In the subgenus Arthrodendron, a type of stem first recognised by Williamson and named by him Calamopitys[632], the principal medullary rays consist of prosenchymatous cells (i.e. elongated pointed elements) and not parenchyma. These elongated elements are not pitted like tracheids, and they are shorter and broader than the xylem elements. In some examples of this subgenus the primary rays are bridged across at an early stage by the formation of secondary interfascicular xylem, and in others they persist as bands of ray tissue, as in Arthropitys. Other characteristics of Arthrodendron are the abundance of reticulated instead of scalariform tracheids in the secondary wood, and the large size of the infranodal canals.
Fig. 83, D represents part of a transverse section of Arthrodendron; in this stem the rays have been occupied by interfascicular xylem at a very early stage of the secondary growth. The section from which fig. 83, D is drawn was described by Williamson in 1871; the complete section shows about 80 carinal canals and primary xylem groups. The prosenchymatous form of the principal medullary rays is seen in fig 83, C, and the reticulate pitting on the radial wall of a tracheid is shown in fig. 83, B. Fig. 83, A illustrates the large infranodal canals as seen in a tangential section of a stem. The same section shows also the course of the vascular bundles characteristic of Calamites as of Equisetum, and the position of outgoing leaf-traces is represented by unshaded areas in the black vascular strands.
Fig. 83. Calamites (Arthrodendron).
- Tangential section (diagrammatic) showing the course of the vascular strands, also leaf-traces and infranodal canals.
- Radial face of a tracheid.
- Prosenchymatous elements of a principal medullary ray.
- Transverse section of the wood. (After Williamson.) No. 36 in the Williamson Collection.
The subgenus Arthrodendron is very rarely met with, and our information as to this type is far from complete[633].
The third subgenus Calamodendron has not been discovered in English rocks, and our knowledge of this type is derived from French and German silicified specimens[634]. There is the same large hollow pith surrounded by a ring of collateral bundles with carinal canals, as in the two preceding subgenera. The tracheids are scalariform and reticulate, and the secondary medullary rays consist of rows of parenchymatous cells which are longer than broad, as in Arthropitys and Arthrodendron.
Fig. 84. Calamites (Calamodendron) intermedium, Ren.
Transverse section through two vascular bundles.
a, a, xylem tracheids, b, b, bands of prosenchyma, c, medullary ray. (After Renault.)
The most characteristic feature of Calamodendron is the occurrence of several rows of radially disposed thick-walled prosenchymatous elements (fig. 84, b) on either flank of each wedge-shaped group of xylem. Each principal ray is thus nearly filled up by bands of fibrous cells on the sides of adjacent xylem groups, but the centre of each principal ray is occupied by a narrow band of parenchyma (fig. 84, c). The relative breadth of the xylem and prosenchymatous bands has been made use of by Renault as a specific character in Calamodendron stems. Fig. 84 is copied from a drawing recently published by this French author of a new species of Calamodendron, C. intermedium[635]. In this case the bands of fibrous cells, b, are slightly broader, as seen in a transverse section of the stem, than the bands of xylem tracheids, a. The narrow band, c, consists of four rows of the parenchymatous tissue of a medullary ray. At the inner end of each group of tracheids there is a large carinal canal.