The tracheids of the nodal wood are often reticularly pitted, and so differ in appearance from the ordinary scalariform elements. It is rare to find the phloem clearly preserved, but in specimens where it has been possible to examine this portion of the vascular bundles, it is found to consist of elongated cambiform cells and sieve-tubes. An unusually perfect specimen has been described by Renault[608] in which the phloem elements are preserved in silica. Fig. 72, _D_, is copied from one of Renault’s drawings, the sieve-tubes, _s_, _s_, show several distinct sieve-plates on the lateral walls of the tubes, reminding one to some extent of the sieve-tubes in a Bracken Fern. The cells, _p_, _p_, associated with the sieve-tubes are square-ended elongated parenchymatous elements. Another characteristic feature illustrated by longitudinal sections is the nodal diaphragm; except in the smallest branches the interior of each internode is hollow, and the ring of vascular bundles is separated from the pith-cavity by a band of parenchymatous tissue. At each node this parenchyma extends across the central cavity in the form of a nodal diaphragm, as in the stem of _Equisetum_. By far the greater number of the petrified fragments of _Calamites_ afford proof of cambial activity, and possess obvious secondary tissues. In exceptionally perfect specimens the xylem tracheids are found to be succeeded externally by a few flattened thin-walled cells which are in a meristematic condition (fig. 72, _A_, _c_); these constitute the cambium zone, and it is the _secondary structure_ that results from the activity of the meristematic cells that we have now to consider. [Sidenote: SECONDARY THICKENING.] In petrified examples of branches in which the secondary thickening has reached a fairly advanced stage, the wood is usually the outermost tissue preserved, the more external tissues having been detached along the line of cambium cells. It is only in a few cases that we are able to examine all the tissues of older examples.