leaf-trace passes out from the rhizome stele the vascular cylinder is interrupted by the formation of a foliar gap (_Platyzoma_[735], fig. 239, is an exception). This fact has been emphasized by Jeffrey[736] who draws a distinction between the Lycopodiaceous type of stele, which is not broken by the exit of leaf-traces, and the fern stele in which foliar gaps are produced: the former he speaks of as the cladosiphonic type (_Lycopsida_) and the latter as the phyllosiphonic (_Pteropsida_). [Illustration: FIG. 238. Stele of _Trichomanes scandens_: _px_, protoxylem; _s_, endodermis. From Tansley, after Boodle.] [Illustration: FIG. 239. _Platyzoma microphylla_. _l.t._, leaf-trace; _i.e._, internal endodermis. (After Tansley; modified from Boodle.)] The transition to a hollow cylinder of xylem from a protostele may be described as the result of the replacement of some of the axial conducting tracheae by parenchyma or other non-vascular tissue consequent on an increase in diameter of the whole stele and the concentration of the true conducting elements towards the periphery[737]. The occurrence of the internal cylinder of phloem, pericycle, and endodermis in a solenostele is rendered intelligible by a study of fern seedlings and by a comparative examination of transitional types connecting protosteles and solenosteles through medullated protosteles and steles of the _Lindsaya_ type. A further stage in stelar evolution is illustrated by what is termed the dictyostele, the arrangement of vascular tissue characteristic of _Nephrodium Filix-mas_, _Cyathea_ (fig. 240), _Polypodium vulgare_ and many other common ferns. [Illustration: FIG. 240. _Cyathea Imrayana._ (From Tansley after de Bary.) (Sclerenchyma represented by black bands.)] 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.