The Present Position of Palaeozoic Botany. Iß7 



xylem-ring is broken up, more or less completely, into distinct bundles. 

 These bundles, however, never pass out into the leaves, but in all 

 cases the single, usually collateral leaf-trace is detaclied from the 

 stele without giving rise to any leaf-gap. 



In most species there was a considerable development of centri- 

 fugal secondary wood, consisting of tracheides and medullary rays, 

 with a marked radial arrangement of the elements. In a few species 

 (e. g. Lepidodendroii Harcourtii, the first fossil Lycopod discovered 

 with structure preserved) no secondary wood has yet been observed. 

 Almost without exception both primär}^ and secondary tracheides are 

 of the scalariform type. Although the presence of primary phloem 

 can be recognised with certaint3^ some doubts have been expressed 

 as to the production of secondary phloem by the cambium. In certain 

 cases (LepidopMoios fuliginosus and Lepidodendron obovatum ^)) the secon- 

 dary xylem may be largely, or even wholly, parenchymatous. In all 

 cases, even where secondary vascular tissues have not been observed, 

 there was an extensive formation of periderm, chiefly in the form of a 

 phelloderm probably produced on the inner side of the generative layer. 



The leaves show marked xerophytic adaptations; the vascular 

 bundle was surrounded by a sheath of tracheal transfusion tissue, and 

 the stomata were commonly sheltered in two deep grooves on the 

 lower surface of the leaf. In the curious genus Sigillariopsis the leaf 

 is traversed through most of its length by two vascular bundles, a 

 unique case among Lj'copods. According to Renault, the French 

 species of this genus is further remarkable for the occurrence of 

 pitted, as distinguished from scalariform tracheides. 



An interesting feature in the leaves of the Palaeozoic Lycopods 

 is the very general presence of a ligule, situated, like that of the 

 recent Ligulatae, on the upper side of the leaf-base and usually seated 

 in a deep pit. 



As will be gathered from the short summary just given, there is 

 no essential structural difference between Sigillaria and other Lepido- 

 dendreae, for the division of the xylem-ring into separate bundles is 

 by no means constant for the genus, or even for the species. In habit, 

 however, the Sigillarias must liave been peculiar, for the stem appears 

 to have branched but sparingly, or even, in some cases, not at all, 

 the tall upright trunk terminating, like a Xanfhorrhoeo, in a sheaf of 

 long, grass-like leaves. The leaves were usually arranged in con- 

 spicuous vertical series, marked, in a large section of the genus, by 

 the presence of prominent ribs. 



Our knowledge of Stigmaria. which represents the subterranean 

 parts of the Lepidodendreae, is still very impeifect, although fossils 



Scott, Annals of Botany, July 1906. 



