Stele (biology)

In the biology of vascular plants, the stele is a vital central structure. It is also known as the vascular stele or the vascular cylinder. This part is located in the center of a plant's root or stem. The stele contains tissues that come from the procambium, which is a specialized plant tissue. These tissues include the vascular tissue used for transport. In some plants, the stele also contains ground tissue called the pith. Some steles include a pericycle, which marks the outermost boundary of the stele. Just outside this boundary lies the endodermis. The endodermis is the innermost layer of the plant's cortex.

Scientists have studied the stele to understand plant evolution and function. In the late 19th century, French botanists P. E. L. van Tieghem and H. Doultion developed the concept. They used the stele as a model to study the relationship between roots and shoots. They also used it to discuss how plant shapes changed over time. Today, researchers continue this work in new ways. Plant molecular biologists study the genetics and developmental pathways of the stele. They want to know how tissue patterns are formed. Meanwhile, physiologists study how the anatomy of a stele affects organ function. They look at how different sizes and shapes change how a plant works.

The simplest type of stele is the protostele. This arrangement was found in the earliest vascular plants. A protostele consists of a central core of xylem tissue. This xylem is surrounded by a region of phloem tissue. Some protosteles have an endodermis to regulate water flow. There are three main types of protostele. The first is the haplostele, which has a simple cylindrical xylem core. A centrarch haplostele has protoxylem in the center of a metaxylem cylinder. This type is common in the rhyniophyte grade, such as the plant Rhynia.

The second type of protostele is the actinostele. In this version, the central core is lobed or fluted rather than a simple cylinder. This type is found in many club moss species, such as Lycopodium. Actinosteles are typically exarch. This means the protoxylem is located external to the metaxylem. They consist of several or many patches of protoxylem at the tips of the metaxylem lobes. This exarch arrangement is a defining trait of the lycophyte lineage. The third type is the plectostele. In a plectostele, the xylem appears as plate-like regions in a transverse section. These plates are surrounded by phloem and look like alternating bands. These plates are connected when viewed in a longitudinal section. Some modern club mosses still have plectosteles in their stems.

A more complex arrangement is the siphonostele. Unlike the solid protostele, a siphonostele has a central region of ground tissue called a pith. The vascular tissue forms a hollow cylinder around this pith. Siphonosteles often have interruptions called leaf gaps. These gaps occur where large leaves, or megaphylls, originate. Siphonosteles can be classified by where the phloem is located. If phloem is only external to the xylem, it is called ectophloic. If phloem is both external and internal to the xylem, it is called amphiphloic. Many ferns and some Asterid flowering plants have an amphiphloic stele.

Within the siphonostele group, there are even more specific types. A solenostele is an amphiphloic siphonostele with limited leaf gaps. Specifically, it contains no more than one leaf gap in any transverse section. This type is mainly found in fern stems today. Some ferns have a more complex version called a dictyostele. In a dictyostele, the leaf gaps are large and overlapping. This makes the vascular tissue look like many isolated islands of xylem surrounded by phloem. Each of these small units that serves a single leaf is called a meristele. This structure is found only in the stems of ferns.

Most modern seed plants possess a different arrangement called a eustele. A eustele is considered a derived version of a siphonostele. In this arrangement, the primary vascular tissue is organized into vascular bundles. These bundles usually form one or two rings around the central pith. The bundles in a eustele can be collateral, meaning phloem is only on one side of the xylem. They can also be bicollateral, where phloem is on both sides. This bicollateral arrangement is seen in some Solanaceae plants. The eustele is also found in the roots of monocot flowering plants.

Finally, there is a unique variant of the eustele found in certain monocots. Plants like maize and rye have an atactostele. In an atactostele, the vascular bundles are numerous and scattered throughout the stem. While it looks very different from a ring of bundles, it is actually a variant of the eustele. Understanding these different patterns helps scientists map the history of plant life. From the solid cores of early plants to the complex bundles of modern seeds, the stele shows how plants evolved to transport water and nutrients efficiently.