In avascular plant, thestele (also calledvascular stele orvascular cylinder) is the central part of theroot orstem[1] containing the tissues derived from the procambium. These includevascular tissue, in some cases ground tissue (pith) and apericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies theendodermis, which is the innermost cell layer of the cortex.
The concept of the stele was developed in the late 19th century byFrenchbotanistsP. E. L. van Tieghem andH. Doultion as a model for understanding the relationship between theshoot androot, and for discussing the evolution of vascularplant morphology.[2] Now, at the beginning of the 21st century, plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele.[citation needed] Moreover, physiologists are examining how the anatomy (sizes and shapes) of different steles affect the function of organs.
The earliestvascular plants had stems with a central core of vascular tissue.[3][4] This consisted of a cylindrical strand ofxylem, surrounded by a region ofphloem. Around the vascular tissue there might have been anendodermis that regulated the flow of water into and out of the vascular system. Such an arrangement is termed a protostele.[5]
There are usually three basic types of protostele:
Siphonosteles have a central region of ground tissue called thepith, with the vascular strand comprising a hollow cylinder surrounding the pith.[9] Siphonosteles often have interruptions in the vascular strand where leaves (typically megaphylls) originate (called leaf gaps).
Siphonosteles can be calledectophloic (phloem present only external to the xylem)[citation needed] or they can beamphiphloic (with phloem both external and internal to the xylem).[9] Among living plants, many ferns and someAsterid flowering plants have an amphiphloic stele.
An amphiphloic siphonostele can be called asolenostele, or this term may be used to refer to cases where the cylinder of vascular tissue contains no more than one leaf gap in any transverse section (i.e. has non-overlapping leaf gaps).[9] This type of stele is primarily found infern stems today. Where there are large overlapping leaf gaps (so that multiple gaps in the vascular cylinder exist in any one transverse section), the termdictyostele may be used.[10] The numerous leaf gaps and leaf traces give a dictyostele the appearance of many isolated islands of xylem surrounded by phloem. Each of the apparently isolated units of a dictyostele that serve a single leaf can be called ameristele.[11] Among living plants, this type of stele is found only in the stems of ferns.
Most seed plant stems possess a vascular arrangement which has been interpreted as a derived siphonostele, and is called aeustele – in this arrangement, the primary vascular tissue consists ofvascular bundles, usually in one or two rings around thepith.[12] In addition to being found in stems, the eustele appears in theroots ofmonocot flowering plants. The vascular bundles in a eustele can be collateral (with the phloem on only one side of the xylem) or bicollateral (with phloem on both sides of the xylem, as in some Solanaceae).
There is also a variant of the eustele found in monocots likemaize andrye. The variation has numerous scattered bundles in the stem and is called anatactostele (characteristic of monocot stems). However, it is really just a variant of the eustele.[12][13]
