Fig. 215. Bothrodendron mundum (Will.).

Reference was made in Volume I. (p. 133) to the so-called paper coal of Carboniferous age from Central Russia, which consists of masses of thin strips of cuticle of Bothrodendron stems. The figures published by Zeiller[637] show that the plant possessed an epidermis consisting of polygonal cells interrupted by spirally disposed gaps marking the position of leaves; the gaps measure 0·5–1·5 mm. in breadth and agree, therefore, with the size of the leaf-scars of the smaller forms of Bothrodendron. The specimens from the Russian mines were first figured by Trautschold and Auerbach[638] as Lepidodendron tenerrimum and afterwards referred by Zeiller to Bothrodendron punctatum[639]. Nathorst[640], however, states that an examination of the Russian material leads him to retain the name originally proposed; he records the same type from Upper Devonian rocks of Spitzbergen. The chief interest of these Russian specimens is their manner of preservation, which Renault has described as the result of bacterial action; he claims to have recognised the actual bacteria associated with the cuticular membranes[641].

Anatomy of vegetative shoots of Bothrodendron.

In 1889 Williamson[642] described several specimens of petrified shoots from the Coal-Measures of Halifax which he named Lepidodendron mundum: these are now known to be branches of a Bothrodendron. The discovery was made by Mr Lomax[643] who found specimens showing the external characters of Bothrodendron and the anatomical characters of Lepidodendron mundum. In some of the smaller twigs, the stele consists of a solid core of xylem with external protoxylem; but in the majority of specimens the centre of the xylem is replaced by parenchymatous tissue, either as a small axial strand or, as in the specimen shown in [fig. 215], D, a wide pith, the elements of which are arranged in regular vertical series. A diagrammatic section of a small axis is represented in [fig. 215], A: this branch, 2 mm. in diameter, is composed of a broad outer cortex consisting exclusively of primary tissue the outer cells of which are smaller and have thicker walls than the more internal elements. The leaf-traces, lt, are accompanied by a strand of delicate tissue, the parichnos. The stele is almost solid; the tissues in contact with the xylem have not been preserved but the inner cortex is represented by a few layers of small parenchymatous cells, c1. The larger section shown in [fig. 215], D, was cut from a specimen from Dulesgate of which the smooth surface exhibits the characteristic leaf-scars of Bothrodendron. The section measures 3 cm. in its longest diameter and the stele has a breadth of 3 mm. The outer cortex has a smooth surface and is composed of rather thick-walled cells succeeded by a zone of secondary elements. The middle cortex has disappeared and the space is partially occupied by Stigmarian rootlets, s, and crushed patches of cortical tissue. The position of a leaf-scar is seen at a; this is more clearly shown in the enlarged drawing fig. E.

In his account of Lepidodendron mundum, Williamson[644] described a section in which the primary wood is surrounded by a considerable thickness of secondary xylem; a diagram of this is shown in [fig. 215], C. An examination of the section led me to compare the structure of the outer cortical cells, characterised by radial rows of tangentially elongated elements, with the outer cortex of Stigmaria. It has recently been shown by Weiss[645] that this and other similar sections present several points of agreement with Stigmaria, particularly with Stigmaria Brardi as described by Renault. At s in [fig. 215], C, a vascular strand is seen passing through the outer cortex; this is almost certainly the bundle of a rootlet: in the sections described by Weiss rootlets are shown in a similar position. The chief anatomical features of the Stigmaria-like organs of Bothrodendron are:—the considerable development of secondary xylem, the structure of the outer cortex, which is practically identical with that of Stigmaria ficoides, and the association of groups of short transfusion tracheids with the bundles of the rootlets. It is very probable that the absence of secondary xylem in the vegetative shoots of Bothrodendron is merely an accident and not a real distinction between the aerial and subterranean branches of the plant; a supposition rendered probable by the occurrence of secondary xylem in the axis of the cone described by Watson. As Weiss points out, there are certain differences between the true Stigmaria and the corresponding organ of Bothrodendron; the secondary xylem in Bothrodendron is not broken up by broad medullary rays as in the common Stigmaria, and in Bothrodendron the occurrence of a ring of primary xylem is another peculiarity.

In the vegetative shoots of Bothrodendron mundum the stele differs from those of Lepidodendron in the narrower primary xylem ring and in the large size of the metaxylem tracheae; from Lepidodendron Harcourtii and L. fuliginosum the xylem is distinguished by its smoother outer face which consists of numerous narrow xylem elements.

Fig. 216. Bothrostrobus. l, ligule. (After Watson.)

Cones of Bothrodendron (Bothrostrobus[646]).