390 EOUISETALES 



stated by Scott: 1 he remarks that "the Calamite, so far as anatomy goes, 

 is simply an Equisetum with secondary thickening." The secondary increase 

 commences at the nodes, and extends thence through the internodes. 

 This again adds point to the similarity with Equitc/itm, since the trace of 

 secondary increase present in Equisetum is seen at the nodes, though it 

 does not extend into the internodes. The result of the secondary growth 

 in Calamites may be a woody mass of great bulk, and varying in the 

 details of its structure : into these matters it is unnecessary to enter here : 

 it will suffice to quote further from Scott- that "we may therefore express 

 the general characteristics of the Calamarian vascular system by the state- 

 ment that the whole arrangement is of the type of Equisetum but more 

 varied, and sometimes more complex " ; and, further, that 3 " the position 

 of the branches with reference to the nodes and leaf-traces was precisely 

 the same in Calamites as in the recent Equisetum" Thus, as regards 

 stelar problems the two stand together, and the hypothesis put forward 

 by G wynne- Vaughan for the elucidation of the stelar structure in Equisetum 

 should find its application in Calamites also. It will now be shown that 

 certain facts derived from these fossils strongly support it. 



In his Pflanzen-palaeontologie (p. 205) Potonie established a comparison 

 between the secondary vascular tissues of the Calamarieae and the Spheno- 

 phyllaceae by mentally doing away with the central mass of primary 

 xylem that exists in the latter. Gwynne-Yaughan suggested that by 

 inverting this procedure, and considering it possible that the ancestors 

 of Equisetum may have possessed a xylem that extended to the centre of 

 the stem, one is led to derive their structure, as it exists at present, 

 from the modification of a stele with a solid central mass of centripetal 

 xylem such as that of Sphenophyllum or of certain Lepidodendreae. To 

 illustrate the nature of the modifications that such a stele would have to 

 undergo, a series of parallel developments was pointed out by Gwynne- 

 Vaughan within the latter group, viz. Lepidodendron Rhodumnense^ Selaginoides, 

 Harcourtii, Si^/ltar/a spi/iosa, and Menardi: here parenchyma appears in 

 the xylem, and gradually increases in quantity until only an attenuated 

 peripheral ring of xylem remains, which then becomes more or less broken 

 up into separate strands. This suggestion raises the question whether any 

 Calamarian stem is known in which the hypothetical primary xylem is 

 better represented, and is shown to be centripetal in its development ? 



At the very same meeting at which G wynne- Vaughan developed his 

 theory Scott described a new species, Calamites petiycurensis^ which gave 

 the requisite answer. It comes from the Calciferous sandstone of Burnt- 

 island. 1 The interest depends on the fact that each vascular bundle 

 possesses a distinct arc of centripetal wood on the side next the pith. 

 The carinal canals are present as in an ordinary Calamite, and contain, as 

 usual, the remains of the disorganised protoxylem. They do not, however, 



1 .SVW/V.v, p. 23. 2 /,.<-., p. 25. 



:: /..'., p. 31. 4 Scotl, />;//. Ass. Report, 1901, p. 849. 



