The most detailed account hitherto published of the anatomy of _Lepidodendron vasculare_ is that by the late M. Hovelacque[268], based on material from the Lower Coal-Measures of England. [Illustration: FIG. 148. _Lepidodendron vasculare_ Binney. A. Transverse section. (Based on a section 2·5 cm. in diameter, in the Cambridge Botany School Collection.) B. Longitudinal section. (Drawn from a section in Dr Kidston’s Collection.)] The small shoot, represented somewhat diagrammatically in fig. 148, A, illustrates the anatomical features of a typical example of the species: the shoot has a diameter of 2·5 cm. and its central cylinder (_x_-_sc_) is 2·5 mm. in width. Noticeable features are (i) the small size of the central cylinder (or stele) in proportion to the diameter of the branch, (ii) the production at a comparatively early stage of growth of a zone of secondary wood, _x_², which gradually assumes the form of a complete cylinder of unequal breadth, surrounding the primary xylem, _x_, (iii) the formation of a secondary cortical tissue by a meristematic cylinder (phellogen, _pl_) situated close to the leaf-cushion region of the outer cortex. On the outer edge the stele consists of narrow tracheae some of which show in longitudinal section the spiral form of thickening characteristic of most protoxylem elements: towards the centre of the stele the diameter of the tracheae gradually increases and parenchymatous cells become associated with the elongated scalariform elements. In the central region the stele is composed of parenchymatous tissue arranged in vertical series of short cells, interspersed with short tracheae distinguished by the greater thickness of their walls and by their scalariform and reticulate thickening bands. Some of these short tracheae are shown in vertical section in fig. 149, B: the fine and broken lines connecting adjacent thickening bands probably represent the remains of the original wall. These delicate bands, which have been figured in various species of lepidodendroid plants[269], are worthy of notice in connexion with the recent work of Mr Gwynne-Vaughan[270] who has shown that in many recent ferns the scalariform bands in the xylem elements are not connected by a thin pit-closing membrane, but are separated from one another by open spaces. In the Lepidodendron tracheae we seem to have a stage in which the intervening membrane is in process of absorption. It is, however, possible that the threads may be the result of contraction and splitting of the membrane during drying or decay. [Illustration: FIG. 149. _Lepidodendron vasculare_. _a_, immature tracheae; _m_, meristem; _mr_, medullary ray; _x_, xylem. A. Longitudinal section through the edge of the secondary wood. B. Short tracheae in the centre of the stele. (From a specimen from the Halifax Hard bed in Dr Kidston’s Collection.)]