The vascular system[749] of the stem constitutes a highly complex dictyostelic or polycylic type which may consist of as many as nine concentric series of strands of xylem surrounded by phloem, with large sieve-tubes and a pericycle which abuts on the parenchymatous ground-tissue without any definite endodermal layer. A peculiarity in the vascular strands is that the first-formed elements of the phloem lie close to the edge of the xylem, the metaphloem being therefore centrifugal in its development. The ground-tissue is devoid of mechanical tissue and is penetrated by roots, a few of which arise from the outer vascular strands while others force their way to the surface from the more internal dictyosteles. Leaf-traces, consisting of several strands, are given off from the outermost cylinder and a segment of the second dictyostele moves out to fill the gap formed in the outermost network, while the gap in the second cylinder receives compensating strands from the third. A few layers below the surface of the petiole there is a ring of thick-walled elements (_s_, fig. 243), and in both petiole and stem numerous mucilage ducts and tannin-sacs occur in the ground-tissue. It has been shown by Farmer and Hill[750] that in some of the vascular strands in an _Angiopteris_ stem a few secondary tracheae are added to the primary xylem by the activity of the adjacent parenchyma. The vascular bundles in the petiole form more or less regular concentric series; they have no endodermis and are characterised also by the large size of the sieve-tubes (_st_, fig. 243). [Illustration: FIG. 242. A–D. _Angiopteris evecta_. A. Apex of sporangium showing “annulus.” B. Sori. C. Sporangium. D. Section of sporangium, showing the two lateral bands of thick-walled cells. E. _Danaea_: _a_, roof of synangium, with pores; _b_, sporangial cavities; _v_, vascular bundle; _i_, indusium. (D, after Zeiller.)] The roots of Marattiaceous ferns (fig. 244) are characterised by the larger number of xylem and phloem groups; the stele is polyarch and not diarch, tetrarch or hexarch as in most Leptosporangiate ferns. [Illustration: FIG. 243. _Angiopteris evecta_. Section of petiole (considerably reduced) and of a single vascular bundle (magnified): _px_, protoxylem; _st_, sieve-tubes.] [Illustration: FIG. 244. _Angiopteris evecta_. Transverse section of root, with part of the stele magnified: _s_, sieve-tubes; _p_, phloem; _px_, protoxylem.] _Archangiopteris_. This monotypic genus, discovered by Mr Henry in South Eastern Yunnan, was described by Christ and Giesenhagen in 1899[751]. The comparatively slender rhizome has a fairly simple vascular system[752]. The simply-pinnate leaves bear pinnules like those of _Danaea_, but the sori agree with those of _Angiopteris_ except in their greater length and in the larger number of sporangia.