_a. Stems._ A transverse section of a young twig, such as is represented in fig. 71, illustrates the chief characteristics of the _primary structure_ of a young branch of _Calamites_. The figure has been drawn from a section originally described by Hick[605] in 1894. A very young Calamite twig bears an exceedingly close resemblance to the stem of a recent _Equisetum_. The axial region of the stem may be occupied by parenchymatous cells, or the absence of cells in the centre may indicate the beginning of the gradual formation of the hollow pith, which is one of the characteristics of _Calamites_. The student of petrified Palaeozoic plants must constantly be on his guard against the possible misinterpretation of Stigmarian ‘rootlets,’ which are frequently found in intimate association with fossil tissues. The intrusion of these rootlets is admirably illustrated by a section of a Calamite stem in the Williamson Collection (No. 1558) in which the hollow pith, 2 cm. broad, contains more than a dozen Stigmarian appendages. [Illustration: FIG. 71. Transverse section of a young Calamite stem. _c_, carinal canals; _mr_, primary medullary rays; _a, b, and d_, cortex; _e_, epidermis. From a section in the Manchester Museum, Owens College, × 60.] In the figured specimen of a Calamite twig (fig. 71) there is a clearly marked differentiation into a cortical region and a large stele or central cylinder. The pith-cells are already partially disorganised, but there still remain a few fairly large parenchymatous cells internal to the ring of vascular bundles. The few irregular projections into the cavity of the large pith consist of small fragments of cells, which may be the result of fungal action. Mycelia of fungi are occasionally met with in the tissues of older Calamite stems.