Such a vascular cylinder is called a haplostele, and the axis containing it is said to be haplostelic. In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing.
In many Pteridophytes the solid haplostele is maintained throughout the axis.
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.
To this type of steIn having a ground-tissue pith, whether with or without internal phloem, is given the name siphonostele to distinguish it from the solid haplostele characteristic of the root, the first-formed portion of the stem, and in the more primitive Pteridophytes, of the whole of the axis.
There is good reason to believe that the haplostele is primitive in the evolution of the vascular system.