The specimen represented in fig. 73 illustrates very clearly the extension of the hollow pith up to the inner surface of the vascular ring; the disorganisation of the pith-cells which had already begun in the twig of fig. 71 has here advanced much further. The bluntly rounded projections represent the prominent primary xylem strands, each of which is traversed by the characteristic carinal canal. Alternating with the wedge-shaped groups of secondary xylem, x, we have the broad principal medullary rays, mr, which become slightly narrower towards the outside. The inner face of each of these wide rays has a concave form, due to the less resistent nature of the medullary-ray cells as compared with the stronger xylem. The regularly sinuous form of the inner face of the vascular cylinder enables one to realise how the Calamite-casts (figs. 82, 99, and 101) have come to have the regular ridges and grooves on their surface. The broad ridges on the cast mark the position of the wide medullary rays, while the grooves correspond to the more prominent ends of the vascular strands. The tissues external to the wood have not been preserved in the example shown in fig. 73. Some silicified specimens described by Stur[609] from Bohemia and now in the Museum of the Austrian Geological Survey, Vienna, admirably illustrate the connection between the surface features of a Calamite cast and the anatomy of the stem.
Fig. 73. Transverse section of a Calamite stem.
mr, medullary ray. After Williamson.
x, x, xylem. (No. 1933 A.A. in the Williamson Collection.)
ARTHROPITYS.
In the large section of a calcareous nodule diagrammatically shown in fig. 17 II. (p. 85) the secondary wood of a slightly flattened Calamite is the most prominent plant fragment. The pith-cavity has been almost obliterated by the lateral compression of the woody cylinder, but the presence of the carinal canals along the inner edge of the wood may still be readily recognised. The appearance presented by a transverse section of the secondary wood of a Calamite is that of regular radial series of rather small rectangular tracheids, with occasional secondary medullary rays consisting of narrow and radially elongated parenchymatous cells. The principal rays[610] in the Arthropitys type of a Calamite stem are often found to gradually decrease in breadth as they pass into the secondary wood, until in the outer portion of the wood the primary medullary rays are practically obliterated by the formation of interfascicular xylem.
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.
Fig. 74.
- 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. - 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.