PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 3. PLANT TISSUES

3.1. The Concept of Tissues. Principles of Tissue Classification

In the course of evolution, which was associated with the adaptation of attached thalloid organisms to land conditions, the Differentiation of the plant body was accompanied by an Increasing complexity of its internal Structure. Groups of Cells emerged that performed specific Functions. The earliest known land plants in the fossil record—rhyniophytes—already possessed complexes of cells of various types. As early as 1671, N. Grew suggested calling groups of similar cells Tissues.

N. Grew, as well as M. Malpighi, identified the main morphological features of various Cell groups. In 1807, H. Link (1707–1851) introduced METABOLISM/2.html">THE CONCEPT OF parenchymatous and prosenchymatous cells. Cells that expand more or less evenly in all directions, with approximately equal diameters or a length-to-width ratio not exceeding two, were designated as parenchymatous. Such cells have an isodiametric or tabular shape. Cells in which the length exceeds the width by more than two times were termed prosenchymatous. Typically, they are part of the conducting and mechanical tissues. This means that from a morphological standpoint, plants consist of just Two Types of cells. However, in the differentiated body of higher plants, groups of cells emerged that are similar not only in shape but also in function. Such stable complexes of cells that are similar in origin, structure, and functions performed, and occupy a specific position in the plant body, are called tissues. There are Different types of tissues and various approaches to their Classification. When characterizing tissues, one can use either a single important feature or the entire complex of features that emphasize not only The structure of the cells forming the tissue, but also its functional characteristics.

Taking into account a single leading feature, tissues are subdivided as follows:

1) by the shape of the cells that form the tissue—parenchymatous tissues (ground parenchyma, epidermis) and prosenchymatous tissues (conducting elements, mechanical fibers).

2) by the physiological state of the cells—dead and living tissues. In dead tissues, the Cytoplasm dies off, but The Cell wall remains and continues to play an important role (tracheids, vessels, wood fibers). The cells of living tissues contain cytoplasm and exhibit cyclosis.

3) by the degree of Cell Differentiation—undifferentiated (meristematic) tissues with high totipotency, which is The ability to realize the entire genetic program of the Organism, and permanent tissues differentiated by function (protective, ground, conducting, etc.).

4) by the time and features of formation—primary, secondary, and tertiary. Primary tissues originate from primary, embryonic, or apical Meristems (epidermis, collenchyma). Secondary tissues form from secondary meristems or from permanent tissues (secondary conducting tissues, cambium, phellogen). Tertiary tissues develop in very old plants as a complex of secondary protective tissues and various dead tissues located between them (rhytidome). Secondary tissues provide secondary thickening and are characteristic mainly of gymnosperms and angiosperm dicots. They are absent in mosses, horsetails, clubmosses, the vast majority of ferns, and angiosperm monocots.

5) by the degree of complexity—simple tissues, consisting of a single cell type (collenchyma, parenchyma, rhizodermis), and complex tissues, comprising cells of several types (xylem, phloem, periderm).

All the aforementioned approaches are relevant to classification to some extent. However, the most rational and widely recognized classification of tissues is based on Anatomical and physiological principles, i.e., on the STRUCTURE OF THE tissues and the functions they perform. This type of tissue classification was first proposed in the late 19th century by G. Haberlandt (1828–1878). He distinguished 9 systems taking into account the maximum physiological activity of tissues. Currently, some authors adhere to this classification, but in most cases, morphologists use a simplified classification according to which all tissues of higher plants are grouped into 6 types:

1) meristematic tissues, or meristems;

2) protective tissues, or dermal/boundary tissues;

3) ground tissues (parenchymatous);

4) mechanical tissues, or armature (supporting) tissues;

5) conducting tissues;

6) secretory structures (a collective group).

Along with their main function, tissues can often perform additional ones, which makes it possible to consider them as polyfunctional.

3.2. Anatomical, Morphological, and Functional Features of Meristematic Tissues

The plant body is formed As a result of The activity of meristematic tissues, or meristems (from the Greek meristos — divisible). It is due to meristems that plants grow and form new roots, shoots, and flowers.

