. Fossil plants : for students of botany and geology . Paleobotany. X] ARTHROPITYS. 321 Some of tte features to be noticed in longitudinal sections of Calamite stems have already been de- scribed, 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 vas- cular bundles is shown by the black lines which represent the xylem tracheids, bifur- cating and usually alternating at each node. Between the xylem strands are the broad principal medullary rays. At
. Fossil plants : for students of botany and geology . Paleobotany. X] ARTHROPITYS. 321 Some of tte features to be noticed in longitudinal sections of Calamite stems have already been de- scribed, 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 vas- cular bundles is shown by the black lines which represent the xylem tracheids, bifur- cating and usually alternating at each node. Between the xylem strands are the broad principal medullary rays. At & 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*. An outgoing branch, as seen in a tan- gential section of a stem, consists of a parenchymatous pith surrounded by a ring of vascular bundles, in which the charac- teristic 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 1 specimen 132*** in the Williamson Collection. s. 21. Pig. 81. Calamites, Longitudinal sec- tion (R, radial; T, tangential) of a, small branch. 6, position of a lateral branch. Prom a specimen (no. 1937) in t
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishercambr, bookyear1898
