reparatory strand deeper in the medullary tissue. In this species there is no evidence that an outgoing trace is accompanied by an arc of secondary xylem, but that such was the case is rendered highly probable by the discovery of an arc of wood added by the cambium to a leaf-trace in _Pitys Withami_[761]. The radial disposition of the pith-cells, many of which appear to be secretory, around each primary xylem-strand (fig. 487, B) is a characteristic feature. A difficult problem, namely the relation of primary xylem and phloem, is raised by the fact that the primary strands are composed exclusively of xylem and are in most cases separated from the secondary wood by several parenchymatous cells. Scott favours the view that the primary phloem was originally at some distance from the primary xylem, the cambium being formed nearer to the phloem, an arrangement foreign to recent Gymnosperms. It is perhaps conceivable that the primary conducting strands included no true phloem. [Illustration: Fig. 487. _Pitys antiqua._ A, radial section of secondary xylem; B, transverse section showing the inner edge of the secondary wood and two primary-xylem strands with protoxylem, _px_. (Kidston Coll.)] The secondary xylem consists of tracheids with 4–5 alternate rows of hexagonal pits on the radial walls (fig. 487, A) and not infrequently on the tangential faces. Near the ends of the tracheids the pits are occasionally more scattered and separate and may be reduced to a single row[762], but on the whole the pitting is essentially Araucarian. The medullary rays are generally 4 cells in breadth, but may be 7 cells broad. In depth the rays extend to 70 cells. As seen in fig. 487, B, the inner ends of the rays are especially broad owing to the tangential dilatation of the cells. The innermost secondary tracheids are characterised by pitting intermediate between spiral and reticulate. There are no regular rings of growth in the wood. _Pitys primaeva_ Witham. This species[763], also founded on material from the Calciferous sandstone of Berwickshire, differs from _P. antiqua_ in the broader and shorter form and greater breadth of the medullary rays (fig. 488, A, B), also in the larger tracheids and in the less crowded arrangement of the bordered pits in which the circular form sometimes replaces the hexagonal type. The structure of the pith is not known, but Scott was able to recognise in the partially preserved pith of a branch indications of primary-xylem groups and other features pointing to a close resemblance to _P. antiqua_[764]. A piece of stem originally described by Williamson[765] as _Lyginodendron_ (?) _anomalum_ from the Lower Carboniferous volcanic ash of the Isle of Arran would appear to be closely related to, if not specifically identical with, _P. primaeva_ and, as Scott says, it should at least be included in the same genus.