In some solenostelic ferns, and in many dictyostelic ones additional vascular strands are present which do not form part of the primary vascular tube.
In the haplostelic ferns the leaf-trace appears as a single strand with a tendency to assume the shape of a horseshoe on cross-section, and this type is also found in the more primitive solenostelic types.
Polycycly was derived independently from monocycly in solenostelic and in dictyostelic forms. In the formation of the stem of any fern characterized in the adult condition by one of the more advanced types of vascular structure all stages of increase in complexity from the haplostele of the first-formed stem to the particular condition characteristic of the adult stem are gradually passed through by a series of changes exactly parallel with those which we are led to suppose, from the evidence obtained by a comparison of the adult forms, must have taken place in the evolution of the race, There is no more striking case in the plantkingdom of the parallel between ontogeny (development of the individual) and phylogeny (development of the race) so well known in many groups of animals.
The xylem and phloem also, rarely form perfectly continuous layers as they do in a solenostelic fern.
The development of the pfothalli is in general similar to that of the Salviniaceae, though the resemblance may be homoplastic. The stem in the less reduced forms is solenostelic with sclerenchymatous ground tissue occupying the centre of the stele.