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).