40 PTERIDOSPERMEAE [CH. 



representing the xylem halves of collateral bundles separated 

 from the primary phloem strands by the intervening cylinder 

 of secondary wood. Two of the primary xylem strands in lateral 

 contact are seen in fig. 404, C; the other four occur as separate 

 bundles. Each primary xylem strand contains a small group of 

 spirally thickened protoxylem elements (px) associated with a 

 few parenchymatous cells. The large primary tracheids internal 

 to the protoxylem are characterised by multiseriate bordered 

 pits on their walls, while those external to the protoxylem, which 

 are in contact with the innermost secondary tracheids, have 

 scalariform pitting. The dark patch s (fig. 404, C) is a portion of 

 the large group of sclerenchymatous cells, shown in figs. 402, 403. 

 The perimedullary xylem strands of mesarch structure are the 

 lower portions of leaf -traces and, as Scott points out, 'each of 

 the bundles surrounding the pith is, in fact, a sympodium, com- 

 posed of the united lower ends of successive adjacent leaf -traces.' 

 The larger of the two bundles shown in fig. 404, C, is on the point 

 of passing out to a leaf, while the smaller strand is on its way 

 to a higher level before bending outwards through the secondary 

 wood. Slightly beyond the middle of the secondary xylem there 

 is an arc of narrower tracheids comparable with an incomplete 

 annual ring. Although zones or arcs of narrow tracheids are not 

 uncommon in the wood of Lyginopteris there is no satisfactory 

 evidence of regularly recurring seasonal changes. On the outer 

 face of the secondary wood are a few leaf-trace strands pursuing 

 a vertical course in the pericycle region; but the structure and 

 behaviour of these bundles are more clearly illustrated in the 

 stem reproduced in fig. 403. The tissue between the crushed 

 phloem and pericycle and the outer cortex (fig. 402) consists of 

 radially compressed parenchyma with scattered secretory cells 

 separated from the more internal tissue by a narrow band of 

 periderm formed by a phellogen in the outer part of the pericycle. 

 A larger and better preserved stem, 3-7 cm. in diameter, is 

 seen in fig. 403. In this stem the pith of parenchyma and scattered 

 sclerenchymatous nests is larger in proportion to the stele than 

 in Binney's type-specimen. From the inner edge of the secondary 

 xylem several primary xylem-strands project as rounded wedges 

 or tangentially elongated groups where two traces are laterally 



