In roots or rootlets smaller than that shown in fig. 91 _B_, the primary xylem may extend to the centre of the stele, and form a continuous axial strand; in such examples the structure may be diarch, triarch or tetrarch. The origin of the cambium agrees with that in recent roots, the cells immediately external to the protoxylem tracheids become meristematic, as also those internal to the phloem. Another root-character is seen in the endogenous origin of lateral members. Good examples of branching roots are figured by Williamson[695] and by Williamson and Scott[696]. Older roots[697] are usually found in a decorticated condition. A transverse section of root in which secondary thickening has been active for some time presents on a superficial view a close resemblance to a stem of _Calamites_, but a careful comparison at once reveals important points of difference. The specimen diagrammatically sketched in fig. 92 illustrates very clearly the origin of a root from the node of a Calamite stem. The section has passed through a stem in a tangential direction, showing the characteristic arrangement of the vascular bundles _x_, and principal medullary rays _m_. The small leaf-traces, _t_, _t_, afford another feature characteristic of a Calamite stem. The portion of stem to the right of the figure has been slightly displaced, and between this piece and the root _R_, one of the ubiquitous Stigmarian appendages, _s_, has inserted itself. At _R_ a fairly thick and decorticated root is seen in oblique transverse section; at the upper end the root tracheids are seen in direct continuity with the xylem of the stem. In the centre of the root is the large solid pith surrounded by twelve bluntly pointed xylem groups, composed in the main of radially disposed scalariform elements with narrow secondary medullary rays like those in a stem. Between each xylem group there is a broad medullary ray, which tapers rapidly towards the outside, and is soon obliterated by the formation of interfascicular secondary xylem. At _R′_ a portion of another root is seen in transverse section, and _R″_ the inner part of a single xylem group is shown more clearly. The solid pith and the absence of carinal canals are the two most obvious distinguishing features of the roots. [Illustration: FIG. 92. Tangential section through a node of _Calamites_, showing a root in organic connection with the stem. _R_, _R′_. Root (_Astromyelon_) in transverse and oblique section, _x_, xylem; _m_, primary medullary ray; _t_, leaf-trace; _s_, Stigmarian appendage. _R″_, the inner portion of one of the xylem wedges of _R′_ more highly magnified. Sketched from a section in the Cambridge Botanical Laboratory Collection.]