XXIX] LYGINOPTERIDEAE 43 



early stage in the growth of the plant, producing several layers 

 of secondary tissue, which is regarded as periderm and forms a 

 conspicuous feature in Lyginopteris stems ; it appears as a com- 

 paratively dark sinuous band where it bends outwards to wrap 

 round the leaf-traces in their almost vertical course through the 

 pericyclic region (fig. 403). The periderm is clearly seen at p 

 close to the crushed secondary phloem of the dividing leaf-trace 

 in fig. 404, D. All the leaf -traces seen in fig. 403 beyond the 

 secondary wood are still within the deep-seated periderm and, as 

 Williamson and Scott showed, each leaf- trace after emerging from 

 the secondary wood remains in the pericycle-zone for a length of 

 five internodes as it very gradually inclines qutwards. Once 

 free from this region the twin bundles bend more sharply towards 

 the petiole. Stated briefly, the history of each leaf-trace from 

 the perimedullary region to the leaf-base is as follows: at the 

 outer edge of the pith a single trace consists of a mesarch xylem 

 bundle with one protoxylem strand; it passes vertically through 

 five internodes and then bends out through a foliar gap in the 

 xylem-cylinder, and the primary tracheids receive additions from 

 the cambium of the stele on their outer face. As the trace leaves 

 the secondary xylem it bends upwards and, as seen at d, fig. 403, 

 begins to divide into twin bundles. As the trace passes higher 

 the bisection of the protoxylem and metaxylem is completed and 

 the secondary xylem-arcs are gradually lost until the separate 

 strands of each pair are reduced to single mesarch bundles composed 

 wholly of primary tracheids. As the trace bends outwards through 

 the cortex the phloem gradually encircles each xylem-strand 

 until a concentric structure is substituted for the collateral dis- 

 position of the conducting tissue. At the same time the proto- 

 xylem strands divide and occupy a position near the inner edge 

 of the metaxylem. On reaching the petiole or after passing some 

 distance up the axis of the frond, the twin bundles unite and 

 usually form a V-shaped vascular strand (figs. 404, E; 405, A). 

 The single meristele subsequently divides into two equal portions 

 preparatory to the bifurcation of the petiole (fig. 406). 



The inner cortex, consisting of parenchymatous tissue and 

 many secretory cells with an occasional group of sclerenchymatous 

 elements in place of the abundant nests of this tissue in the peri- 



