CALLUS WOOD.
The development of cork in a younger Calamite stem is clearly shown in a specimen described by Williamson and Scott in their Memoir of 1894. In a transverse section of the stem several large cells of the inner cortex are seen to be in process of division by tangential walls, and giving rise to radially arranged periderm tissue[617].
The section diagrammatically sketched in fig. 80 is that of a Calamite twig in which the wood appears to have been injured, and the wound has been almost covered over by the formation of callus wood. The young trees in a Palaeozoic forest might easily be injured by some of the large amphibians, which were the highest representatives of animal life during the Carboniferous period, just as our forest trees are often barked by deer, rabbits, and other animals. Fissures might also be formed by the expansion of the bark under the heating influence of the sun’s rays[618]. Such a specimen as that of fig. 80 gives an air of living reality to the petrified fragments of the Coal period trees. It is well known how a wound on the branch of a forest tree becomes gradually overgrown by the activity of the cambium giving rise to a thick callus, which gradually closes over the wounded surface in the form of two lips of wood which finally meet over the middle of the scar. The two lips of callus are clearly shown in the fossil branch arching over the tear in the wood just beyond the ring of carinal canals. The tissue external to the wood represents the imperfectly preserved cortex. A section which was cut parallel to that of fig. 80 shows a continuous band of wood beyond the wound, and the latter has the form of a small triangular gap; this section appears to have passed across the wound where it was narrower and has already been closed over by the callus. The formation of a rather different kind of callus wood has been described by Renault[619] and by Williamson and Scott[620], in stems where aborted or deciduous branches have been overgrown and sealed up by cambial activity.
Fig. 80. Diagrammatic sketch of a transverse section of a Calamite twig, showing callus wood. From a specimen in the Cambridge Botanical Laboratory Collection. × ca. 10.
Fig. 81. Calamites. Longitudinal section (R, radial; T, tangential) of a small branch. b, position of a lateral branch. From a specimen (no. 1937) in the Williamson Collection. Slightly enlarged.
Some of the features to be noticed in longitudinal sections of Calamite stems have already been described, at least as regards younger branches. The specimen shown in fig. 81 illustrates the general appearance of a stem as seen in tangential and radial section. In the lower portion, T, the course of the vascular bundles is shown by the black lines which represent the xylem tracheids, bifurcating and usually alternating at each node. Between the xylem strands are the broad principal medullary rays. At b a branch has been cut through on its passage out from the parent stem, just above the nodal line. In tangential sections of Calamite stems one frequently sees both branches and leaf-trace bundles (fig. 83, A), passing horizontally through the wood and enclosed by strongly curved and twisted tracheids. In the upper part of the figure (81, R), the section has passed through the centre of the stem, and the wood is seen in radial view; each node is bridged across by a diaphragm of parenchymatous cells capable of giving rise to a surface layer of periderm[621].
An outgoing branch, as seen in a tangential section of a stem, consists of a parenchymatous pith surrounded by a ring of vascular bundles, in which the characteristic carinal canals have not yet been formed, but if the section has cut the branch further from its base, there may be seen a circle of irregular gaps marking the position of the carinal canals. Such gaps are often occupied by thin parenchyma, and contain protoxylem elements. The outgoing branches, as seen in a tangential section of a Calamite stem, are seen to be connected with the wood of the parent stem by curved and sinuous tracheids, which give to the stem-wood a curiously characteristic appearance[622], as if the xylem elements had been pushed aside and contorted by the pressure of the outgoing member. A tangential section through a Pine stem[623] in the region of a lateral branch presents precisely the same features as in Calamites. The branches are given off from the stem immediately above a node and usually between two outgoing leaf-trace bundles.
RHIZOME OF CALAMITES.