PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 4. THE DEVELOPMENT OF PLANT BODY FORM

4.5. Evolutionary Complexification of Plant Anatomical Structure. Evolution of Stele Types

As the External Structure of plants becomes more complex, their internal Organization likewise evolves. First and foremost, we should note the Increasing complexity of the central cylinder, or stele.

The fundamentals of the stellar theory were established by the French botanist P. Van Tieghem in the late 19th century. He defined METABOLISM/2.html">THE CONCEPT OF the "stele" (from the Latin stela and Greek stele meaning pillar or Column) as a composite of primary Vascular Tissues and the pericycle. Van Tieghem successfully demonstrated that the stele serves as a universal structural element in the stems and roots of vascular plants, yet exhibits diverse structural variations across different taxa. The stellar theory was further advanced through the works of several researchers, most notably the German scientist W. Zimmermann and the Russian botanists K. I. Meyer and A. L. Takhtajan, who showed that stellar architecture varies among taxa both over the course of evolution and throughout ontogeny.

The most primitive type of stele is considered to be the protostele (from the Greek protos meaning first), which was identified in rhyniophytes. It consists of a solid core of vascular elements featuring xylem in the center, surrounded by the phloem. A protostele with a circular cross-section is frequently termed a haplostele (from the Greek haplos meaning single). The protostele is enclosed and protected by the cortex. In extant plants, a protostelic central cylinder can be found in the rhizome-like axes of Psilotum and in the Organs of young, primitive ferns such as Botrychium, Ophioglossum, and Helminthostachys.

The diversification and complexification of stele types are closely linked to The Emergence of leaves and lateral branches, which necessitated an upgraded Vascular System. The British botanist F. O. Bower (1855–1948) pointed out that The rate of substance transport is proportional to the contact surface area between living and dead conducting tissues. Through comparative studies of the central cylinder in fossil and extant plants, researchers have identified various stele types across different taxa and established their evolutionary relationships. Current evidence indicates that the complexification of steles occurred by increasing the contact area between the xylem and phloem, as well as via vitalization (from the Latin vitalis meaning alive)—the incorporation of a large volume of living parenchymatous tissue within the central cylinder.

Alterations in the contact area between the xylem and phloem led to The formation of the actinostele (from the Greek aktis meaning ray), which is characterized by a lobed or star-shaped xylem core entirely surrounded by phloem. This type of stele has been discovered in Asteroxylon and is also found in young shoots of certain extant lycophytes. In older lycophyte shoots, the central cylinder consists of xylem dissected into separate ribbon-like strands, with phloem situated around them. The pericycle is barely distinguishable from the innermost layer of the cortex, the endodermis. This particular type of stele is termed a plectostele (from the Greek plectos meaning plaited or interwoven). The protostele, actinostele, and plectostele are commonly regarded as variations of the protostele. Protosteles, actinosteles, and plectosteles are characteristic of Representatives of the microphyllous evolutionary Lineage.

In representatives of the macrophyllous evolutionary lineage, the complexification of stellar structure is associated with The process of vitalization. Two Types of siphonostele (from the Greek siphon meaning tube)—ectophloic and amphiphloic (also known as a solenostele, from the Greek solon meaning pipe)—could have originated from the protostele. In a siphonostele, a pith composed of living parenchymatous tissue develops in the center of the xylem and begins to perform a storage function. In an ectophloic siphonostele, the phloem lies outside the xylem and is enclosed by the pericycle, with the entire central cylinder protected by the cortex. The innermost layer of the cortex—the endodermis—consists of thin-walled Cells. The Displacement of the xylem toward the periphery increased the mechanical strength of the axial organs. The ectophloic siphonostele (or simply siphonostele) is typical of certain modern ferns that produce a small number of relatively small leaves, such as Botrychium and Marsilea. In some ancient pteridophytes with large leaves ranging from 2 to 6 meters in length (e.g., Osmunda, marattioid ferns), a solenostele develops in young organs. It possesses specific structural differences from the siphonostele: the pith is surrounded by an internal endodermis, followed outward by vascular tissues comprising both outer and inner phloem with the xylem sandwiched between them. The outer phloem is bordered by the pericycle, which defines the outer boundary of the central cylinder, followed in turn by the cortex.

