[Illustration: FIG. 54. A. Plan of the vascular bundles in the stem of an _Equisetum_; _b_, branches passing out to buds (after Strasburger); _l_, vascular strands passing to the leaf-segments. B. Longitudinal section through a node of _E. arvense_ L. (after Duval-Jouve; × 20). Explanation in the text.] There are certain points connected with the vascular bundles in the nodal region of a shoot, which have an important bearing on the structure of fossil equisetaceous stems. Fig. 54 B represents a diagrammatic longitudinal section through the node of a rhizome of _Equisetum arvense_ from which a root _h_ is passing off in a downward direction, and a branch in an upward direction. The black band _c_ in the parent stem shows the position of the vascular strands; in the region of the node the vascular tissue attains a considerable thickness, as seen at _d_ in the figure. The bands passing out to the left from _d_ go to supply the branch and root respectively. The increased breadth of the xylem strands at the node is due to the intercalation of a number of short tracheids. Fig. 55, 4 shows a transverse section through a mature node of _Equisetum maximum_; _px_ marks the position of the protoxylem and _e_ that of the endodermis. On comparing this section with that of the internodal vascular bundle in fig. 52, D, the much greater development of wood in the former is obvious; the carinal canal of the internodal bundle is absent in the section through a node. The disposition of the xylem tracheids in fig. 55, 4 shows a certain regularity which, though not very well marked, suggests the development of wood elements as the result of cambial activity. Longitudinal sections through the nodal region demonstrate the existence of “cells similar to those of an ordinary cambium, and a cell-formation resulting from their division which is similar to that in an ordinary secondary thickening.”[490] The short tracheids which make up this nodal mass of xylem differ from those in the internodal bundle in their smaller size, and in being reticulately thickened. There is, therefore, evidence that in the nodes of some _Equisetum_ stems additional xylem elements are produced by a method of growth comparable with the cambial activity which brings about the growth in thickness of a forest-tree[491]. The significance of these statements will be realised when the structure of the extinct genus _Calamites_ is described and compared with that of _Equisetum_.