Fig. 14. Sigillaria, with black carbonized bark partially removed.
There can be no doubt that these tubercules must once have supported leaflets. They are true leaf-scars, like those on the Scotch fir, and the lozenge-shaped knobs on the bark of lepidodendron. But of the form of these leaves we are still in ignorance, for no part of the plant, save the stem and roots, has yet been found. The sigillaria must have been a tree that could not long withstand maceration, for not only are its leaves gone, but, in many cases, the outer bark has partially or wholly decayed, leaving a scarcely distinguishable mass of carbonized matter.[23] When this outer rind is peeled off, the inner surface of the stem is seen to be ridged, furrowed, and tuberculed in the same way, but the markings are much less distinct than on the outside. The bark sometimes attains the thickness of an inch, and is always found as a layer of pure coal enveloping the stem where it stands erect, or lying as a flat cake without any central cylinder where the stem is prostrate. (See [Fig. 14].)
[23] Another proof of the looseness of the texture of this ancient vegetable may be gathered from the almost invariable truncation of even the largest erect stems; they are snapped across at the height of a few feet from their base. The famous "Torbanehill Mineral" contains many such fragmentary stems, often of considerable thickness. Their interior consists of the same material as the surrounding bed, and displays many dissevered plants that may have been washed into the decaying trunks. For the internal structure of sigillaria see Dr Hooker's Memoir, and the authorities therein cited.
Another remarkable feature in this carboniferous plant is that it appears to have had no branches along its stem. Trunks have been found four and five feet in diameter, and have been traced to a distance of fifty, sixty, and even seventy feet, without any marks of branches being detected. Brongniart examined the portion of one stem, which, at its thicker end, had been broken across, but still measured a foot in breadth. It ran for forty feet along the gallery of a mine, narrowing to a width of not more than six inches, when it divided into two, each branch measuring about four inches across. The sigillaria stems, accordingly, must have shot up, slim and straight, to a height of sometimes seventy feet before they threw out a single branch. We know nothing of the coronal of these strangely-formed trees. From Brongniart's observations, it would seem that the upper part of the stem, like that of the lepidodendron, was dichotomous, that is, it branched out into two minor stems; but how these were disposed is unknown. We are wholly ignorant, too, of the foliage of these branches, though, from the general structure of the plant, as well as from the number of fern-fronds often found around the base of the stems, it has been conjectured that the sigillaria was cryptogamous, and, like the tree-ferns, supported a group of sweeping fronds. If so, it differed in many respects from every known member of the cryptogamic tribes.
Putting together, then, all that we know of the exterior of the sigillaria, we find that it was a tall slender tree, with, palm-like, a clump of foliaged branches above, its stem bristling thickly, in at least its upper part, with spiky leaves, and its roots equally hirsute, shooting out to a distance of sometimes forty feet through the soft muddy soil. Future researches may bring us better acquainted with this ancient organism. In the meanwhile, enough of it is known to mark it out as one of the most ornate forms of vegetation that the world has ever seen.
In addition to the above, the coal strata have yielded many other fragmentary remains, to which names have been given, but of which very little is known. It is pleasant, amid such a wide sea of doubt and uncertainty, to alight upon some well-known form of whose affinities there can be no question, since it still finds its representatives in living nature. Of such a kind are the coniferous stems occasionally met with in the sandstones of the coal-measures. .
It is now many years since the operations of the quarryman in the carboniferous sandstones of Edinburgh and Newcastle disclosed the remains of huge gnarled trunks deeply imbedded in the rock. The neighbourhood of the latter town yielded, in 1829,[24] the stem of a tree seventy-two feet long, without branches, but roughened with numerous knobs, indicative of the places whence branches had sprung. At Craigleith, near Edinburgh, a trunk thirty-six feet long, and three feet in diameter at the base, was disinterred in the year 1826. Since then, several others have been found in the same neighbourhood; some of them sixty and even seventy feet in length, and from two to six in breadth. They were, for the most part, stripped of roots and branches, and lay at a greater or less angle among the white sandstone beds, which they cut across obliquely. It was unknown for some time to what division of the vegetable kingdom these trunks should be referred. Their irregular branched surface and undoubted bark indicated a higher kind of structure than that possessed by any of the other carboniferous plants; but the conjecture remained unverified until an ingenious and beautiful method was discovered of investigating their internal organization. Two Edinburgh geologists, Mr. Nichol and Mr. Witham, succeeded in obtaining slices of the plants sufficiently transparent to be viewed under the microscope by transmitted light, and in this way their true structure was readily perceived. The method of preparing these objects was simply as follows:—A thin slice of the plant to be studied was cut by the lapidary, or detached by the hammer. One side having been ground down smooth, and polished, was cemented by Canada balsam to a piece of plate-glass, and the upper surface was then ground down and polished in like manner, so as to leave the slice no thicker than cartridge-paper.[25] When the preparation was then placed under a magnifying power, the minute cells and woody fibre of the plant could be detected as clearly as those of a recent tree. The Craigleith fossils were in this way recognised as belonging to the great coniferous family, and to that ancient[26] division of it which is, at the present day, represented by the pine of Norfolk Island—"a noble araucarian, which rears its proud head from 160 to 200 feet over the soil, and exhibits a green and luxuriant breadth of foliage rare among the coniferæ."[27] Some of these plants have yielded faint traces of the annual rings shown so markedly in the cross section of our common forest-trees; whence it would appear, that even as far back as the times of the coal-measures, there were seasons of alternate heat and cold, though probably less defined than now.
[24] Witham's Foss. Veget. p. 31.
[25] For a more detailed description of the process, see Witham's Foss. Veget. p. 45.
[26] The solitary lignite of the Lower Old Red Sandstone, seems to have been araucarian. Miller's Footprints of the Creator, p. 203.