940 PRINCIPLES OF STRATIGRAPHY 



The structure of the stem is, however, more Hke that of lycopods, 

 but the cone again suggests affinities with the Equisetales. The 

 sporangiophores, however, spring from the bracts instead of the 

 axis. The class combines the characters of ferns, lycopods, and 

 Equisetales. Their nearest living relatives are probably the Psilo- 

 tales (Psilotum and Tmesipteris) formerly classed with the Lyco- 

 podiales. 



The Lycopodincr are represented by three living and one extinct 

 orders. The Lycopodiales are represented by two genera — Phyllo- 

 glossum with one species, and Lycopodium with nearly a hundred. 

 Selaginella with between 300 and 400 species and Isoetes with 

 about 50, mostly aquatic, species are each the sole representatives 

 of their respective orders. The modern genera are small forms, 

 but the extinct orders contained some of the largest Palaeozoic 

 trees, reaching 100 feet or more in height. A general external 

 characteristic of these plants is the simple form of the leaves, 

 which are generally of small size, while the sporangia are situated 

 on the upper surface of the sporophylls. In structure the stem is 

 a single cylinder (monostelic) with a centripetal development of 

 woody tissue (xylem-). The earliest, or protoxylem, is at the periph- 

 ery of the stele. In Selaginellalcs the stem contains one, two, 

 or several stele, while the Lepidodendrales are monostelic, as in 

 Lycopodium. A section of a Lepidodendron stem shows the central 

 pith, often destroyed, surrounded by a zone of primary wood, and 

 outside of this, in most cases, a zone of secondary wood, sharply 

 defined from the inner zone by the layer of protoxylem. In some 

 of the smaller species the wood was solid, without central pith. 

 The cortex or bark surrounds this and is bounded externally by the 

 persistent leaf scars. In Sigillaria the ring of primary wood is 

 narrower. The leaf scars are arranged spirally in Lepidodendron, 

 but in vertical rows in Sigillaria. Both were attached to large 

 creeping root-stocks or stigmaria, which were provided with 

 numerous cylindrical "roots" which penetrated the soil on all 

 sides. 



The spores of lycopods are formed in sporangia of considerable 

 size, which are situated on the upper surface and near the 

 base of the sporophylls. These are arranged in definite terminal 

 cones, or they may resemble the foliage leaves and occur in alter- 

 nate zones with them. In Selaginella the sporophylls are arranged 

 radially in the cones, these terminating the branches. A single 

 sporangium is borne on the axis just above the insertion of each 

 sporophyll. Large and small spores (mega- and micro-spores) 

 occur in this genus, but in Lycopodium they are all of one kind. 



