X] 



ARTHROPITYS. 



321 



Some of the 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 6 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 

 ^ E.g. specimen 132*** in the Williamson Collection. 

 8. 21 



Fig. 81. Calamites, 

 Longitudinal sec- 

 tion (R, radial; T, 

 tangential) of a 

 small branch, h, 

 position of a lateral 

 branch. From a 

 specimen (no. 1937) 

 in the Williamson 

 Collection. Slightly 

 enlarged. 



