Our knowledge of the minute structure of the tissues abutting on the secondary xylem is far from complete.
The xylem is succeeded by a zone of delicate cells which was the seat of meristematic activity. It is noteworthy that in a section figured by Williamson[600] there is the same disparity in size between the outermost elements of the xylem and the adjacent cells of the meristematic zone as in Lepidodendron stems. Beyond this region an imperfectly preserved lacunar tissue occurs like that which I have called the secretory zone in Lepidodendron stems; but information as to the structure of this part of Stigmaria is much more incomplete than in the case of the aerial shoots. The middle cortex was of the same lacunar type as in the stems, and the fact that it is never well preserved in large Stigmarian axes suggests that it may have been even more richly supplied than in the aerial stems with an aerating system of spaces. The outer cortex, consisting in young examples of large-celled parenchyma, became at an early stage of growth the seat of cambial activity which resulted in the production of radially placed series of secondary elements (fig. 210, H, p). The outer and older elements of this secondary cortex are more tangentially stretched than the inner cells, a necessary result of the position of the phellogen on the internal edge of the tissue and of the increasing girth of the axis.
In comparatively young Stigmarian axes the outer cortex already possesses a band of secondary radially disposed cells characterised by the greater tangential extension of the more external elements; usually this tissue terminates abruptly on the inner edge and the line of separation no doubt marks the position of the phellogen. Occasionally some delicate secondary elements are preserved internal to the phellogen, and these in young specimens form a narrow cylinder composed in part of radially elongated cells showing signs of recent tangential divisions. In its earlier stage of activity the phellogen seems to form a greater amount of secondary tissue on the outside, but this is clearly not of the nature of cork, the tissue which occupies a corresponding position in recent plants. The primary cortex shows no signs of shrinkage or collapse as would be the case were it cut off from the vascular system by a zone of impermeable cork.
[Fig. 210], G, represents a piece of the external tissue of a specimen in which the slightly flattened xylem cylinder measures 1·4 × 1 cm.; the inner cortex has disappeared and fragments only of the middle cortex are preserved. The outer cortex, with an average breadth of 2 mm., consists superficially of primary parenchyma with a somewhat uneven surface and with a rootlet attached here and there; a short distance below the surface is a band of conspicuous cells, b, characterised by dark contents suggesting very imperfectly preserved fungal hyphae, but the nature of the substance filling the cells cannot be made out with certainty. It is, however, interesting to find that this dark band constitutes an obvious feature ([fig. 210] H, b); its position is comparable with that of the dark-walled cells in the outer cortex of rootlets. A short distance internal to this dark band tangentially elongated cells form the outermost elements of the secondary cortex; these become gradually narrower towards the interior and pass into radial series of smaller cells of uniform size, as seen on the inner edge of [fig. 210], G. At the inner boundary of this tissue, just below the region shown at the bottom of the drawing, was situated the phellogen. Such traces of tissue as occur on the inner side of the line where splitting has usually occurred, consist of thinner elements with recently formed tangential walls and probably represent an early stage in the development of phelloderm.
A much older section is shown in part in [fig. 210], E. The secondary xylem cylinder, St, is shown in the lower part of the section; beyond this is a band of secondary tissue which reaches in some places a breadth of 6 cm. The greater part of this tissue consists of phelloderm of very uniform structure made up of radial series of cells: this is interrupted in most parts of the section by a gap crowded with intruded rootlets (a portion of this is enlarged in [fig. 210], D). Beyond this gap the secondary tissue consists of radial series of cells characterised by the considerable tangential elongation of many of the elements, precisely like the tissue figured by Williamson. In all probability the gap represents a line of weakness due to the phellogen, and if this is the case it is clear that in an old Stigmaria the phelloderm exceeded in amount the tissue formed external to the phellogen. The secondary tissue on the inner side of the phellogen is characterised by numerous irregular concentric lines superficially resembling rings of growth in the wood of a Conifer: these are, however, not the result of any periodic change in external conditions, but are apparently due to crushing of the tissue and are possibly, to some extent, the result of the presence of secretory strands like those in the phelloderm of Lepidodendron. The surface of this older rhizome retains patches of primary tissue, and an occasional rootlet, as at r, [fig. 210], E, is seen in connexion with the cortex; the cortex has been vertically fissured as the result of secondary growth and presents an appearance like that shown in Lepidodendron Wünschianum and L. Veltheimianum (figs. [181], A, and [186], A).
