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. The annexed figure, _Lign. 37_, represents the transverse section of a small Stigmaria, with the axis displaced from its natural situation; this circumstance, as well as the corresponding external groove, has arisen from compression, by which the tough cylinder has been forced from its original position in the middle of the soft cellular tissue, to one side. [Illustration: Lign. 37. Transverse Section of Stigmaria ficoides; _nat._ (_M. Adolphe Brongniart._) This specimen shows that the cylinder (_a_ in _Lign. 36_) is formed of bundles of vascular tissue, disposed in rays.]