XVIII] ZYGOPTERIDEAE 21 



out through the cortex into the petiole. The vascular supply to the axillary 

 bud resembles structurally that of the main stele, but on a smaller scale: 

 it separates from the inner face of the trace on its course through the cortex, 

 in a manner very similar to that seen in the living Hymenophyllaceae 

 (Vol. I, p. 70, Fig. 63). The roots draw their vascular supply directly from 

 the main system of the shoot. Finally small isolated vascular strands may 

 be given off from the leaf-trace either below or above its actual separation 

 from the stele: each enters a small scale-like appendage called an aphlebia, 

 and dividing into a few strands supplies its small expanse. The aphlebiae 

 may be held as peculiar basal pinnae, partly on the ground of their form 

 and position, partly on that of their vascular supply. 



The stele o{ Ankyropteris is protostelic, but it differs from the simplest of 

 these by its corrugated form, and by the differentiation of the xylem. Two 

 regions are clearl}' distinguishable, the peripheral or outer-xylem has 

 relatively large scalariform tracheides which form arcs following the incurved 

 outer surfaces of the stele, but deepening those curves: the inner-xylem is 

 composed of narrower tracheides, mostly scalariform, mixed with paren- 

 chyma, and it forms a narrow five-rayed star. From each of its rays a band 

 of small tracheides containing protoxylem-elements extends outwards up 

 the middle of each arm to each several leaf-trace. Phloem surrounded the 

 xylem of the stele, having a single or even a double series of sieve-tubes. 

 There is no pith. 



The leaf-trace of A)ikyroptcris Grayi at its departure from the stele 

 spreads out laterally, and consists also of central and peripheral regions: the 

 latter supplies three groups of protoxylem: one group retains a median 

 position, and gives off two lateral groups each enclosed by narrow tracheides 

 with parenchyma, which distending forms a loop at either cusp of the cres- 

 centric trace. These enlarge as the trace passes into the petiole, still enclosed 

 by the phloem. Each cusp spreading into adaxial and abaxial curves, the 

 result is the H-shaped meristele so characteristic of the Zygopterideae: 

 indeed this was the leading character of the old genus Zygopteris (Fig. 326). 

 The cortex was indurated, and hairs were borne on the outer surface. Those 

 about the base of the petiole were developed as stiff bristles (Kidston Coll. 

 1985). In ^. corrugata they were not only transversely septate, but longi- 

 tudinal walls also appeared in their cells, as is seen in modern Ferns, 

 e.g. Dipteris. (See Vol. I, p. 198. Fig, 185, 2, 3.) 



The stems of other Zygopterideae show variants on this structure, but 

 still with differentiated xylem. In Diplolabis Rbmeri the stele is cylindrical 

 (diam. 3'5 mm.), and there is no parenchyma in the xylem. In Metaclepsyd- 

 ropsis duplex the stele contains an oval mass of xylem composed of a dense 

 band of outer wide tracheides surrounding an inner-xylem of the nature of 

 a "mixed pith" (diam. 2*2 mm.). In this plant the stem was an elongated 



