In a vascular plant, the stele is the central part of the root or stem containing the vascular tissue.

The simple stems of non-vascular plants such as mosses or kelp are purely mechanically structural - such as holding up the plant's phyllodes . If any substances are transported up or down the stem, it is by simple diffusion from cell to cell, possibly assisted by osmosis.

The stele is considerably more complex than this. As well as its structural purpose, it also contains a central core of xylem surrounded by a living growing layer, the vascular cambium. Outside of this is a layer of phloem, sometimes arranged in bundles. Outside this is another growing layer, the cork cambium, which is in turn overlain with a protective outer layer; in woody plants and trees this may be composed of dead tissue, bark.

As well as growing in length as the plant grows, the stele of some species of gymnosperms and perennial dicotyledons can grow in girth as well, gradually growing more stout throughout the life of the plant. This process is called secondary thickening and is, for example, the origin of the growth rings in tree trunks.

The xylem is composed of the dead casings of elongated cells arranged into pipes, which carry water (usually also containing dissolved nutrients) from the roots to the other parts of the plants, such as the leaves (generally the primary sites of photosynthesis), the reproductive organs and the growing point or points, the meristem.

The phloem is structurally similar but composed of still-living cells which transport sap - also a water-based solution, but additionally rich in sugars made by the photosynthetic areas, which are transported to non-photosynthetic parts of the plant such as the roots or into storage structures such as tubers or bulbs.

Stele or stela also means an inscribed stone slab. See Stela.