The Present Position of Palaeozoic Botany. 191 



structure iu which, at first sight, C'ycadean characters appear to pre- 

 dominate (Fig. 18). There is a pith, of considerable size, surrounded 

 by a zone of wood and bast, with a layer of cambium, sometimes 

 perfectly preserved, between the two; the greater part of both wood 

 and phloem shows a regular radial seriation of the elements, and is 

 clearly of secondary origin, the structure resembling that of the cor- 

 responding tissues in a recent Cycad. Around the pith however, 

 several distinct strands of primary wood are evident, a character not 

 met with in the vegetative stem of Cycads. 



The primary xylem-strands belong to the leaf-trace system of the 

 plant; they pass out through the zone of secondary wood into the 

 pericycle, which they traverse for some distance, here, of course, 

 assuming the character of complete collateral bundles. During its 



Fig. 19. Lyginodendron oWmmium. Double leaf-trace bundle and adjacent tissues. 



X, centripetal; jS centrifugal xyleni; px. protoxylem; pli, phloem of bundle; 



p/i^, part of phloem of stele; pd., periderm; ss, secretory sacs. X about 40. 



passage through the pericycle each leaf-trace divides into two. The 

 leaf-trace bundles of Lyginodendron have precisely the structure of the 

 foliar bundles of recent Cycads, for their xylem is of the mesarch type, 

 the centripetal portion exceeding the centrifugal in amount (see the 

 comparative figures 19 & 20). The occurrence of this structure in the 

 stem of Lyginodendron suggested a search for mesarch bundles in axial 

 organs of Cycadaceae, and they were found to occur in the peduncles 

 of the cones of Stangeria and some other genera.^) It may be pointed 

 out that the tracheides of Lyginodendron. like those of almost all 

 Pteridosperms investigated, are characterized by multiseriate bordered 

 pits. Fig. 18 gives a general idea of the structure of the stem, which 



^) Scott, The Anatomical Characters presented by the Peduncle of Cj'cadaceae. 

 Ann. of Bot., Vol. 11, 1897. 



