FOSSIL VASCULAR CRYPTOGAMS 



117 



B 





Although the genus Sigillaria is still placed by some writers among 

 Gymnosperms, its true place is undoubtedly near to Lepidodendron 

 in the order Selaginellacese ; the structure of the stem presents no 

 important difference 

 from that of Lepido- 

 dendron, while the 

 fructification known as 

 Sigillariostrobus bears a 

 remarkable resemblance 

 to Lepidostrobus. 



The remains of 

 various species of Sigil- 

 laria occur in enormous 

 quantities in the coal 

 measures ; and they 

 constituted one of the 

 predominant forms of 

 vegetation of the period. 

 The stems rivalled va 

 height and thickness 

 those of Lepidoden- 

 dron, and were covered, 

 like them, with the 

 scars of fallen leaves in 

 linear series. They were 

 simple or dichotomously 

 branched. The scars 



are circular, ovate, or hexagonal from mutual compression. In the 

 section known as Leiodermaria the cushions which occur in other forms 

 are wanting, and the scars stand out at a considerable distance from one 

 another on the smooth surface of the stem. The leaves, which are 

 occasionally found still attached to the branches, were narrow, linear, 

 and sedge-like, up to as much as one and a half feet in length, with a 

 projecting midrib. According to Van Tieghem the stem of Sigillaria 

 differs from that of the Lepidodendre^, and indeed from that of all 

 other Vascular Cryptogams, in the leaf-trace bundles being ' diploxy- 

 lous ' — that is, in the central cylinder having an external secondary and 

 centrifugal as well as an internal primary and centripetal xylem. Renault 

 regards the Rhytidolepida, or Sigillariae with stem exhibiting raised 

 cushions as well as scars, as Cryptogamic ; the LeioderinariecE^ or smooth- 

 stemmed Sigillariae, as Gymnospermic ; but this view is not supported 

 by a careful examination of the structure. . . 



'^kjm 



c 



Fig. 89- — A, B, C, portions of surface of stem of different species 

 oi Sigillaria; D, Leioderj7iaria. (After Solms-Laubach. } 



