Plants of the Palaeozoic Period

The abundance in Upper Triassic floras of large-leaved species of the class Ginkgoales is a noteworthy character; the genus Baiera represented by large wedge-shaped leaves cut into linear segments and, in the more typical species, without a leaf-stalk, though already in existence in the Palaeozoic era began to play a prominent role in the vegetation of the Keuper stage ; it persisted through the Rhaetic and Jurassic periods and survived into the Cretaceous.

Among other gymnosperms reference may be made to Podoza mites (fig. 3), a tree with leaves similar to those of species of the living conifer Agathis, but distinguished from all recent gen era by bearing its short, fertile, seed-bearing leaves in loose catkin like clusters. Podozamites is an example of a genus which be gan its career in the Triassic period and became a cosmopolitan and abundant plant in Rhaetic and Jurassic floras. Our knowl edge of the conifers is scanty; V oltzia survived from the early Triassic floras and with it some other members of the group which, through lack of material, cannot be closely compared with modern types.

In illustration of the salient features of the Triassic vegetation of Gondwanaland a brief reference is made to the plant-beds of South Africa. Sedimentary strata included in the Stormberg series have yielded a flora which shows many points of contact with the floras of the northern hemisphere. V oltzia, Baiera, Piero phyllum, Pseudoctenis, Thinnfeldia, Schizoneura, Neocalamites, genera of true ferns and plants with fern-like fronds occur with Glossopteris and other genera peculiar to southern Triassic floras. One of the most remarkable constituents of the South African vegetation is the genus Rhexoxylon founded on petrified wood; this was at first believed to be allied to a Palaeozoic group of gymnosperms, the Medulloseae, but an examination of additional specimens led J. Walton to compare it with certain species of Dadoxylon, a genus of conifers. The stem of Rhexoxylon reached a length of several metres and a diameter of 25cm.; it is charac terized by a large pith with secretory ducts and some scattered vascular bundles, but more especially by the unusual arrangement of the wood, which was divided into wedge-shaped masses sepa rated by bands of softer tissue and bears a very striking resem blance to the structure of some Dicotyledonous climbers in the tropical forests of the present day. Rhexoxylon is recorded from Triassic and probably Rhaetic beds in South Africa, and from the Antarctic continent. Nothing is known of the leaves or reproduc tive organs; the most interesting feature is the close agreement in the structure of the stem with that of certain living plants be longing to an entirely different section of the vegetable kingdom.

From a comparison of the fossil plants obtained from South Africa, India, Australia and South America it has been established that in its main features the Triassic vegetation of Gondwana land was fairly uniform; many of the genera are common to both hemispheres and the distinction between the northern and south ern continents was less obvious than in the latter part of the Palaeozoic era.

Rhaetic Floras.

The Rhaetic floras, richer than those of the Keuper series, are preserved in deposits formed for the most part in the estuaries of rivers and represented in many widely separated regions. Between the late Triassic plants and those fur nished by the overlying Rhaetic rocks the difference is compara tively slight; there is a greater wealth of material in the Rhaetic beds and, though new forms occur, the general facies is similar to that of the Keuper vegetation. A rich Rhaetic flora has been de scribed from Tongking; another equally rich flora has left abun dant remains in southern Sweden and more recently a no less lux uriant flora has been described from Rhaetic rocks in the Scoresby Sound district in Greenland (Harris, 1926). Collections of the same geological age have been made from central Europe and from many other regions. Though it is not easy precisely to cor relate the rocks of Gondwanaland with those in the northern hemisphere, we know that in its broader features the vegetation which flourished south of the Tethys sea agreed with that of the rest of the world. This statement is not intended to give the im pression of a vegetation unaffected by differences in climate; there were undoubtedly regional peculiarities, but there were many gen era which appear to have had a remarkably wide geographical range. In the Rhaetic flora of Tongking Glossopteris is a connect ing link with the Permo-Carboniferous vegetation of Gondwana land, and the recently re-recorded occurrence of the genus as far north as Lat. 7o° N. in Greenland demonstrates clearly its capacity as a traveller. In the Tongking flora we find also Equise tites and Neocalamites represented by species closely allied to northern forms. Among the ferns are many splendid examples of Dictyophyllum and Clathropteris, suggestive of the fronds of Malayan species of Dipteris (fig. 5, the smaller fronds) ; also of the Osmunda family. The various forms of simple Taeni opteris leaves recalling those of the hart's tongue (Scolopen drium), may be the foliage of Cycadophyta and not true ferns. Pterophyllum seems to have been one of the most conspicuous members of the Cycadophyta, a class represented by other genera such as Pti/ophyllum and Otozamites. The fronds known as Ptilo phyllum are characterized by two rows of linear segments giving them an appearance similar to that of the leaves of the Mexican cycad Dioon: Otozamites fronds are distinguished by the eared base of the segments which are occasionally relatively broad. Species of the cosmopolitan Podozamites also occur. It is pointed out by Dr. du Toit, in his recent account of the older Mesozoic floras of South Africa, that while the Tongking flora contained sev eral plants specifically identical with South African and Aus tralian forms, the absence in the Far Eastern vegetation of Gink goites, Thinnfeldia and other common Rhaetic genera is an inter esting feature.