An interesting feature observed in some specimens of older Calamite branches is the development of periderm or cork. This is illustrated on a large scale by a unique specimen originally described by Williamson in 1878[616]. Figs. 78 and 79 represent transverse and longitudinal sections of this stem. This unusually large petrified stem was found in the Coal-Measures of Oldham, in Lancashire. In the slightly reduced drawing, fig. 78, the large and somewhat flattened pith, _p_, 4·2 cm. in diameter, is shown towards the bottom of the figure. Surrounding this we have 58 or 59 wedge-shaped projecting xylem groups and broad medullary rays; the latter soon become indistinguishable as they are traced radially through the thick mass of secondary wood, 5 cm. wide, composed of scalariform tracheids and secondary medullary rays (fig. 78, 3). The secondary wood presents the features characteristic of _Calamites_ (_Arthropitys_) _communis_ (Binney). External to the wood there is a broken-up mass, about 5·5 cm. wide composed of regularly arranged (fig. 78, 2) and rather thick-walled cells; this consists of periderm, a secondary tissue, which has been developed by a cork-cambium during the increase in girth of the plant. The more delicate cortical tissues have not been preserved, and the more resistant portion of the bark has been broken up into small pieces of corky tissue, among which are seen numerous Stigmarian appendages, pieces of sporangia and other plant fragments. These associated structures cannot of course be shown in the small-scale drawing of the figure. [Illustration: FIG. 78. 1. Transverse section of a thick Calamite stem. _p_, pith; _x_, secondary wood; _c_, bark. (⅔ nat. size.) 2. Periderm cells of bark. 3. Xylem and medullary rays. (2 and 3, × 80.) From a specimen in the Williamson Collection (no. 79).] In the radial longitudinal section (fig. 79) we see the pith with the projecting wood and the remains of a diaphragm at the node. The mottled or watered appearance of the wood is due to numerous medullary rays which sweep across the tracheids. The periderm elements, as seen in longitudinal section, are fibrous in form. [Illustration: FIG. 79. Longitudinal section of the specimen stem in fig. 78. From a specimen in the Williamson Collection, British Museum (no. 80). ⅔ nat. size.] [Sidenote: CALLUS WOOD.] The development of cork in a younger Calamite stem is clearly shown in a specimen described by Williamson and Scott in their Memoir of 1894. In a transverse section of the stem several large cells of the inner cortex are seen to be in process of division by tangential walls, and giving rise to radially arranged periderm tissue[617].