150 D. H. Scott. 



solid vascular cj'linder, without pith, aud iu no way broken up into 

 bundles. The form of the stele is triquetrous, the initial groups of 

 spiral tracheides Ij'ing at the prominent angles, so that the develop- 

 ment of the primary wood was centripetal. In some forms there is 

 a pair of protoxjlem-groups at eacli angle, and in certain cases, es- 

 pecially in the axis of the cone, in S. cimeifolium, the groups of each 

 pair may be so widely separated that the triquetrous form of the 

 stele is lost. The phloem appears to have occupied the bays between 

 the angles of the wood. It is only in quite young specimens that 

 this primary structure is found unaltered; secondary growth soon be- 

 gan, and the great majority of the stems show a more or less exten- 

 sive zone of secondary wood and bast, produced by the agency of a 

 normal cambium, the cells of which are sometimes preserved. The 

 traclieides, apart from those of the protoxylem, bear multiseriate pits 

 in most species, but in the Lower Carboniferous S. insigtw they are 

 scalariform. The same species — the oldest known with structure 

 preserved — is further peculiar in possessing true medullary rays, 

 while in other forms they are for the most part replaced by a reti- 

 culum of radial and vertical strands of xylem-parenchyma. The cortex 

 was thrown off in the older stems by the formation of an internal 

 periderm. 



At least two leaf-trace bundles started from each angle of the 

 stele, and, forking within the cortex, supplied tlie dichotomously-veined 

 leaves or their separate segments. The roots were in some cases at 

 least diarch, and like the stem, formed secondary vascular tissues. 



The recent discovery of secondary wood in the base of the stem 

 of Psilotum \) emphasizes the anatomical comparison between that genus 

 and SphenophijJhnn; otherwise the latter is of a very isolated type. 



As regards the fmcUfication the most characteristic feature is 

 the fact that the sporophylls possessed ventral and dorsal lobes. Usu- 

 ally the former alone were fertile, constituting the sporangiophores, 

 while the dorsal lobes were developed as sterile bracts; in one spe- 

 cies, however, both dorsal and ventral lobes were fertile. 



Four types may be distinguished, represented by Sphenophyllum 

 Dmvsoni, Boninanites-) Bömeri, Splienoplnßkmi fertile, and S. majus, which 

 will be briefly described in order. 



The cone described by Williamson (1871, 1874, 1891) as ^oir- 

 manites Dawsoni was subsequently shown by Zeil 1er (1893) to belong 

 to a SpliempliijUum of the type of S. cuneifolmm. The petritied spe- 



1) Boodle. On the Occurrence of Secoudary Xyleui in Psilotum. Annals of 

 Botany, Vol. XVIII p. 505. 1904. 



-) In tliis case the reference tn the genus Sphenophyllum is not absolutely cer- 

 tain, though a close affinity is established. 



