In a vascular plant, the stele is the central part of the root or stem containing the vascular tissue and occasionally a pith. The concept of the stele was developed in the late nineteenth century by P. E. L. van Tieghem as a model for understanding the relationship between the shoot and root, and for discussing the evolution of vascular plant morphology. Now, at the beginning of the twenty-first century, plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele.
The earliest vascular plants had both root and shoot with a central core of vascular tissue. This consisted of xylem in the center, surrounded by a region of phloem tissue. Around these tissues there might be an endodermis that regulated the flow of water into and out of the vascular core. Such an arrangement is termed a protostele.
There are three basic types of protostele:
- haplostele - the most basic of protosteles, with a cylindrical core of vascular tissue. This type of stele is the most common in roots.
- actinostele - a variation of the protostele in which the core is lobed. This type of stele is rare among living plants, but is found in stems of the whisk fern, Psilotum.
- plectostele - a protostele in which interconnected plate-like regions of xylem are surrounded and immersed in phloem tissue. Many modern club mosses (Lycopodiopsida) have this type of stele within their stems.
Plants that produce complex leaves also produce more complex stelar arrangements. The hormones produced by the young leaf and its associated axillary bud affect the development of tissues within the stele producing leaf gaps. These plants have a pith in the center of their stems, surrounded by a cylinder containing the vascular tissue. This stelar arrangement is termed a siphonostele.
There are three basic types of siphonostele:
- solenostele - the most basic of siphonosteles, with a central core of pith enclosed in a cylinder of vascular tissue. This type of stele is found only in fern stems today.
- dictyostele - a variation of the solenostele caused by dense leaf production. The closely arranged leaves create multiple gaps in the stelar core. Among living plants, this type of stele is found only in the stems of ferns.
- eustele - the most common stelar arrangement in stems of living plants. Here, the vascular tissue in arranged in vascular bundles, usually in one or two rings around the central pith. In addition to being found in stems, the eustele appears in the roots of monocot flowering plants.
Siphonosteles may be ectophloic, with the phloem tissue positioned on one side of the xylem and closer to the epidermis. They may also be amphiphloic, with the phloem tissue on both sides of the xylem. Among living plants, many ferns and some Asterid flowering plants have an amphiphloic stele.
There is also a variant on the eustele found in monocots like maize and rye. The variation has numerous scattered bundles in the stem and is called an atactostele. However, it is really just a variant of the eustele.
- Gifford, Ernest M. & Foster, Adriance S. (1988). Morphology and Evolution of Vascular Plants, (3rd ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1946-0.