400 SPHENOPHYLLALES. A. SPHENOPHYLLEAE 



early, so that the unaltered primary state is seen only in small twigs. 

 Here a protostelic structure is seen, without any pith or conjunctive 

 parenchyma. The primary xylem is of triangular form, the groups of 

 protoxylem, either single or double, being at the projecting angles; or 

 the angles may be duplicated, and a hexarch form be attained. The 

 vascular system is strictly cauline : it passes through the nodes without any 

 appreciable change of structure, a point of interest for comparison with 

 the Equisetal structure as interpreted by Gwynne-Vaughan. 1 A peculiarity 

 of some importance for further comparison is shown in the primary wood 



D 



FIG. 216. 



A = a leaf-whorl of Sphenophylhun ctmeifolium, and one leaf of it somewhat enlarged. 

 B a. leaf- whorl of Sphenophllynm tenerrimum. C Sphenophyllum verticillatum. 

 (From Potonie's Lehrbuch der Pflanzenpalciontologie.) D = " Trizygia " speciosa. 

 Royle, from the Glossopteris-facies, (after O. Feistmantel.) 



of the ancient species, S. insigne, from the calciferous sandstone : here a 

 canal is formed at each of the three angles of the primary wood, pre- 

 sumably by disorganisation of the protoxylem, as in the Equisetales 2 

 (Fig. 217). The cambial activity commences immediately outside the 

 primary wood, and results in a broad zone of secondary wood, which 

 completely surrounds the primary : it is traversed by continuous medullary 

 rays in S. insigne, but in the later species these are represented only by 

 little groups of thin-walled cells, which form, nevertheless, a continuous 

 system. Outside the wood lie the phloem and the cortex, the latter 

 showing a formation of periderm, which may be repeated, resulting in 

 a scaly bark. 



Compare Williamson and Scott, Phil. Trans., vol. clxxxv., part, ii., p. 922. 

 2 Scott, Studies, p. 88. 