The cytological characteristics of meristematic tissues are closely related to the functions they perform. The cells of apical meristems are small, more or less isodiametric, polyhedral, closely packed, and lack intercellular spaces. They have a thin primary wall containing a small amount of Cellulose, which is capable of stretching. Meristem cells are characterized by high metabolic activity. They contain dense cytoplasm with a fairly large Nucleus (relative to the small cell size). The cells contain many Ribosomes, but a negligible number of Plastids; Mitochondria are poorly differentiated. Ergastic substances (reserve substances) are generally absent. Vacuoles are small and inconspicuous.

Meristem cells are capable of active mitotic division throughout the life of the plant or for a fairly long time, which distinguishes them from animal educative tissues. Meristems are preserved for such a long period due to the fact that they possess one (horsetails) or several (angiosperms) so-called initial cells. These cells have the appearance of polyhedra (usually with 14 faces). The initial cells of meristems retain the ability to divide throughout the plant's life and produce only meristematic cells. They divide an unlimited number of times in different planes. As a rule, each subsequent division occurs parallel to one of the faces of the 14-hedron. The remaining daughter cells form the ground meristem, the cells of which divide a limited number of times and eventually turn into permanent tissues—conducting, protective, mechanical, ground, etc.

Depending on their origin and the time of appearance in ontogeny, two groups of meristems are distinguished: Primary and secondary.

3.2.1. Primary Meristems

Primary meristems originate from the moment of zygote division and the Initial Stages of embryo (proembryo) formation. Until the proembryo develops into a sufficiently large spherical body, all of its cells possess meristematic activity and divide in various directions. From the moment the lateral elements of the embryo—the cotyledons—are formed, the first localization of the meristem occurs at the apical (from Latin apex—tip) pole of the stem. Between the cotyledons, the SHOOT apical meristem (apex), represented by the primary apical meristem, differentiates. On the opposite side, the ROOT apical meristem differentiates, also formed by the primary meristem. It is precisely due to the activity of apical meristems that the root and shoot grow in length. During branching, each lateral root and lateral shoot possesses its own apical meristems and initials, enabling these Organs to grow longitudinally. Apical meristems drive the so-called primary growth of axial organs, extending them not only in length but also in thickness.

The apical meristems of the root and stem represent quite complexly differentiated tissues (Fig. 13). In 1868, the German botanist J. Hanstein (1822–1880), while studying the root apex, discovered three distinct zones within it, which he named the dermatogen, periblema, and plerome. Thus, the histogen theory—The Theory of tissue-forming layers (from Greek histos—tissue and genos—origin)—was born. According to Hanstein's views, each histogen possesses its own initial cell. Hanstein believed that various root elements (topographic zones) develop from each layer of the root apical meristem. Since the dermatogen is single-layered and its cells are capable of dividing only anticlinally (perpendicular to the organ's surface), it gives rise to the primary protective tissue of the root—the rhizodermis.

Fig. 13. I. Structure of the apical meristem of growth cones (A–B). II. Scheme of apex differentiation According to the theories of J. Hanstein (C),

Class="center">J. Buder and A. Schmidt (D), A. Foster (E): A—root of common sunflower (Helianthus annuus); B—stem of mare's tail (Hippuris vulgaris): 1—root cap, 2—initial Cells of the apical meristem, 3—dermatogen, 4—periblema, 5—plerome, 6—calyptrogen, 7—protoderm, 8—tunica, 8a—protoderm, 9—corpus, 10—mantle, 11—zone of central mother cells, 12—flank meristem zone, 13—medullary zone

The periblema consists of several layers of meristematic cells that divide in various directions and form a specific topographic zone of the root—the primary cortex. It is composed of parenchymal tissue. The most robust layer of the apical meristem is the plerome. It forms the most critical topographic zone of the root—the central cylinder, or stele. The central cylinder has a complex structure. During early Selection/3.html">Stages of development, special types of meristems—the pericycle and procambium—form within it from the peripheral cells of the plerome, alongside The Development of the central cylinder parenchyma. Thus, the pericycle and procambium are derivatives of the apical meristem.

