The vascular region is surrounded by a parenchymatous cortex, which in younger plants, or in the apical region of an older plant, forms the surface of the stem to which the leaf-stalks are attached. From the peripheral steles, or from the more external bands of the vascular network, roots are given off which pass in a sinuous vertical course through the cortex, appearing on the surface between the leaf-bases. In older stems, after leaf-fall, the tissue immediately external to the vascular region produces secondary parenchyma with which the roots become intimately associated by their outermost cells. As a result of the secondary cortical development and the gradual increase in the number of roots invading the cortical tissue from above, the stem is enclosed by a cylinder of roots and associated parenchymatous tissue of secondary origin. In still older portions of a stem the more external roots are free from the stem-cortex and form a thick felted mantle, which increases in thickness towards the base of the tree. The roots (fig. 296, E) are polyarch, 5–10 groups of xylem alternating with strands of phloem, and similar in structure to those of recent species of _Marattia_ and _Angiopteris_; the stele is enclosed by an inner cortex of compact or lacunar tissue containing secretory sacs, and this is surrounded by a cylinder of mechanical tissue. In one or two instances secondary xylem has been observed wholly or partially enclosing the root-stele[1065]. [Illustration: FIG. 296. A. _Psaronius infarctus_ (P, peripheral steles; L, leaf-traces). B. _P. infarctus_, longitudinal tangential section through the peripheral region of the stem. C. _P. coalescens._ D. _P. musaeformis._ E. _P. asterolithus_ (root). (A—C, E, after Zeiller; D, after Stenzel.)]