The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith is known as a solenostele.
In this use the term loses, of course, its morphoI logical value, and it is better to call such a segment of a broken-up I stele a meristele, the whole solenostele with overlapping leaf-gaps being called a dictyostele.
This type of stern is therefore often spoken of as protoslelic. In the Ferns there is clear evidence that the amphiphloic haplostele or protostele succeeded the simple (ectophloic) protostele in evolution, and that this in its turn gave rise to the solenostele, which was again succeeded by the dictyostele.
The anatomy of the stele in the stem exhibits on the whole a progression from a solid protostele through a tubular solenostele to one or more circles of separate steles derived by the breaking up of the solenostele.
The stem in the more primitive forms has a tubular stele (solenostele); for the most part two to many steles, arranged in a ring (dictyostele).