Stigmaria. Lign. 36.—This extended notice of the structure of the Sigillariæ,.will enable us to understand the nature of the fossil vegetables termed Stigmariæ, or spotted-stems, which abound in the beds of under-clay of most coal-fields, as stated in a former part of this work (ante, [p. 81].). These bodies when uncompressed are of a cylindrical form, from one to six or seven inches in diameter, and of considerable length—sometimes upwards of twenty feet—and gradually diminish in size towards their termination. The surface is marked with distinct pits or areolæ, of a circular or oval form, with a small tubercle in the centre of each, disposed around the stem in a quincunx and somewhat regular order. When broken transversely, a small cylindrical axis is seen to extend in a longitudinal direction through the stem, like a medullary column; it seldom occupies the centre, but lies near to one side, and parallel with a depression on the outer surface of the fossil. This internal body is often loose, and removable; its surface is covered with interrupted, irregular, longitudinal, ridges, which leave corresponding depressions on the walls of the cavity in which it was inclosed. [Lign. 36], represents a fragment exhibiting the characters above described.
Lign. 36. Stigmaria ficoides.
Carboniferous. Derbyshire.—1/4 nat.
| Fig. | 1.— | Portion of a stem, with some of the rootlets (formerly considered as leaves) extending into the surrounding clay. The internal axis is seen at a; and the corresponding groove on the portion of external surface that remains. |
| 2.— | An outline of one of the rootlets, with a tubercle to show the mode of its attachment by a ball and socket joint to the root. |
When Stigmariæ are observed in the under-clay, to which stratum they are principally confined, long, tapering, subcylindrical fibres, often several feet in length, are seen affixed to the tubercles with which the surface is covered; their form and mode of attachment are shown in [Lign. 36]. Instances occur in which several Stigmariæ spring from a common centre, of a dome-like form, whence they radiate in every direction (Bd. pl. lvi. fig. 8), and the main branches divide and subdivide till they are lost in the surrounding rock.
The nature of these fossil vegetables was long a perplexing question, for no specimens had been found in connexion with any of the stems, branches, or foliage, that abound in the coal deposits. At length, the discovery of a dome-shaped mass, to which were attached numerous Stigmariæ, seemed to afford a clue to the solution of this botanical problem; and it was concluded by the eminent authors of the "Fossil Flora of Great Britain," that the original belonged to a tribe of plants which inhabited swamps, or still and shallow lakes, and were characterized by a low truncated stem, having long horizontal branches beset with cylindrical, and, probably, succulent leaves, that either trailed on the surface of the swamp, or floated in the water.
But within the last few years, the occurrence in various carboniferous deposits of erect stems of Sigillariæ, has shown that the Stigmariæ are nothing more than the roots of these and other congenerous trees; an opinion maintained by the Rev. H. Steinhaur more than thirty years ago, and subsequently affirmed by M. Adolphe Brongniart, who found, on examining microscopically the internal structure of a silicified specimen in which the vascular tissue was preserved, that it bore as close an analogy to that of the Sigillariæ, as exists between the roots and trunks of certain dicotyledonous trees.