SPHENOPTERIS BIFIDA.
(Burdiehouse.)
Even among the vegetable organisms of our Coal Measures, already partially described and figured, much remains to be accomplished in the way of restoration. Portions of Sphenopteris bifida, for instance, a fern of the Lower Carboniferous rocks have been repeatedly figured; but a beautiful specimen on the table, which exhibits what seems to be the complete frond of the plant, will give, I doubt not, fresh ideas respecting the general framework, if I may so speak, of this skeleton fern, to even those best acquainted with the figures; and an elaborate restoration of its contemporary, Sphenopteris affinis (see frontispiece) which I completed from a fine series of specimens in my collection, will be new, as a whole, to those most familiar with this commonest of the Burdiehouse fossils. From comparisons instituted between minute portions of this Sphenopteris and a recent fern, it has been held considerably to resemble a Davallia of the West Indies; whereas it will be seen from the entire frond that it was characterized by very striking peculiarities, exemplified, say some of our higher botanical authorities, to whom I have submitted my restoration, by no fern that now lives. The frond of Davallia Canariensis, though unlike in its venation, greatly resembles in general outline one of the larger pinnæ of Sphenopteris affinis; but these pinnæ form only a small part of the entire frond of this Sphenopteris. It was furnished with a stout leafless rachis, or leaf-stalk, exceedingly similar in form to that of our common brake (Pteris aquilina). So completely, indeed, did it exhibit the same club-like, slightly bent termination, the same gradual diminution in thickness, and the same smooth surface, that one accustomed to see this part of the bracken used as a thatch can scarce doubt that the stipes of Sphenopteris would have served the purpose equally well; nay, that were it still in existence to be so employed, a roof thatched with it, on which the pinnæ and leaflets were concealed, and only the club-like stems exposed, row above row, in the style of the fern-thatcher, could not be distinguished, so far as form and size went, from a roof thatched with brake. High above the club-like termination of the rachis the stem divided into two parts, each of which, a little higher up, also divided into two; these in turn, in at least the larger fronds, also bifurcated; and this law of bifurcation,—a marked, mayhap unique, peculiarity in a fern,—regulated all the larger divisions of the frond, though its smaller pinnæ and leaflets were alternate. It was a further peculiarity of the plant that, unlike the brake, it threw off, ere the main divisions of its rachis took place, two pinnæ placed in the alternate order, and of comparatively small size. The frond of Sphenopteris bifida was of a more simple form than that of its larger congener, and not a little resembled a living fern of New Zealand, Cœnopteris vivipara. It was tripinnate; its secondary stems were placed directly opposite on the midrib, but its tertiary ones in the alternate arrangement; and its leaflets which were also alternate, were as rectilinear and slim as mere veins, or as the thread-like leaflets of asparagus. Like the fronds of Cœnopteris when not in seed, it must have presented the appearance of the mere macerated framework of a fern. I need scarce remark that, independently of the scientific interest which must attach to restorations of these early plants, they speak powerfully to the imagination, and supply it with materials from which to construct the vanished landscapes of the Carboniferous ages. From one such restored fern as the two now submitted to the Association, it is not difficult to pass in fancy to the dank slopes of the ancient land of the Lower Coal Measures, when they waved as thickly with graceful Sphenopteres as our existing hill sides with the common brake; and when every breeze that rustled through the old forests bent in mimic waves their slim flexible stems and light and graceful foliage.
In 1844, when Professor Nicol, of Marischal College, Aberdeen, appended to his interesting "Guide to the Geology of Scotland," a list of the Scottish fossils known at the time, he enumerated only two vegetable species of the Scotch Oolitic system,—Equisetum columnare and Pinites or Peuce Eiggensis; the former one of the early discoveries of our distinguished President, Sir Roderick Murchison; the latter, of the late Mr. William Nicol of Edinburgh. Chiefly from researches in the Lias of Eathie, near Cromarty, and in the Oolites of Sutherland and the Hebrides, I have been enabled to increase the list from two to rather more than fifty species,—not a great number, certainly, regarded as the sole representative of a flora; and yet it may be deemed comparatively not a very small one by such as may remember, that in 1837, when Dr. Buckland published the second edition of his "Bridgewater Treatise," Adolphe Brogniart had enumerated only seventy species of plants as occurring in all the Secondary formations of Europe, from the Chalk to the Trias inclusive. In a paper such as the present I can of course do little more than just indicate a few of the more striking features of the Scottish flora of the middle Secondary ages. Like that of the period of the true Coal, it had its numerous coniferous trees. As shown by the fossil woods of Helmsdale and Eigg, old Oolitic Scotland, like the Scotland of three centuries ago, must have had its mighty forests of pine;[54] and in one respect these trees seem to have more nearly resembled those of the recent pine forests of our country than the trees of the coniferous forests of the remote Carboniferous era. For while we scarce ever find a cone associated with the coniferous woods of the Coal Measures,—Lindley and Hatton never saw but one from all the English coal fields, and Mr. Alexander Bryson of Edinburgh, only one from all the coal fields of Scotland,—tree-cones of at least four different species, more probably of five, are not rare in our Scottish deposits of the Lias and Oolite. It seems not improbable that in the Carboniferous genera Pinites, Pitus, and Anabathra, which approach but remotely to aught that now exists, the place of the ligneous scaly cone may have been taken, as in the junipers and the yews, by a perishable berry; while the Pines and Araucarians of the Oolite were, like their congeners in recent times, in reality coniferous, that is, cone-bearing trees. It is another characteristic of these Secondary conifers, that while the woods of the Palæozoic periods exhibit often, like those of the tropics, none of the dense concentric lines of annual growth which mark the reign of winter, these annual lines are scarce less strongly impressed on the Oolitic woods than on those of Norway or of our own country in the present day. In some of the fossil trees these yearly rings are of great breadth; they seem to have sprung up in the rich soil of sheltered hollows and plains, and to have increased in diameter from half an inch to three quarters of an inch yearly; while in other trees of the same species the yearly zones of growth are singularly narrow,—in some instances little more than half a line in thickness. Rooted on some exposed hill side, in a shallow and meagre soil, they increased their diameter during the twelvemonth little more than a line in the severer seasons, and little more than an eighth part of an inch even when the seasons were most favorable. Further, whether the rings be large or small, we ordinarily find them occurring in the same specimens in groups of larger and smaller. In one of my Helmsdale specimens, indicative generally of rapid growth, there are four contiguous annual rings, which measure in all an inch and two twelfths across, while the four contiguous rings immediately beside them measure only half an inch. "If, at the present day," says a distinguished fossil botanist, "a warm and moist summer produces a broader annual layer than a cold and dry one, and if fossil plants exhibit such appearances as we refer in recent plants to a diversity of summers, then it is reasonable to suppose that a similar diversity formerly prevailed." The same reasoning is of course as applicable to groups of annual layers as to single annual layers; and may we not venture to infer from the almost invariable occurrence of such groups in the woods of this ancient system, that that ill-understood law of the weather which gives us in irregular succession groups of colder and warmer seasons, and whose operation, as Bacon tells us, was first remarked in the provinces of the Netherlands, was as certainly in existence during the ages of the Oolite as at the present time?
CONIFERS?
