SLENDER CLIMBERS WITH COMPLICATED CONES

ONE characteristic group of Palaeozoic plants must be mentioned briefly : the delicate Sphenophylls. The ribbed and jointed stems bearing whorls of wedge-shaped, sometimes fringed or forked leaves, indicate a relationship with the tree horse-tails. The stems had a solid woody axis, which produced secondary wood, but never reached a very great size, and they were probably trailing or climbing plants. The cones of most members of the group were complex, and do not suggest any close affinity with other grovips. One of the Lower Carboniferous forms, known as Cheirostrobus, of which only a few specimens have ever been found, at Pettycur in Scotland, is quite the most complex fructification known in any spore-plant, either fossil or recent, with whorls of elaborately segmented scales bearing well-protected spore-sacs. So far as structure is concerned, the group apparently reached its climax early in the seed-fern era; representatives are still to be found up to the close of the era in rocks of all parts of the world, but after that it died out completely.

The complexity of structure exhibited at such an early stage as the Lower Carboniferous by plants like Cheirostrobus is a very striking and interesting phenomenon. It suggests for one thing that such plants must be the result of a very long period of evolution, but in this connection there are two points to be remembered : first that we have very little idea of the rate of evolution in living beings; second, that the apparent suddenness with which groups rose to importance

or developed in organisation is often due to an inevitable foreshortening as we look back over geological time. Thus between the Lower Devonian, in which so far as we know only relatively simple plants flourished, and the Lower Carboniferous, with Cheirostrobus and other highly specialised types, there was an interval of many million years—a far longer period, probably, than that covered by the whole evolution of man from anthropoid apes. Whether or not Cheirostrobus was the result of comparatively rapid evolution from some earlier type of jointed spore-bearing plant, one lesson seems clear : that extreme complexity and specialisation lead inevitably to extinction.

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