REMARKABLE AND PUZZLING EXTINCT PLANTS

INTERESTING and important as the primitive Devonian land-plants are, the co-existence of other types must not be overlooked. Many of these are so imperfectly known that their affinities are still quite uncertain; some of them are so puzzling that they do not seem to fit into any of the main groups of the vegetable kingdom. Thus there are certain large stem fragments, sometimes three feet in diameter and apparently showing rings of growth, which are found in Upper Silurian and Devonian beds of Europe and America; they were first called Prototaxites, from a supposed relationship to the ancestors of the yew, but this view was soon discarded.

The loosely interwoven tubular tissue of which the fossil is composed is rather like that of some living brown alga?

(such as the large Antarctic Lessonia), and for long Prototaxites was accepted, though rather doubtfully, as a colossal fossil seaweed. When, later on, the supposed seaweed was found definitely associated with land-plants the question of its nature had to be reconsidered, and there was even a suggestion that it might be a fungus. But nothing has yet been found to connect the chunks of petrified ‘stem ‘(as they are still presumed to be) with any leaves or other appendages, nor with any reproductive organs, and Prototaxites remains, in the words of W. II. Lang, ‘one of the most remarkable and puzzling extinct plants.’

The dominant plants in the age of the early land-floras were lowly types of vascular plants, mostly belonging to groups which are now extinct. However, as D. H. Scott has put it, ‘that each new type arose much earlier than its full manifestation is obvious, and there is often direct evidence of precursors in an age previous to the transformation of the flora.’ One of these precursors in the Middle Devonian, discovered long ago by Hugh Miller, author of The Old Red Sandstone and The Testimony of the Rocks, was a stem with its internal structure preserved which Miller regarded as an undoubted cone-bearing tree. Whether it really belongs to the group of Gymnosperms (which includes the conifers) is uncertain, but the elaboration of its structure indicates a higher type of organisation than the primitive Middle Devonian plants of Rhynie. Looked at from the present, it may well have belonged to an archaic group; three hundred and fifty million years ago it was perhaps a forerunner of the flora which in Upper Devonian times was to extinguish most of the primitive land-plants and usher in a new era.

The Middle Devonian beds of Germany have yielded a series of plants, including stems anatomically resembling the one found by Hugh Miller, which also suggest that the vegetation was very far advanced and diversified compared with the earlier land-flora. Again, however, there is no definite evidence of seed-plants, but there are leafy and woody plants of various kinds, some with small leaves and some with compound fronds which suggest an approach to the fern type. Some have obscurely pointed stems and whorled leaves, with fructifications in loose spikes or cones; these may be the forerunners of that group of fern-allies (or pteridophytes) which to-day is represented only by the horse-tails.

There is abundant scope for the fossil-collector in exploring

the plant-bearing beds of the Old Red Sandstone in Scotland and the Welsh borders; new discoveries of high botanical importance undoubtedly await the student in these rocks.

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