Sigillaria ([Plate IV. Figs. 1 and 2]) is perhaps the most important plant in the coal formation. The name is derived from sigillum, a seal, to indicate the seal-like markings in the stem. It is found in all coal-shales over the world. Schimper mentions 83 species. It occurs in the form of lofty stems, 40-50 feet high, and 5 feet broad (Figs. 33 and 34). Many stems of Sigillaria may be seen near Morpeth, standing erect at right angles to the planes of alternating strata of shale and sandstone (Fig. 33). They vary from 10 to 20 feet in height, and from one to three feet in diameter. Sir W. C. Trevelyan counted 20 portions of these trees within the length of half-a-mile, of which all but four or five were upright. Brongniart mentions similar erect stems as being found near St. Etienne. The stem of Sigillaria is fluted in a longitudinal manner, like a Doric column, and has a succession of single scars, which indicate the points of insertion of the leaves (Figs. 35, 36, and 37). When the outer part of the stem separates like bark, it is found that the markings presented by the inner surface differ from those seen externally. This has sometimes given rise to the erroneous multiplication of species and even of genera. Sigillaria elegans, as figured by Brongniart in Archives du Museum, i. 405, has a stem consisting of a central cellular axis or medulla, surrounded by a vascular cylinder, and this is invested by a thick cellular cortical layer, the outer portion composed of fusiform cells of less diameter than those of the inner portion. What Brongniart calls medullary rays are mere cracks or separations in the wedges traversed by vessels. In its structure it resembles its root Stigmaria, and must be referred to Lycopodiaceæ, along with Lepidodendron, Halonia, Ulodendron, etc. The small round sporangia of Sigillaria are borne in a single patch on the somewhat enlarged bases of some of the leaves. (See Carruthers on Structure and Affinities of Sigillaria, in Journ. Geol. Soc. Aug. 1869.)
Fig. 38. Fig. 39.
Fig. 38. Stigmaria ficoides, root of Sigillaria, giving off rootlets, which have been compressed.
Fig. 39. Stigmaria ficoides (S. Anabathra of Corda), which is the root of a Sigillaria. The markings are the points whence rootlets proceed.
It has been ascertained by Professor King and Mr. Binney of Manchester, that the plant called Stigmaria (Fig. 38) is not a separate genus, but the root of Sigillaria ([Plate IV. Figs. 1 and 2]). The name is derived from στίγμα, a mark, indicating the markings on the axis. It is one of the most common productions of the coal-measures, and consists of long rounded or compressed fragments, marked externally by shallow circular, oblong, or lanceolate cavities (Fig. 39) in the centre of slight tubercles, arranged more or less regularly in a quincuncial manner ([Plate III. Fig. 7]). The cavities occasionally present a radiating appearance. The axis of the fragments is often hollow, and different in texture from the parts around. This axis consists of a vascular cylinder or woody system, penetrated by quincuncially arranged meshes or openings, through which the vascular bundles proceed from the inner surface of the cylinder to the rootlets ([Plate III. Figs. 8 and 9]). From the scars and tubercles arise long ribbon-shaped processes, which were cylindrical cellular roots, now compressed (Fig. 38). The vascular cylinder of Stigmaria is composed entirely of scalariform tissue, pierced by meshes for the passage, from the inner surface of the cylinder, of the vascular bundles which supply the rootlets. (Carruthers in Geol. Proc., Aug. 1869.) Stigmaria ficoides (Fig. 38) abounds in the under-clay of a coal-seam, sending out numerous roots from its tubercles, and pushing up its aerial stem, in the form of a fluted Sigillaria. On the Bolton and Manchester Railway Mr. Binney discovered Sigillarias standing erect, and evidently connected with Stigmarias which extended 20 feet or more.[9] Stigmaria is regarded by Schimper as roots, not of Sigillaria only, but of Knorria longifolia (one of the Lepidodendreæ). The base of the stem of this species of Knorria is Ancestrophyllum, and the upper part is Didymophyllum Schottini of Goeppert. Professor King and others suppose that the Fern-like frond called Neuropteris is connected with Sigillaria, but this is a mere conjecture, set aside by the discovery of leaves attached to a species allied to Sigillaria elegans, which establishes that the long linear leaves described under the name Cyperites are the foliage of this genus. Goldenberg has figured the fructification, which consists of small sporangia like those of Flemingites, borne on the basis of but slightly modified leaves. This establishes the opinion that Sigillaria was an acrogenous plant belonging to Lycopodiaceæ. Brongniart reckons it as representing an extinct form of Gymnosperms, and King, having erroneously associated the Cyclopteris with it, places it between the Ferns and Cycadaceæ. Mr. Carruthers informs me that he has examined the stem of a true fluted Sigillaria, with the tissues preserved, and that these agree with the structure of Lepidodendron, a position in which he had already placed it from the structure of its fruit.
Fig. 40. Fig. 41.
Figs. 40 to 44 exhibit the stems and fructification of Lepidodendron. Fig. 40. Bifurcating stem of Lepidodendron obovatum (elegans), showing the scale-like scars, and the narrow-pointed leaves, resembling those of Lycopodium, but much larger. Fig. 41. Stem of Lepidodendron crenatum, with the scars of its leaves.
Fig. 42. Fig. 43.