Some of the features to be noticed in longitudinal sections of Calamite stems have already been described, at least as regards younger branches. The specimen shown in fig. 81 illustrates the general appearance of a stem as seen in tangential and radial section. In the lower portion, _T_, the course of the vascular bundles is shown by the black lines which represent the xylem tracheids, bifurcating and usually alternating at each node. Between the xylem strands are the broad principal medullary rays. At _b_ a branch has been cut through on its passage out from the parent stem, just above the nodal line. In tangential sections of Calamite stems one frequently sees both branches and leaf-trace bundles (fig. 83, _A_), passing horizontally through the wood and enclosed by strongly curved and twisted tracheids. In the upper part of the figure (81, _R_), the section has passed through the centre of the stem, and the wood is seen in radial view; each node is bridged across by a diaphragm of parenchymatous cells capable of giving rise to a surface layer of periderm[621]. An outgoing branch, as seen in a tangential section of a stem, consists of a parenchymatous pith surrounded by a ring of vascular bundles, in which the characteristic carinal canals have not yet been formed, but if the section has cut the branch further from its base, there may be seen a circle of irregular gaps marking the position of the carinal canals. Such gaps are often occupied by thin parenchyma, and contain protoxylem elements. The outgoing branches, as seen in a tangential section of a Calamite stem, are seen to be connected with the wood of the parent stem by curved and sinuous tracheids, which give to the stem-wood a curiously characteristic appearance[622], as if the xylem elements had been pushed aside and contorted by the pressure of the outgoing member. A tangential section through a Pine stem[623] in the region of a lateral branch presents precisely the same features as in _Calamites_. The branches are given off from the stem immediately above a node and usually between two outgoing leaf-trace bundles. [Sidenote: RHIZOME OF CALAMITES.]