tree-like di mensions and in other characters differed widely from the corre sponding diminutive plants of our own time. We know also that nearly all the fern-like shrubs and trees were not true ferns but plants for which it has been necessary to institute a new group name, the Pteridosperma (see Palaeozoic section, above), because they produced seeds in place of the spore-cases (sporangia) which are characteristic of modern ferns. Some of the trees were similar in habit and in the structure of the wood to existing members of the Araucaria family, but they differed from all living gymno sperms in the nature of the reproductive organs. There were a few plants with leaves agreeing in form with the fronds of cycads, the surviving representatives in the vegetation of the present day of the Cycadophyta, a class which played an increasingly impor tant role as the Mesozoic floras succeeded one another. Similarly a wanderer in the Palaeozoic forests would note here and there trees with relatively broad, wedge-shaped leaves resembling in form and in venation those of the maidenhair tree (Ginkgo). In the absence of flowering plants (Angiosperms) and in the unfa miliar features of most of the commoner trees, the forests of the Coal age formed a striking contrast to the woodlands of the mod ern world. The vegetation of the latter part of the Carboniferous period persisted in diminished numbers into the Permian period which is the last chapter of Palaeozoic history. At the close of the Carboniferous period changes in the earth's crust, which were sufficiently widespread and disturbing to be described as a geo logical revolution, created a new environment ; humid swamps were transformed into relatively dry and hilly regions or into arid wastes in which inland seas like the Caspian replaced estu aries and fresh-water lakes. This shifting of the geological back ground is reflected in the clearly marked change in the character of the vegetation. The plants from Permian rocks are less nu merous and less varied than those preserved in the coal seams and associated sediments of the latter part of the Carboniferous period. At the end of the Palaeozoic era the development of the plant world suffered a severe check; many of the Carboniferous trees failed to survive ; a few new forms were evolved, but on the whole there was no marked alteration in the main botanical features of the depauperated floras.