Jurassic Floras

There were frequent though comparatively slight oscillations of the earth's crust in the course of the Jurassic period which are registered in the varying nature of the sedimentary rocks—series of beds rich in the shells of marine creatures associated with de posits formed in estuaries and lakes. The period is subdivided into stages grouped as Lower, Middle, and Upper Jurassic. A considerable interval of time separated the stages represented by the plant-bearing beds of Lower Jurassic age, such as occur in England on the coast of Dorset, from Upper Jurassic dirt beds (old surface-soils) near Lulworth cove in the same county. Beds intermediate in age are exposed on the Yorkshire coast. It is un necessary to follow the changes in the rise and fall of individual species from the earlier to the later stages of the period ; there were differences in detail, but the main features of the vegetation so far as we know remained fairly constant. Ferns, Cycadophyta, Ginkgoales and conifers continued to be the ruling dynasties from the Rhaetic through the Jurassic period.

Cretaceous Floras.

In the south of England and in some other parts of the world there is no sharp line of division between the last phase of the Jurassic period and the dawn of the Cre taceous period. In the Weald district of Kent and Sussex, in northern France, Germany and Belgium a series of fresh-water beds contains records of a time when rivers carried into a large lake logs of wood, twigs, leaves, and other samples of the vegeta tion which grew on the adjacent land. From the relics of this flora preserved in the cliffs at Ecclesbourne, near Hastings, on the south coast of England, and at other localities in western Europe it is possible to form a picture of the plant-world at the beginning of the Cretaceous period. The differences between this Wealden flora in northern Europe and the preceding Jurassic floras are comparatively slight : a few new forms are met with, for exam ple a plant known as Weichselia Mantelli usually regarded as a fern with spreading fronds bearing small and thick leaflets ; an other fern Onychiopsis psilotoides, probably allied to the living genus Onychium, a member of the Polypodiaceae which is rep resented in the Far East and in other regions ; also a fern of pe culiar habit referred to the genus Ternpskya, which may be a member of the Schizaeaceae, recorded from several European lo calities and from Montana; species of conifers agreeing in the needle-like leaves and in the cones with members of the family Abietineae which includes pines, firs, cedars, larches and other genera. The genus Sagenopteris, though not yet extinct, was much less in evidence in early Cretaceous than in Jurassic floras. In the Wealden vegetation of northern Europe there were no trees, so far as is known, other than conifers. The vegetation had still the Jurassic facies at least in the main. Reference has already been made to the structure of the fertile shoots of the Rhaetic genus Wielandiella and the Jurassic Williamsoniella, two examples of the extinct Bennettitales which in their comparatively slender, forked stems differ widely from all existing cycads.

One of the outstanding features of the early Cretaceous floras is the abundance of cycadean plants, a feature also of Jurassic vegetation. Most of the stems are of the type known as Cycadeo idea; the splendid specimen (fig. 9) from Upper Jurassic rocks of southern England, Cycadeoidea gigantea, agrees closely with stems of some modern cycads in its bulky form and in the closely packed persistent bases of old leaf-stalks which give a character istic appearance to the plant. This species differs from most ex amples in the absence of fertile branches. The first typical repre sentative of the genus is one described by Carruthers from Lower Cretaceous beds in the Isle of Wight named Bennettites Gibson ianus and now included in Cycadeoidea. Embedded among the leaf-bases are several lateral branches bearing a terminal "flower" and numerous short, linear leaves. In this species the flowers appear to be unisexual and female. The discovery in Dakota and in other districts in North America of hundreds of stems preserved in wonderful perfection supplied material which was described by Dr. Wieland in two well-illustrated volumes.

Some of the stems are unbranched; others consist of several tuberous branches. A remarkable feature is the extraordinary abundance of fertile shoots on a single stem. The flowers are usu ally bisexual, some bearing male and female organs which were apparently functional ; others with female organs only and with scars showing the former presence of male organs. A fertile branch ends in a depressed dome or in a tapered cone (fig. ioA) covered with two kinds of appendages, fertile ovule-bearing or seed-scales, each surrounded by a rosette of steril.! inter-seminal scales with swollen, flat tips (fig. soB) forming a protective layer over the whole "fruit." Below the female portion was a circle of 10-20 pinnately branched stamens bearing rows of compound sporangia (pollen-sacs) with numerous pollen-grains. The stamens ' were at first infolded round the flower-axis ; after expanding and shedding the pollen they fell off. The fertile branch bore numerous hairy bracts, which surrounded a conical axis, bearing short female organs (ovuliferous scales) and interseminal scales enclosed by a series of infolded leaves (stamens) bearing rows of compound sporangia' (pollen-sacs). The seeds contained an embryo with two cotyledons. The male organs may be compared on a small scale with the fertile leaves of modern ferns of the family Marat tiaceae; while the female organs are of a type that is peculiar to the Bennettitales (e.g., Wielandiella, Williamsoniella, W illiam sonia, Cycadeoidea). The largest example of a flower so far re corded is one described by Dr. Marie Stopes from Lower Cre taceous beds in the Isle of Wight which produced thousands of small seeds on a single "fruit." Cycadeoidea in its reproductive apparatus differs fundamentally from existing cycads though in the form and structure of the stem it is similar to them. It has been suggested that the general resemblance of the bisexual flow. ers to those of a magnolia may be an expression of actual affinity and that the angiosperms, which replaced the Cycadophyta as the ruling dynasty in the early days of the Cretaceous period may be connected with them by descent. The chief difficulty in the way of accepting this view is the lack in the flowers of the Bennetti tales of any structure definitely comparable with an ovary. A few leaves discovered in strata on the Atlantic border of the United States of America, in the so-called Potomac series the lowest members of which are approximately equivalent to the English Wealden series, bear some resemblance to those of certain modern flowering plants ; but their precise nature is uncertain. On the other hand Lower Cretaceous beds along a strip of the west Greenland coast on Disko island, on the coast of the adjacent mainland and on Upernivik island (Lat. 71° N.) have yielded sev eral Wealden species. A species of Ginkgoites, ferns allied to Matonia and Dipteris, several forms of Gleichenites, closely allied to tropical and sub-tropical Gleichenias and to fossil members of the family recorded from Jurassic rocks, also some Wealden spe cies of gymnosperms, associated with many leaves practically identical in form and venation with those of living flowering plants. Plane trees, magnolias, oaks, also leaves described as Dalbergites because of their striking resemblance to the foliage of the tropical genus Dalbergia, a member of the Leguminoseae. It is significant that in no other part of the world do we find a greater abundance and variety of angiosperms preserved side by side with ferns and gymnosperms, which appear to be specifically indistinguishable from Wealden species from northern Europe, North America and many other parts of the world. It would seem, therefore, that many of the broad-leaved trees may have had their origin in the early days of the Cretaceous period on an Arctic continent whence in later times they spread farther and farther towards the south.