302 MORPHOLOGY OF ANGIOSPERMS 



clusters of sclerenchymatous tissue. These are also found in 

 the foliar gaps and in the pith. In the case of Lyginodendron 

 the primary wood is comparatively poorly developed and occurs- 

 as distinct islands along the margin of the medulla. The sec- 

 ondary wood is characterized by the regular radial seriation of 

 its elements and is abundant, but, in common with many other 

 fossil Pteridophytes with secondary growth, shows no indica- 

 tion of annual rings. The continuity of the woody zone is 

 completely interrupted at intervals by the foliar gaps which 

 subtend the outgoing leaf-traces. 



Fig. 110, P, is. a photograph of part of the ligneous zone 

 of L. Oldhamium. The protoxylem, distinguished by the small 

 size of its elements, is seen to be embedded in the primary wood. 

 Most of the primary metaxylem lies on the medullary side of 

 the protoxylem, and a smaller portion between it and the sec- 

 ondary wood. Hence the primary bundle is mesarch, as is often 

 the case in the Ferns and their allies. Another important fili- 

 cinean feature is the presence of well-marked foliar gaps. 



Fig. 110, Q, taken from Williamson and Scott, 6 shows an 

 interesting departure from the usual state of affairs in L. Old- 

 hamium', a primary wood-bundle is present, and external to it 

 is the usual secondary wood. In this case, however, there is 

 secondary wood and phloem on the medullary side of the bundle 

 as well. The condition represented in the figure is quite un- 

 usual in L. Oldhamium ; but, as has been shown by Seward, is 

 of common occurrence in L. robustum. The facts just described 

 are of particular interest, because Scott 6 has made a specific 

 comparison between the central cylinders of Lyginodendron and 

 Osmunda ; and indeed, if we imagine a secondary zone of wood 

 present in the latter genus and the primary wood-bundles cor- 

 respondingly reduced in size, the resemblance becomes very 

 close. The occurrence of internal phloem and secondary wood 

 is paralleled by the discovery of internal phloem in 0. cinna- 

 momea. 



The forms described above sufficiently illustrate the variety 

 of structure in the stem of the Cycadofilices, and it now be- 

 comes necessary to discuss their phylogenetic significance. First 

 of all is to be noted the fact that they represent the three types 

 of stelar structure described at the beginning of the chapter : 

 Heterangium being protostelic like Gleichenia ; Medullosa sipho- 



