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the thin bark and firm wood with which we are familiar in our modern trees, it has a hard external rind, then a great thickness of cellular matter with rope-like bands of fibres, constituting an inner bark, while in the centre is a firm, woody axis of comparatively small diameter, and
FIG. 34.-Sigillaria Lorwayana, Dawson. a, Zones of fruit-scars. b, Leafscar enlarged. c, Fruit-scar enlarged. See appended note.
somewhat intermediate in its structures between that of the Lepidodendra and those of the cycads and the taxine conifers. Thus a great stem, five feet in diameter, may consist principally of cellular and bast fibres with very little true woody matter. The roots of this tree are
perhaps its most singular feature. They usually start from the stem in four main branches, then regularly bifurcate several times, and then run out into great
cylindrical cables, running for a long distance, and evidently intended to anchor the plant firmly in a soft and
oozy soil. They were furnished with long, cylindrical rootlets placed regularly in a spiral manner, and so articulated that when they dropped off they left regular rounded scars. They are, in short, the Stigmaria, which we have already met with in the Erian (Figs. 38, 39). In Fig. 33 I have endeavoured to restore these strange trees. It is not wonderful that such plants have caused much botanical con
part of stem of S. Brown troversy. It was long before botanists could be convinced that
their roots are properly roots at all, and not stems of some aquatic plant. Then the structure of their
FIG. 38.-Stigmaria root, seen from above, showing its regular divisions. From "Acadian Geology."
stems is most puzzling, and their fruit is an enigma, for while some have found connected with them cones supposed to resemble those of lycopods, others attribute to them fruits like those of yewtrees. For years I have been myself gathering materials from the rich coal-formation deposits of Nova Scotia in aid of the solution of these questions, and in the mean time Dr. Williamson, of Manchester, and Renault and other botanists in France, have been amassing and studying stores of specimens, and it is still uncertain who may finally be the fortunate discoverer to set all controversies at rest.
FIG. 39.-Portion of bark of Stigmaria, showing scars of attachment of rootlets.
that the true solution consists in the fact that there are many kinds of Sigillaria. While in the modern forests
of America and Europe the species of any of our ordinary trees, as oaks, birches, or maples, may almost be counted on one's fingers, Schimper in his vegetable paleontology enumerates about eighty species of Carboniferous Sigillaria; and while on the one hand many of these are so imperfectly known that they may be regarded as uncertain, on the other hand many species must yet remain to be discovered.* Now, in so vast a number of species there must be a great range of organisation, and, indeed, it has already been attempted to subdivide them into several generic groups. The present state of the question appears to me to be this, that in these Sigillariæ we have a group divisible into several forms, some of which will eventually be classed with the Lepidodendra as lycopods, while others will be found to be naked-seeded phænogams, allied to the pines and cycads, and to a remarkable group of trees known as Cordaites, which we must shortly notice.
Before considering other forms of Carboniferous vegetation, let us glance at the accumulation of coal, and the agency of the forests of Sigillaria therein. Let us imagine, in the first instance, such trees as those represented in the figures, growing thickly together over vast swampy flats, with quantities of undergrowth of ferns and other plants beneath their shade, and accumulating from age to age in a moist soil and climate a vast thickness of vegetable mould and trunks of trees, and spores and sporecases, and we have the conditions necessary for the growth of coal. Many years ago it was observed by Sir William Logan that in the coal-field of South Wales it was the rule with rare exceptions that, under every bed of coal, there is a bed of clay filled with roots of the Stigmaria, already referred to as the root of Sigillaria. This dis
*In a recent memoir (Berlin, 1887) Stur has raised the number of species in one subdivision of the Sigillariæ (the Favularia) to fortyseven!
covery has since been extended to all the coal-fields of Europe and America, and it is a perfectly conclusive fact as regards the origin of coal. Each of these "underclays," as they are called, must, in fact, have been a soil on which grew, in the first instance, Sigillariæ and other trees having stigmaria-roots. Thus, the growth of a forest of Sigillaria was the first step toward the accumulation of a bed of coal. More than this, in some of the coarser and more impure coals, where there has been sufficient earthy matter to separate and preserve impressions of vegetable forms, we can see that the mass of the coal is made up of flattened Sigillariæ, mixed with vege
FIG. 40.-Vegetable tissues from coal. a, Sigillaria and Cordaites. b, Calamodendron.
table débris of all kinds, including sometimes vast quantities of lepidodendroid spores, and the microscopic study of the coal gives similar results (Fig. 40). Further, on the surfaces of many coals, and penetrating the shales or sandstones which form their roofs, we find erect stumps of sigillaria and other trees, showing that the accumulation of the coal terminated as it had begun, by a forestgrowth. I introduce here a section of a few of the numerous beds of coal exposed in the cliffs of the South Joggins, in Nova Scotia, in illustration of these facts. We can thus see how in the slowly subsiding areas of the coal-swamps successive beds of coal were accumulated, alternating with beds of sandstone and shale (Figs. 41, 42). For other details of this kind I must refer to papers mentioned in the sequel.