XVl] SIGILLARIA 223 



also contribute the important fact that the leaf- traces in passing 

 through the phelloderm bifurcate and enter the leaf as two 

 distinct vascular strands. This double bundle has been referred 

 to in the description of Sigillaria leaves. (Page 214.) 



Although our knowledge of the anatomy of Sigillaria 

 has been considerably extended since Williamson^ drew atten- 

 tion to our comparative ignorance of the subject, there are 

 several points on which information is either lacking or very 

 meagre. As regards the stele, it is in all types so far in- 

 vestigated, of the medullated type and constructed on the 

 same plan as that of Lepidodendron Wiinschianum, L. Velt- 

 heimianum, and other species. Secondary xylem was developed 

 at an early stage of growth, and its relation to the primary 

 xylem, from which as Kidston points out in his description of 

 S. elegans, it may be separated by a few parenchymatous ele- 

 ments, is like that in Lepidodendron. The tendency of the 

 outer face of the secondary xylem to present a crenulate 

 appearance in transverse sections may, as Scott thinks'', be a 

 feature of some diagnostic importance, but this is not a constant 

 character in the genus. In origin and in their mesarch structure, 

 the leaf-traces closely resemble those of Lepidodendron. The 

 earlier account of the structure of the leaf-traces of Sigillaria, 

 which were described as possessing both centrifugal and centri- 

 petal wood, led Mettenius' to draw attention to an important 

 anatomical resemblance between this genus and modern Cycads. 

 This comparison was, however, based on a misconception ; the 

 Cycadean leaf-trace, consisting solely of primary wood, is not 

 strictly comparable with those of some species of Sigillaria, in 

 which one part of the xylem is primary and another secondary. 

 The occasional presence of secondary xylem in Sigillarian leaf- 

 traces is matched in some Lepidodendra\ and cannot be 

 accepted as a distinguishing feature. 



The origin of the leaf-traces from the middle of the sinuses 

 on the edge of the primary xylem is regarded as a difference ; in 

 Lepidodendron the leaf-traces are said to arise in some species 

 from the sides of the crenulations ; but, as already pointed out, 



1 Williamson (72). ^ Scott (08) p. 227. 



■' Mettenius (60). ■* e.g. L. Wiinschianum (fig. 181, B, It). 



