54 LYCOPODIALES [9^- 



as affording an exception, in the endarch structure of the 

 xylem, to the usual exarch plan of the stelar tissues. This 

 species is the only one in which any indication of the produc- 

 tion of secondary xylem elements has so far been recorded. 

 Bruchmann^ has shown that, in the small tuberous swelling- 

 which occurs at the base of the young shoot (hypocotyl), a 

 meristematic zone is formed round the axial vascular strand and 

 by its activity a few secondary tracheids are added to the 

 primary xylem. With this exception Selaginella appears to 

 have lost the power of secondary thickening, the possession 

 of which constitutes so striking a feature of the Palaeozoic 

 Lycopods. Another type is represented by 8. inaequalifolia, 

 an Indian species, the shoots of which may have either a 

 single stele or as many as five, each in its separate lacuna. 

 The homophyllous 8. laevigata var. Lyallii Spr., a Madagascan 

 species, affords a further illustration of the variation in plan of 

 the vascular tissues within the genus. There is a considerable 

 difference in structure between the erect and creeping shoots ; 

 in the former there may be as many as 12 — 13 steles, which 

 gradually coalesce before the vertical axis joins the creeping 

 rhizome to form one central and four peripheral steles. In the 

 rhizome there is usually a distinct axial stele without proto- 

 xylem, surrounded by an ill-defined lacuna and enclosed by a 

 cylindrical stele (solenostele)^ usually two tracheae in width 

 with four protoxylem strands on its outer edge. The continuity 

 of the tubular stele is broken and, in transverse section, it 

 assumes the form of a horseshoe close to the base of an erect 

 shoot to which a crescentric vascular strand is given off. 

 Harvey- Gibson^ has figured a section of the rhizome of this 

 type in which the axial vascular strand is represented by a 

 slight ridge of tracheae (fig. 131, C, t) projecting towards the 



^ Bruchmann (97). 



' The term solenostele, first used by Van Tieghem and revived by Gwynne- 

 Vaughan, may be applied to a stem in which the vascular tissue has the form of 

 a hollow cylinder with phloem and endodermis on each side of the xylem. Ab 

 each leaf-trace is given off the continuity of the vascular tube is interrupted. 

 See Gwynne-Vaughan (01) p. 73. 



' Harvey-Gibson (94) PI. xii, fig. 93. 



