X] SECONDARY THICKENING. 309 



cambiform cells and sieve-tubes. An unusually perfect speci- 

 men has been described by Renault ^ in which the phloem 

 elements are preserved in silica. Fig. 72, D, is copied 

 from one of Renault's drawings, the sieve-tubes, s, s, show 

 several distinct sieve-plates on the lateral walls of the tubes, 

 reminding one to some extent of the sieve-tubes in a Bracken 

 Fern. The cells, p, p, associated with the sieve-tubes are 

 square-ended elongated parenchymatous elements. Another 

 characteristic feature illustrated by longitudinal sections is 

 the nodal diaphragm; except in the smallest branches the 

 interior of each internode is hollow, and the ring of vascular 

 bundles is separated from the pith-cavity by a band of paren- 

 chymatous tissue. At each node this parenchyma extends 

 across the central cavity in the form of a nodal diaphragm, as 

 in the stem of Equisetum. 



By far the greater number of the petrified fragments of 

 Calamites afford proof of cambial activity, and possess obvious 

 secondary tissues. In exceptionally perfect specimens the 

 xylem tracheids are found to be succeeded externally by a few 

 flattened thin-walled cells which are in a meristematic con- 

 dition (fig. 72, A, c); these constitute the cambium zone, and 

 it is the secondary structure that results from the activity of 

 the meristematic cells that we have now to consider. 



In petrified examples of branches in which the secondary 

 thickening has reached a fairly advanced stage, the wood is 

 usually the outermost tissue preserved, the more external 

 tissues having been detached along the line of cambium cells. 

 It is only in a few cases that we are able to examine all the 

 tissues of older examples. 



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 

 1 Eenault (93), PI. xlvii. fig. i. 



