Fig. 241.
- Angiopteris evecta. (Considerably reduced.)
- Marattia fraxinea. Stipule. M.S.
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]).
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.
Fig. 243. Angiopteris evecta. Section of petiole (considerably reduced) and of a single vascular bundle (magnified): px, protoxylem; st, sieve-tubes.