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.
Fec. io.Diarch haplostele of .5 Lyco podium.
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.
Haplostele, the segments of inner endodermis, pericycle, phloem and ~ Pig.