If a solenostele is interrupted by leaf-gaps at intervals sufficiently close to cause overlapping, a transverse section at any part of the stele will show apparently separate curved bands of concentrically arranged xylem and phloem, which on dissection are seen to represent parts of a continuous lattice-work or a cylinder with the wall pierced by large meshes. The manner of evolution of the dictyostele has been ably dealt with by Gwynne-Vaughan[738] and other authors. In a few ferns, e.g. Matonia pectinata[739], a transverse section of the stem ([fig. 237], B) reveals the presence of two or in some cases three concentric solenosteles with a solid protostele in the centre: this polycylic type may be regarded as the expression of the fact that in response to the need for an adequate water-supply to the large fronds, ferns have increased the conducting channels by a method other than by the mere increase of the diameter of a single stele. Fig. 237, A, shows the vascular tissue of a petiole of Matonia in transverse section.
The two genera of Osmundaceae, Todea and Osmunda, are peculiar among recent ferns in having a vascular cylinder composed of separate strands of xylem varying considerably in shape and size, from U-shaped strands with the concavity facing the centre of the stem and with the protoxylem in the hollow of the U, to oval or more or less circular strands with a mesarch protoxylem or without any protoxylem elements ([fig. 221], A, B). These different forms are the expression of the change in contour or in structure which the parts of the lattice-work undergo at different levels in the stem[740]. Beyond this ring of xylem bundles is a continuous sheath of phloem of characteristic structure. A transverse section of a stem of Osmunda regalis may show 15 or more xylem strands; in O. Claytoniana there may be as many as 40. In Todea barbara ([fig. 221], B) the leaf-gaps are shorter, and in consequence of the less amount of overlapping the xylem cylinder becomes an almost continuous tube. The recent researches of Kidston and Gwynne-Vaughan[741] have resulted in the discovery of fossil Osmundaceous stems with a complete xylem ring, the stele being of the medullated protostele type; in another extinct member of the family the stele consists of a solid xylem core. The Osmundaceous type of stele is complicated in O. cinnamomea ([fig. 221], A) by the occurrence of local internal phloem and by an internal endodermis, a feature which leads Jeffrey to what I believe to be an incorrect conclusion that the vascular arrangement found in Osmunda regalis has been evolved by reduction from a stele in which the xylem was enclosed within and without by phloem. New facts recently brought to light enable us to derive the ordinary Osmundaceous type from the protostele and solenostele. It is worthy of remark that the Osmundaceae occupy a somewhat isolated position among recent ferns; their anatomy represents a special type, their sporangia differ in several respects from those of other leptosporangiate ferns and in some features Osmunda and Todea agree with the Eusporangiate ferns. The possession of such distinguishing characters as these suggests antiquity; and the facts of palaeobotany, as also the present geographical range of the family, confirm the correctness of this deduction.
Before leaving the stelar structure of leptosporangiate fern stems, a word must be added in regard to a type of structure met with in the Hymenophyllaceae. In this family Trichomanes reniforme ([fig. 237], E) may be regarded, as Boodle suggests, as the central type: the stele consists of a ring of metaxylem tracheae, the dorsal portion having the form of a flat arch and the ventral half that of a straight band. This flattened ring of xylem encloses parenchymatous tissue containing scattered tracheae some of which are protoxylem elements. In Trichomanes radicans the rhizome is stouter than in T. reniforme and the stele consists of a greater number of tracheae. The stele is cylindrical like that shown in [fig. 238], but the centre is occupied by two groups of protoxylem and associated parenchyma. In Hymenophyllum tunbrigense the stele is of the subcollateral type; the ventral plate of the xylem ring has disappeared leaving a single strand of xylem with endarch protoxylem and completely surrounded by phloem. Trichomanes muscoides possesses a still simpler stele consisting of a slender xylem strand with phloem on one side only. Reference has already been made to the occurrence in this family of the protostelic type. The Hymenophyllaceae afford a striking illustration of the modification in different directions of stelar structure connected with differences in habit, and of the correlation of demand and supply as shown in the varying amount of conducting tissue in the steles of different species.
The leaf-trace in a great number of ferns is characterised by its C-shaped form[742] as seen in transverse section: this in some genera, e.g. Matonia ([fig. 237], A), is complicated by the spiral infolding of the free edges of the C; in other ferns (e.g. some Cyatheaceae) ([fig. 278], C) the sides of the C are incurved, while in some species the xylem is broken up into a large number of separate strands.
An elaborate treatment of the leaf-traces of ferns was published a few years ago by MM. Bertrand and Cornaille[743] in which the authors show how the various systems of vascular tissue in the fronds of ferns maybe derived from a common type. As Prof. Chodat[744] justly remarks this important work has not received the attention it deserves, the neglect being attributed to the strange notation which is adopted[745].
The roots of ferns are characterised by a uniformity of plan in marked contrast to the wide range of structure met with in the stem and to a less extent in the leaves. The xylem may consist of a plate of scalariform tracheae with a protoxylem group at each end, or the stele may include six or more alternating strands of xylem and phloem.
II. Marattiales (Eusporangiate isosporous Filicales).
The Marattiaceae, the single family of ferns included in the Marattiales, comprise the genera Angiopteris, Archangiopteris, Marattia, Danaea, and Kaulfussia, which are for the most part tropical in distribution. These genera are characterised by eusporangiate sori or synangia, the presence of stipules at the base of the petioles, and by the complex arrangement of the vascular tissue. In view of the fact that many fossil ferns show a close resemblance to the recent Marattiaceae, the surviving genera are briefly described. The prothallus is green and relatively large.
Angiopteris. This genus occurs in Polynesia, tropical Asia, and Madagascar; it is characterised by a short and thick fleshy stem bearing large bipinnate leaves which occasionally show a forking of the rachis[746], a feature reminiscent of some Palaeozoic fern-like fronds. One of the large plants of Angiopteris evecta in the Royal Gardens, Kew, bears leaves 12 feet in length with a stalk 6 inches in diameter at the base. The sessile or shortly stalked and rather leathery linear or broadly lanceolate pinnules have a prominent midrib and dichotomously branched lateral veins. The surface of an old stem is covered with the thick stumps of petioles enclosed by pairs of fleshy stipules ([fig. 241], A) and bears numerous fleshy roots, which hang free in the air or penetrate the soil. The young fronds ([fig. 220], A) exhibit very clearly the characteristic circinate vernation. The proximal part of each primary pinna is characterised by a pulvinus-like swelling. The sporangia, in short linear elliptical sori near the edge of the pinnules, consist of free sporangia ([fig. 242], A–D) provided with a peculiar type of “annulus”[747], in the form of a narrow band of thicker-walled cells, which extends as a broad strip on either side of the apex. An examination of sections through the sporangia of Angiopteris in different planes[748] illustrates the difficulty of determining the precise nature of the annulus in a petrified sporangium which is seen only in one or two planes. Many of the sporangia from the English Coal-Measures, compared by authors with those of Leptosporangiate ferns, are in all probability referable to the Marattiaceous type.
