In fig. 74, _A_, we have a portion of a single xylem group of a thick woody stem. The stem from which the figure has been drawn was originally described by Binney[611] as _Calamodendron commune_; we now recognise it as a typical example of the subgenus _Arthropitys_. The specific term _communis_ was used by Ettingshausen[612] in 1855 in a comprehensive sense to include more than twenty species of the genus _Calamites_, but since Binney’s use of the term it has come to be associated with a definite type of _Arthropitys_ stem, in which the primary medullary rays decrease rapidly in breadth towards the periphery of the wood. The wood of Binney’s stem[613] measures 2·5 cm. across, but the pith-cavity has been crushed to the limits of a narrow band represented in the figure by the shaded portion. The strand of cells, _s_, in the pith is a portion of a Stigmarian appendage (“rootlet”), which penetrated into the hollow stem of the Calamite and became petrified by the same agency to which the preservation of the stem is due. These intruded Stigmarian appendages are of constant occurrence in the calcareous nodules; their intimate association with the tissues of other plants is often a serious source of error in the identification of petrified tissues. The inner portion of one of the xylem groups is shown in fig. 74, _A_. External to the carinal canal, the xylem tracheids are disposed in regular series and associated with numerous narrow secondary medullary rays. The width of the xylem wedge increases gradually as we pass outwards, this is due to the formation of interfascicular xylem, which in the more peripheral portion of the stem extends across the primary medullary rays. The few primary medullary-ray cells shown in the drawing illustrate the characteristic tangentially elongated form and large size of the parenchymatous elements. Williamson and Scott have pointed out that the tangentially elongated form of the medullary-ray cells is the result of active growth, and not merely the expression of the tangential stretching of the stem consequent on secondary thickening. [Illustration: FIG. 74. _A._ Transverse section of part of a Calamite stem. [_Calamites (Arthropitys) communis_ (Binney).] _s_, Stigmarian appendage. _x_, xylem. From a specimen in the Binney Collection, Cambridge, × 50. _B._ Transverse section of a stem. _h_, hypodermal tissue; _c_, inner cortex. From a specimen in the Williamson Collection (no. 62). × 35.] A glance at the complete transverse section of the stem,—of which a small portion is shown in fig. 74 _A_,—suggests the existence of annual rings in the wood, but this appearance of rings is merely the result of compression. The secondary wood of a Calamite does not exhibit any regular zones of growth comparable with the annual rings of our forest trees. [Illustration: FIG. 75. Longitudinal tangential section near the inner edge of the wood of the Calamite of fig. 74.