The form in which a Stigmarian rootlet is usually preserved is shown in [fig. 210], D; the single vascular bundle strand with its endarch protoxylem ([fig. 210], B, px) is enclosed by a ring of inner cortical parenchyma ([fig. 210], F, c1); the cells in immediate contact with the xylem having usually disappeared. Beyond the middle cortical space a second cylinder of parenchyma represents the outer cortex (F, c3) in which a layer of dark-walled cells (b, [fig. 210], F) may be compared with the hypodermal band in the main Stigmarian axis (G, b). These Stigmarian rootlets, usually less than 1 cm. in diameter, are the commonest objects in sections of the calcareous nodules from English coal-seams. A good example of their abundance is shown in [fig. 210], D and E; here they have invaded the space formed by the splitting of the secondary cortical tissues along the line of the phellogen and a few are seen here and there in the deeper layers of the phelloderm (s, [fig. 210], E). Not infrequently the close contact of these ubiquitous rootlets with the tissues of the plant which they have invaded leads to confusion between invader and invaded. Partially decayed tissues lying, probably, under water were penetrated by Stigmarian rootlets in exactly the same way as the roots of recent plants bore through vegetable substances which happen to be in their path. The rootlet bundles are in the first instance composed of the primary tracheae which line the inner edge of the secondary xylem; these receive additions from the meristematic zone, and thus, when seen in the cortex outside the stelar region, are found to consist in part of primary and in part of a fan-shaped group of secondary tracheae. On the other hand, the monarch bundle as it appears in a free rootlet is usually composed entirely of primary elements ([fig. 210], A–C, F). It has been shown by Weiss[601] that in the Stigmarian rhizome of what is probably Lepidodendron fuliginosum, the rootlet bundle is accompanied by a parichnos strand, but this has not been detected in the ordinary Stigmaria ficoides. When free from the parent axis a rootlet usually consists of an outer cylinder of cortex enclosing a broad space in which remnants of lacunar tissue are sometimes seen. The relation of the external features of a well-preserved Stigmarian rootlet-scar to the internal structure of a petrified rootlet is very clearly seen on comparing such sections as those represented in [fig. 210], D, with the form of the scar on a Stigmarian cast. A specimen figured by Hooker[602] in 1848 affords a good illustration of the structure of a rootlet-base as seen in an unusually complete cast; this correlation of anatomical and surface features is clearly described also by Williamson[603] and by Solms-Laubach[604]. It is probable that even during life the rootlets were hollow for a part at least of their length as are the roots of Isoetes ([fig. 133], G).
An interesting discovery was made a few years ago which confirmed a statement by Renault which Williamson was unable to accept, namely that the xylem bundle of a rootlet occasionally gives off a delicate tracheal strand at right angles to the long axis of a rootlet. In some rootlets Weiss[605] found obliquely running delicate strands of xylem, surrounded by a layer of parenchymatous tissue, in the space between the vascular bundle and the outer cortical cylinder. It is clear that a few spiral tracheids are occasionally given off from the protoxylem of a rootlet bundle: these follow an oblique course to the outer cortex, where in some cases they have been traced into connexion with short and spirally marked cells resembling transfusion tracheae ([fig. 210], A). This arrangement may serve as a means of facilitating the passage of water absorbed by the superficial cells into the xylem strand. It should be noticed that, like roots of recent water-plants, the rootlets of Stigmaria had no root-hairs. Fig. 210, F, shows a transverse section of part of a rootlet in which the outer cortical cylinder, c3, is connected, as in the roots of Isoetes, with the sheath surrounding the vascular bundle. A few obliquely cut tracheae are seen in this section traversing the connecting band of parenchyma t, [fig. 210], A.
A point of biological interest in connexion with Stigmaria rootlets is the occasional presence of hypertrophied cells, the large size of which is due to the attacks of a fungus named by Weiss[606] Urophlyctites stigmariae.
In addition to Stigmaria ficoides, which is by far the commonest form, a few other species have been founded on external characters. One of these is represented by Stigmaria stellata, Goepp.[607], characterised by the presence of radially disposed ridges and small tubercles surrounding each rootlet-scar. Kidston refers to Goeppert’s species as a Lower Carboniferous type. We have no evidence as to the meaning of the stellate ridges and tubercles, nor have we any reason to suppose that this form differed essentially in structure from Stigmaria ficoides.