Fig. 75. Longitudinal tangential section near the inner edge of the wood of the Calamite of fig. 74.
x, x, secondary xylem and medullary rays; m, principal medullary ray. From a section in the Binney Collection, × 50.
Before passing to other examples of Calamitean stems, reference may be made to the sections shown in figs. 75 and 76, which illustrate some further points in the structure of Binney’s stems. In fig. 75 the xylem tracheids are shown at x, and between them the secondary medullary rays present the appearance of long and narrow parenchymatous cells; as the section is tangential the characteristic scalariform character of the tracheids is not shown, the ladder-like bordered pits being confined to the radial walls of the tracheal elements. The much greater length than breadth of the cells which form the rays associated with the xylem tracheids, is a characteristic feature in Calamitean stems. The breadth of the principal ray, m, shows that the section has passed through the wood a short distance from the pith; in a tangential section cut further into the wood the breadth of the principal rays would be considerably reduced. The large medullary-ray tissue consists of square-walled parenchymatous cells. The more highly magnified section, in fig. 76, shows a central group of parenchyma containing a few transversely cut tracheids, but the two kinds of elements are not clearly differentiated in the figure; this group of cells is an outgoing leaf-trace which is enclosed by the strongly curved tracheids of the stem. The section is taken from the node of a stem where several leaf-trace bundles are passing out to a whorl of leaves; the few cells intercalated between the tracheids belong to the parenchyma of the secondary medullary rays.
Fig. 76. Longitudinal tangential section of the same Calamite as that of figs. 74 and 75, showing a leaf-trace and curved tracheids at a node. From a section in the Binney Collection, × 100.
ARTHROPITYS. SURFACE FEATURES.
In the small portion of a stem represented in fig. 74 B, the cortical tissues have been partially preserved; at the inner edge, next the hollow pith, there are two xylem groups, each with a carinal canal, and between them is part of a broad “principal” medullary ray[614]. The cambium has not been preserved, but beyond this region we have some of the large cells, c, of the inner cortex; these are followed by a few remnants of a smaller-celled tissue, and external to this part of the cortex there is a series of triangular groups, h, consisting of small thick-walled cells alternating with spaces which were originally occupied by more delicate parenchyma. The darker groups constitute hypodermal strands of mechanical tissue or stereome which lent support to the stem. The surface of a stem possessing such supporting strands would probably assume a longitudinally wrinkled or grooved appearance on drying; the intervening parenchyma, contracting and yielding more readily, would tend to produce shallow grooves alternating with the ridges above the stereome strands.
The complete section of the stem of which a small portion is shown in fig. 74 B, is figured by Williamson[615] in his 12th memoir on Coal-Measure plants. The section was obtained from Ashton-under-Lyne in Lancashire; it illustrates very clearly a method of preservation which is occasionally met with among petrified plants. The walls of the various tissue elements are black in colour and somewhat ragged, and the general appearance of the section is similar to that of a section of a charred piece of stem. It is possible that the Calamite twig was reduced to charcoal before petrifaction by a lightning flash or some other cause.
It is often said that the surface of a Calamite stem was probably marked by regular ridges and grooves similar to those of the pith-cast, and that such external features are connected with the arrangement of the tissues in the vascular cylinder. The indication of grooves and ridges on the bark of fossil Calamites is no doubt the result of the existence in the hypoderm of firm strands alternating with strands of less resistant cells. It is very common to find Calamite pith-casts covered with a layer of coal presenting a ribbed surface, but this is simply due to the moulding of the coaly film on an internal pith-cast. The broad grooves on such a specimen as that of fig. 77 are, on the other hand, probably an indication of the existence of hypoderm bands similar to those in fig. 74 B, h. The specimen from which fig. 77 is drawn shows many interesting features. The figure given by Grand’Eury, of which fig. 77 is a copy, is somewhat idealised, but the various surfaces can be made out in the fossil. The surface of the coaly envelope surrounding the pith-cast, a, is distinctly grooved, but the depressions have nothing to do with the surface features of the wood or the pith-cast; they are no doubt due to the occurrence of alternating bands of thick- and thin-walled tissue in the hypodermal region of the cortex; the peripheral strands of bast cells would stand out as prominent ribs as the stem tissue contracted during fossilisation. At b (fig. 77) we have a view of the wood in which the position of the principal rays is indicated by fine longitudinal lines at regular intervals; the oval projections just below the nodal line are probably the casts of infranodal canals (cf. p. 324). At a the characteristic pith-cast is seen with a small branch-scar on the node. The scar on the middle node, N 2, is probably that of a root, and a root R is still attached to the node, N 3.
