The stele of _Lepidodendron vasculare_, before the addition of any secondary xylem, may be described as a protostele, a term originally proposed by Professor Jeffrey[271], in which the central part of the conducting strand of xylem elements has been converted into rows of parenchyma and short tracheids, the latter being better adapted to storage than to conduction. It is probable that this type of stelar anatomy, which distinguishes _L. vasculare_ from other species, represents a comparatively primitive arrangement forming a transition between the stele of _L. esnostense_, which consists of a solid rod of tracheids, and the stele of _L. Harcourtii_ (fig. 179, A) and other species in which the xylem forms a cylinder enclosing a large parenchymatous pith. Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by an irregular reticulate pitting. The tangential section represented in fig. 148, B, illustrates the appearance of a shoot of _L. vasculare_ in which no secondary xylem is present: the central strand of tissue consists of the parenchyma abutting on the xylem with several leaf-traces (_lt_) passing upwards in an almost vertical course from the outer edge of the stele. The secondary xylem (fig. 148, A, _x_²) consists of radially arranged scalariform tracheae with associated rows of parenchymatous cells which form medullary rays (fig. 149, _mr_). Leaf-traces pass through the medullary rays in the secondary xylem cylinder in a direction at right angles to the primary xylem stele from which they are given off, but at the outer edge of the secondary xylem they bend suddenly upwards and for a time follow a steep and almost vertical course. In well-preserved longitudinal sections the outermost secondary xylem tracheae are seen to be succeeded by a few narrow and vertically elongated elements (fig. 149, A, _a_), which represent young unlignified tracheae: these are followed by shorter parenchymatous cells (_m_) forming part of a meristematic zone from which the secondary xylem receives additions.