TREES THAT WERE TRIUMPHS OF ENGINEERING

THE last important group of Upper Palaeozoic plants is exemplified by the tall trees known as Cordaites. Their lofty branched stems bore large, strap-shaped leaves, sometimes as much as a metre in length. The complicated internal structure of the leaves, with strands of thick fibres to act as girders, show that they were mechanically well adapted to withstand bending and tearing strains. On this point D. H. Scott writes :— ‘It is interesting to note that the construction of the leaves of this extinct race of gymnosperms was, from an engineering point of view, on the same lines as that of the similar leaves of certain Monocotyledons at the present day. Thus, when the conditions were identical, the adaptations of Palseozoic plants were the same as those of plants of similar habit now living.’ The structure of the wood much resembles that of the monkey-puzzle conifers of the present day. The pith was very large (and thus was more like the cycads than the conifers) and was divided by a series of transverse diaphragms. Casts of the pith (comparable with those of the horse-tails mentioned above) are among the commonest fossils of the Coal Measures. The fructifications were of an elaborate structure, rather like catkins in form, and the seeds, like those of the Pteridosperms and the living cycads and the Maidenhair-tree, were provided with a pollen-chamber. The Cordaiteans were certainly an advanced and highly-developed race and they afford evidence of relationship with all the chief groups of gymnosperms.

The flora in the early part of this era, that is to say, in the Upper Devonian and Lower Carboniferous, was somewhat different from that of the Coal Measures and the Permian, which also may be classed together in a general way. Although most of the same plant groups are represented throughout the era, some of them, such as the Cordaites trees just mentioned, and also Sigillaria, attained their main development

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later, while towards the end there is an increase in the conifers and cycads. Many genera are especially characteristic of the earlier period, and one of the most abundant of these, supposed to be a seed-fern, has suggested the name Archceopteris flora. Where marine beds occur in the Lower Carboniferous, reefs of calcareous alga; abound, but in this sketch* we can deal only with the land vegetation of these and later times.

The interesting thing about this Archccopteris flora is that it was world-wide in distribution, like the preceding early land-flora, representatives of it having been found from Spitsbergen to South America and Australia. When we come to the Coal Measures, however, we find that the profuse and varied flora which provided our most important coal deposits was almost confined to the northern hemisphere. Even in the north more than one floral province can be distinguished, and there are many plants in the coal-beds of China which are not found in those of Europe and North America.

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