AT the end of the Palaeozoic Age the great tree club-mosses and tree horse-tails had died out completely; the little Sphenophylls that trailed over them disappeared too; the older types of gymnosperms, such as Cordaites, were replaced by conifers and others which were only beginning to creep in at the close of the Palaeozoic. The familiar-ferns had all gone, and the general appearance of the vegetation must have been completely different. Nevertheless it was also unlike that of to-day, for there were still none of the higher . It is strange, for example, to think of a world without grasslands, but so far we know of no fossil plants which can have carpeted the ground in quite the same way as do the grasses of this latest era in which we live.
The plants which gave character to the vegetation in this era, which we have therefore called the age of gymnosperms, belonged to threeof seed-plants, the conifers, the maidenhair-trees, and the cycads—or better, the cycadophytes. The living cycads are rare and few in number; they are mainly tropical, and have no popular name—some of them have been called sago-palms, but they have nothing to do with the real palms and the sago they produce is of no account. The fossil cycadophytes had very similar, stiff, sometimes palm-like foliage, but their reproductive organs differed considerably.
The age in which thesewere dominant is roughly the Mesozoic era of the geologists, but it ended, so far as the plant life is concerned, about the middle of the Cretaceous period—that is, rather earlier than the conventional line between the Mesozoic and the Tertiary. In actual time, it is safe to say that it lasted a hundred million years—probably not so long as the preceding age of seed-ferns.
PLANT LIFE BECOMES COSMOPOLITAN
THE difference between the vegetation of the northern and the southern hemispheres which was so pronounced at the close of the Palaeozoic era continued into the Mesozoic. In the south, where the distinction between Palaeozoic and Mesozoic—or the age of seed-ferns and the age of gymno-
sperms—is not so easily drawn, the dominant Triassic plants differed markedly from those of the north. The Glossopteris flora, after changing its character very gradually, becomes known as the Dicroidium flora, from the name of a very characteristic plant with forked fronds which is common in most parts of the old continent of Gondwanaland.
Into the problems of the possible changedof continents through crustal drift, or of the former existence of land-bridges across oceans, we cannot enter here, but as a result of migrations and invasions from south to north and vice versa over long periods of time, the vegetation of the whole word seems to have grown much more uniform again in Jurassic times. From Alaska and Siberia to the Antarctic the same kinds of plants were dominant, and the Yorkshire cliffs yield fossils which are indistinguishable from those of China and Japan, or of Graham Land in the far south. This flora lasted on into the early part of the succeeding Cretaceous period.
SOME FAMILIES WHOSE DESCENDANTS SURVIVE TO-DAY THE horse-tails of the Mesozoic need not detain us long. They continued to form a minor element in the vegetation, but though often much larger than the present-day relics of the class, they all seem to have been herbaceous and not woody forms. Except for some obscure fossils in the Triassic, the giant club-mosses were also herbaceous, and quite comparable with the modern club-mosses. These two great classes had passed their zenith; the small herbaceous forms had the tenacity to survive right down to the present day, but they were fixed and gave rise to no new types.
The true ferns were very abundant in this era; the primitive Palaeozoic ferns as well as most of the tree-ferns of the Permian had disappeared, and a great many of the Mesozoic fossils can be fitted into families which still exist. It is very interesting to find, however, that some of the most abundant and widespread of these fossils belong to families which have now only a few living representatives, with a very restricted distribution. The tropical ferns Dipteris and Malonia of Borneo and Malay are the last survivors of families which in Mesozoic times contained various genera with many common species, found in the rocks of almost all parts of the world.
The Royal Ferns, which have had a long history from the Permian to the present day, with a very interesting series of
developments in the structure of the, were abundant in the Jurassic and Cretaceous, and so were the Gleichenias, now confined to the tropics. Several other families of ferns which were well represented in the Mesozoic have survived to the present day. This does not mean that the species, or even the genera, are the same; they may have changed considerably, and we do not know the fossil remains completely enough to be able to compare them fully with living forms. Moreover, there are many fossil ferns of this era whose affinities still puzzle us.