The section (fig. 151, A) measures 7 × 3·8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood (fig. 151, E, _x_ and _x²_). The almost smooth surface of the primary wood (fig. 151, E, _x_) is succeeded by the secondary xylem, _x²_, characterised at its inner edge by the tapered ends of the radial rows of scalariform tracheids between which occur several delicate parenchymatous cells (fig. 151, E, _a_). The occurrence of such isodiametric elements, often exhibiting a delicate spiral thickening band, is a characteristic feature of the boundary between primary and secondary wood in lepidodendroid stems. The secondary wood is penetrated by numerous medullary rays and in some of them are seen strands of narrow spirally thickened tracheae—the leaf-traces—which are in organic continuity with the exarch protoxylem of the primary wood. The leaf-traces are oval and mesarch. The space, _c²_, (fig. 151, A) originally occupied by the delicate middle cortex, is succeeded by a shell of outer cortex composed chiefly of secondary tissue (phelloderm, _pd_) passing towards the inner boundary of this region into the primary outer cortex _g_ (fig. 151, A and C). The radially disposed elements which make up the bulk of the phelloderm are associated with concentric rows of secretory strands, represented by tangentially arranged dots in fig. 151, A: on the outer edge of the phelloderm a few patches of primary cortex are still preserved, as at _c_, fig. A. One of these is shown on a larger scale in fig. B; at _m_ the phelloderm is interrupted by a gap beyond which the cells have thinner walls and show signs of recent division; this is probably the position of the phellogen. The tissue _b_, fig. 151, B, consists of secondary cortex succeeded beyond _d_ by the parenchymatous tissue of the leaf-cushion, in which the remains of a ligule, _l_, are seen in the ligular pit. This section corresponds in position to a line drawn across fig. 150 at the level of _b_. In this specimen we have two kinds of secondary cortical tissue: that formed external to the phellogen, from _m_ to _d_ in fig. 151, B, is less in amount than that produced internal to the phellogen. We cannot make any satisfactory statement as to the nature of this secondary tissue, whether or not any of it agreed in composition with the cork which is usually formed external to the phellogen in recent plants. As the stem of a _Lepidodendron_ grew in girth the leaf-cushions became separated by intervening depressions composed of the secondary cortex formed external to the phellogen, but at a later stage the cushions were thrown off, leaving the outer edge of the phelloderm as the superficial tissue. This exposed tissue became fissured as growth and consequent stretching continued, producing the appearance seen on the surface of the still older stem represented in fig. 153.