218 LYCOPODIALES [CH. 



iv. The structure of Sigillarian stems. 



The first account of the anatomy of Sigillaria we owe to 

 Brongniart' who published a description of the internal structure 

 of an agatised stem, about 4 cm. in diameter, from Autun, which 

 he referred to Sigillaria elegans. It has, however, been shown 

 by Zeiller^ and by Kenault that this petrified fragment belongs 

 to Brongniart's species S. Menardi, which is probably a young 

 form of S. Brardi. Brongniart's specimen, now preserved in 

 the Paris Natural History Museum, is a very beautiful example 

 of a silicified plant : on part of the surface are preserved the 

 hexagonal contiguous leaf-scars, like those shown in fig. 193, A, 

 and on the polished transverse section is seen a relatively large 

 stele consisting of a ring of secondary xylem surrounding 

 a series of crescentic groups of primary xylem (fig. 200, A) 

 enclosing a wide pith occupied by concentric layers of silica. A 

 portion of the outer cortex is preserved, and this is separated 

 from the stele by a broad space filled with siliceous rock. The 

 main features of this type may be described in a few words. 

 The primary xylem differs from that of such Lepidodendron 

 stems as have been described in being made up of groups of 

 scalariform and occasionally reticulate (fig. 200, C) tracheae, 

 having a plano-convex or more or less crescentic form as seen in 

 transverse section. These primary strands, in contact with one 

 another laterally, have their narrowest elements on the outer 

 edge. The leaf-traces are given off from the middle of the 

 abaxial face of each xylem strand (fig. 202, C, It); these pass 

 obliquely outwards through medullary rays and then, as in 

 Lepidodendron, turn sharply upwards before bending outwards 

 again on their way to the leaves. Each leaf-trace consists of a 

 group of primary tracheae to which a few secondary tracheae are 

 added during the passage through the secondary wood. The 

 secondary xylem forms a continuous cylinder of tracheae with 

 scalariform bands on both radial and tangential walls; the 

 medullary rays are numerous and consist of long and narrow 



1 Brongniart (39) ; Renault (96) A. 



2 Zeiller (88) A. p. 586 ; Kidston (05) p. 534. 



