A portion of the cushion below the triangular leaf-scar often shows transverse gaping cracks or depressions (fig. 185, C) such as occur on a smaller scale on the older cushions of a Fir twig (fig. 140). Secondary thickening, as in recent trees, is not confined to the vascular cylinder but at an early stage, frequently before there are any signs of secondary wood, the outer region of the broad cortex becomes the seat of active cell-formation which results in the addition of a considerable thickness to the bark. At a later stage of increase in girth, the leaf-cushions are stretched apart and the original surface-features become obliterated by vertical cracks and by the exfoliation of the superficial tissues[238]. Some species of _Lepidodendron_ produced branches characterised by spiral or vertical series of scars; these in older shoots were replaced by depressions having a diameter of several inches and comparable in appearance, as also perhaps in manner of formation, with the scars left on the stem of a Kauri Pine (_Agathis australis_)[239] on the abscission of lateral branches by a natural process. These shoots, known as _Ulodendron_, are described in a subsequent section. (Page 128.) A fully-grown _Lepidodendron_ must have been an impressive tree, probably of sombre colour, relieved by the encircling felt of green needles on the young pendulous twigs. The leaves of some species were similar to those of a fir while in others they resembled the filiform needles of the Himalayan Pine (_Pinus longifolia_). The occasional presence of delicate hyphae in the tissues of _Lepidodendron_ demonstrates susceptibility to fungal pests. Architecturally, if one may use the term, _Lepidodendron_ owed its power of resistance to the bending force of the wind to its stout outer bark formed of thick-walled elements produced by the activity of a cylinder of cortical meristem (figs. 148, 172, etc.). The vascular axis, of insignificant diameter in proportion to the size of the stem (figs. 152, 153, 172, 181, A), must have played a subordinate part, from a mechanical point of view, as compared with the solid mass of wood of a Pine or an Oak. • • • • • Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of the species of the widely distributed Palaeozoic genus _Lepidodendron_. In spite of the great number of known species of this common member of Carboniferous floras, our knowledge of the type as a whole is deficient in many points, and such information as we possess needs systematising and extending by comparative treatment based on a re-examination of available data. In order to appreciate the meaning of certain external features characteristic of Lepidodendron stems it is essential to have some knowledge of the internal structure.