244 STUDIES IN FOSSIL BOTANY 



anatomical peculiarity, considering that the centripetal 

 primary wood is one of the chief structural characters 

 of Lycopods, both recent and fossil. The case of 

 Stigmaria ficoides is not, however, without analogy 

 within the Order. Thus, in the creeping stem of 

 Selaginella spinosa, the first-formed tracheae are central, 

 so that the whole of the wood is centrifugally developed. 

 The same is the case in the hypocotyl and rhizophores 

 of 5. Kraussiana and other species. 1 In both plants the 

 upper parts of the axis have the normal structure, with 

 centripetal xylem, just as in the case of the Sigillarian 

 or Lepidodendroid stems to which Stigmaria ficoides 

 belonged. As we shall see presently, the absence of 

 centripetal wood does not hold for all forms of 

 Stigmaria. 



The bulk of the xylem in 5. ficoides consists of 

 radially arranged scalariform tracheae, the pits occurring 

 on the tangential as well as on the radial walls of the 

 elements (Fig. 99). In addition to the principal rays, 

 numerous narrow secondary rays traverse the wood ; 

 they may consist of a single radial row of cells, or may 

 be several cells in height ; usually they are one cell 

 thick, sometimes more (Fig. 99, m.r). It is an inter- 

 esting fact that the rays consist partly of spirally or 

 reticulately thickened elements, presumably tracheides, 

 a peculiarity which, as we have seen, is very general 

 in the secondary wood of Lepidodendroid stems. 

 There is here an obvious analogy with the complex 

 organisation of the medullary rays in the Abietineae, 

 though the two structures no doubt arose quite in- 

 dependently. 



1 See the papers by Bruchmann and Harvey Gibson, cited on p. 260. 



