In a vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

, the stele is the central part of the root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

 or stem

Plant stem

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , conifer cones, roots, other stems etc. The internodes distance one node from another...

 containing the tissues derived from the procambium. These include vascular tissue

Vascular tissue

Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...

, in some cases ground tissue (pith

Pith

Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant. In eudicots, pith is located in the center of the stem. In monocots, it extends also into flowering stems and roots...

) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis

Endodermis

The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...

, which is the innermost cell layer of the cortex.

The concept of the stele was developed in the late 19th century by French

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 botanists P. E. L. van Tieghem

Phillippe Édouard Léon van Tieghem

Philippe Édouard Léon Van Tieghem was a French botanist who was a native of Baillleul in the département of Nord....

 and H. Doultion as a model for understanding the relationship between the shoot

Shoot

Shoots are new plant growth, they can include stems, flowering stems with flower buds, and leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop...

 and root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

, and for discussing the evolution of vascular plant morphology

Plant morphology

Plant morphology or phytomorphology is the study of the physical form and external structure of plants. This is usually considered distinct from plant anatomy, which is the study of the internal structure of plants, especially at the microscopic level...

. 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.

Protostele

The earliest vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

s had stems with a central core of vascular tissue. This consisted of a cylindrical strand of xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 , surrounded by a region of phloem

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

. Around the vascular tissue there might have been an endodermis

Endodermis

The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...

 that regulated the flow of water into and out of the vascular system. Such an arrangement is termed a protostele.

There are three basic types of protosteles:

• haplostele - consisting of a cylindrical core of xylem surrounded by a ring of phloem. An endodermis generally surrounds the stele. A centrarch (protoxylem in the center of a metaxylem cylinder) haplostele is prevalent in members of the rhyniophyte grade, such as Rhynia
  Rhynia
  Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
  
  .
• actinostele - a variation of the protostele in which the core is lobed or fluted. This stele is found in many species of club moss (Lycopodium
  Lycopodium
  Lycopodium is a genus of clubmosses, also known as ground pines or creeping cedar, in the family Lycopodiaceae, a family of fern-allies...
  
  and related genera). Actinosteles are typically exarch (protoxylem external to the metaxylem) and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem. Exarch protosteles are a defining characteristic of the lycophyte lineage.
• plectostele - a protostele in which plate-like regions of xylem appear in transverse section surrounded by phloem tissue. In fact, these discrete plates are interconnected in longitudinal section. Some modern club mosses have plectosteles in their stems. The plectostele may be derived from the actinostele.

Siphonostele

Siphonosteles have a region of ground tissue called the pith internal to xylem. The vascular strand comprises a cylinder surrounding the pith. Siphonosteles often have interruptions in the vascular strand where leaves (typically megaphylls) originate (called leaf gaps).

Siphonosteles can be ectophloic (phloem present only external to the xylem) or they can be amphiphloic (with phloem both external and internal to the xylem. Among living plants, many ferns and some Asterid flowering plants have an amphiphloic stele.

An amphiphloic siphonostele can be called a:

• solenostele - if the cylinder of vascular tissue contains no more than one leaf gap in any transverse section (i.e. has non-overlapping leaf gaps). This type of stele is primarily found in fern
  Fern
  A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
  
   stems today.
• dictyostele - if multiple gaps in the vascular cylinder exist in any one transverse section. 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 can be called a meristele. 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 a

• eustele - in this arrangement, the primary vascular tissue consists of vascular bundles
  Vascular tissue
  Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
  
  , usually in one or two rings around the pith. In addition to being found in stems, the eustele appears in the root
  Root
  In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
  
  s of monocot 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 on the eustele found in monocots like maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 and rye

Rye

Rye is a grass grown extensively as a grain and as a forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some whiskeys, some vodkas, and animal fodder...

. The variation has numerous scattered bundles in the stem and is called an atactostele. However, it is really just a variant of the eustele.