The majority of extant pteridophytes are characterized by a dictyostele (from the Greek dictyon meaning net). It forms from a solenostele that is intersected by numerous leaf gaps (associated with leaf traces, which are bundles of vascular tissue supplying the leaves), resulting in a high number of parenchyma-filled leaf gaps. The segment of the stele lying between two adjacent leaf gaps is called a meristele. Each meristele resembles a concentric vascular bundle containing a central core of xylem surrounded by phloem, a pericycle, and a well-defined endodermis. Meristeles vary in size and shape; typically, they are arranged in a ring embedded in parenchymatous tissue. In certain ferns, such as bracken (Pteridium), meristeles may also occur within the cortex, forming one or more concentric rings. Such dictyosteles are referred to as dicyclic or polycyclic dictyosteles, respectively.

Angiosperms, with their highly branched axial organs and abundant leaves, are characterized by two types of stele: eustele and atactostele. The eustele develops from a siphonostele and forms a circular system of collateral bundles arranged in a single row, separated by primary medullary rays. This stelar type is typical of dicotyledonous plants. Cross-sections through the young ROOT and stem reveal quite significant differences in The structure of the eustele. The root lacks a pith, but features a well-developed pericycle and endodermis. In the young stem, the pith is clearly defined, whereas the pericycle and endodermis are poorly distinguished. Furthermore, the formation of xylem and phloem differs between the root and the stem. In monocots, the central cylinder contains A large number of randomly distributed closed collateral bundles. This type of stele is called an atactostele (from the Greek atactos, meaning disordered). It arises because numerous leaf traces enter the stem from the monocot leaf, retaining their independence within the parenchyma of the central cylinder (Fig. 60). In many monocots, the pericycle is very frequently modified into mechanical tissue—pericyclic sclerenchyma—while the cortex is not sharply demarcated from the central cylinder.

Class="center">Fig. 60. Diagram of the evolution of stelar types: 1—protostele; 2—actinostele; 3—plectostele; 4—ectophloic siphonostele; 5—arthrostele; 6—amphiphloic siphonostele (solenostele); 7—dictyostele; 8—eustele; 9—atactostele (primary xylem is shaded, primary phloem is indicated by dots)

All types of steles are evolutionarily interrelated, and each is characteristic of specific taxa of vascular plants. Consequently, stelar theory is of great importance not only for plant Morphology and Taxonomy, but also for understanding phylogenesis as a whole.

4.6. Directions of Evolution. Body Forms in Vascular Plants

As noted earlier, in the course of evolution, the vegetative sphere of plants predominantly developed toward increasing The complexity of the external and Internal Structure of organs and differentiating their Functions.

Modern vascular plants (with the exception of bryophytes) are complexly organized organisms with well-developed tissue systems and vegetative organs, enabling them to inhabit A wide variety of ecological niches.

However, the evolution of vascular plants under terrestrial conditions did not proceed solely along the path of morphophysiological progress. In connection with the transition to an aquatic lifestyle, as well as phenomena of saprophytism and parasitism, some vascular plants underwent a simplification of both morphological and Anatomical Structure. Consequently, three main evolutionary directions can be identified in the Development of Body form in vascular plants: aromorphosis, idioadaptation, and degeneration.

The appearance of vegetative organs, the formation of tissues, and The Development of the stele undoubtedly represent aromorphoses—qualitative leaps that significantly raised the level of plant organization and enabled them to adapt to new environmental conditions.

The diverse leaf structures found in plants of different habitats—such as hydrophytes (Water lilies, pondweeds) and hygrophytes (marsh marigold), which inhabit water-logged environments; xerophytes (heather, stonecrops), which inhabit arid regions; and mesophytes, plants of moderately moist environments—should be regarded as Examples of idioadaptation, i.e., the emergence of specific adaptations in organ Structure and function while preserving the general organizational plan of ancestral forms.

Finally, the plant world provides examples of degeneration—the simplification of an organ's structure, up to its complete disappearance (abortion). In parasitic plants (dodder — Cuscuta, broomrape — Orobanche), leaves and roots disappear. In free-floating aquatic angiosperms (duckweed — Lemna), a root is present, but the root cap is absent and root hairs do not form. In the aquatic fern floating watermoss (Salvinia natans), roots do not form at all; their function is performed by a highly dissected and densely pubescent submerged leaf. Moreover, Salvinia exhibits a simplified stelar type: instead of a dictyostelic central cylinder, it develops a protostelic one. The degeneration of organs or other structures is a specific PLANT RESPONSE TO particular environmental conditions.