LYGINODENDRON 363 



traces. Where one of the circum-medullary xylem- 

 strands is double (Fig. 129), one half is about to pass 

 out to constitute a leaf-trace, while the other will 

 continue the sympodium in a vertical direction. 



We thus see that the entire vascular system of the 

 Lyginodendron stem is built up of leaf-traces, in a 

 manner comparable to that of the Gymnosperms or 

 Dicotyledons, and in some degree to that of Osmunda 

 among the Ferns. 



The more detailed structure of the vascular tissues 

 must now be considered. The primary xylem of the 

 leaf-trace, whether examined at the margin of the pith, 

 or at some point on its outward course, shows a constant 

 arrangement of its elements. The small spiral tracheae 

 (protoxylem) are invariably placed in the interior of 

 the xylem-strand, but nearer its outer than its inner 

 edge (see Figs. 131, 132,/*). Thus the greater part 

 of the primary wood in each bundle was developed 

 centripetally, on the inner side of the protoxylem, while 

 a smaller portion was formed centrifugally, on its outer 

 side. The tracheides adjacent to the spiral primitive 

 elements are scalariform, but the bulk of the primary 

 wood, especially its centripetal portion, is formed of 

 tracheides with bordered pits (cf. Fig. 157, p. 406, from 

 Heterangium). A few parenchymatous cells adjoin the 

 protoxylem (see Figs. 131 and 132), usually on the 

 inner side. The bundles, as we have already seen, are 

 collateral ; this fact is most clearly seen in the outgoing 

 leaf-traces (see Fig. 132), for in the bundles of the 

 circum-medullary ring, xylem and phloem are usually 

 widely separated by the intercalated zone of secondary 

 wood. Vascular bundles with the primitive elements 