Following its discovery, the histogen theory gained widespread recognition and initially played a major role in studying tissue formation across various plants during ontogeny. However, subsequent research showed that it was not universal. Not all plants exhibited three histological zones in the root apex. Following critical remarks by A. de Bary and other authors, interest in the histogen theory waned.

In 1924–1928, the German botanists J. Buder and A. Schmidt developed the tunica-corpus theory (Fig. 13, B). They established that in the stem apex of angiosperms, the meristem is differentiated into only two zones. The tunica is weakly expressed; its outermost layer gives rise to the epidermis, while the remaining layers form the primary cortex. The central cylinder develops from the corpus. According to the tunica-corpus theory, The Role of the shoot apical meristem is significant not only during The formation of the axial organ—the stem—but also in the development of its appendages, namely leaves and buds.

The histogen theory and the tunica-corpus theory do not contradict each other; rather, they illustrate the Specificity of meristems in axial organs subjected to different environmental conditions. While the size and shape of the root growth cone remain constant at various Stages of Ontogeny, these parameters in the stem growth cone vary, showing age-related and seasonal differences.

In 1938, the American botanist A. Foster, while studying the shoot apex of ginkgo (Ginkgo biloba, division Gymnospermae), pointed out that the growth cone is divided into a mantle and a corpus, which are formed by apical meristems. At the same time, he discovered that the corpus is heterogeneous in terms of cell size, division activity, cell wall structure, vacuole development, and staining with histological Dyes. This is because gymnosperms possess a group of initials located not only at the apex of the growth cone (apical cells) but also in deeper-lying layers.

The uppermost layer of apical initial cells forms the mantle. It consists of more or less uniform meristematic cells. This very layer corresponds to the tunica (according to J. Buder and A. Schmidt's theory), but in gymnosperms and angiosperms, the cells of this layer divide differently: in gymnosperms—periclinally (parallel to the organ's surface) and anticlinally (perpendicular to the organ's surface), whereas in angiosperms, divisions are strictly anticlinal.

Beneath the apical meristematic cells lies the zone of central corpus mother cells, which give rise to the medullary and flank zones. The flank zone participates in the Formation of the procambium, part of the primary cortex, and leaf primordia. The cells of the medullary zone form the pith of the stem.

Thus, A. Foster's theory of histocytological zonation complements the tunica-corpus theory of J. Buder and A. Schmidt and attempts to explain the peculiarities of shoot appendage formation through the distinct structure and varying meristematic activity of cells in its apex. All these theories provide insight into the differentiation of the root and stem apices (growth cones, growing points), which is undoubtedly important for understanding the formation of the Water/123.html">Primary Anatomical Structure of axial organs; however, none of these theories is universal.

As derivatives of apical meristems, the procambium and pericycle are primary meristems and possess A number of specific features.

Procambium cells are prosenchymatous, polygonal in cross-section, with well-developed large vacuoles. The procambium lacks initial cells, forms longitudinal strands, and functions as a vascular meristem (from Latin vasculum, diminutive of vas—vessel), giving rise to conducting tissues.

The pericycle is a specialized meristematic tissue that is well-developed in the root. Like the procambium, it lacks initial cells. Most commonly, the pericycle consists of a single row of tabular cells elongated parallel to the root surface.

Primary meristems also include intercalary meristems. They become separated from the apical meristem as zones of permanent, differentiated tissues develop between them. Intercalary meristems lack initial cells; therefore, they function for a limited time, soon cease dividing, and transform into permanent tissues. Intercalary meristems are located at the base of shoot internodes (persisting particularly long in grasses), staminal filaments, leaf primordia, petioles, and flower scapes. They are characteristic of plants whose apices are frequently damaged or where flower or inflorescence primordia are laid down at the apex.

3.2.2. Secondary Meristems

Secondary meristems include the cambium, phellogen, and wound meristems. Secondary plant growth, or secondary thickening, is associated with the emergence and activity of secondary meristematic tissue—the cambium. The cambium can originate from the procambium, pericycle, or parenchyma capable of restoring meristematic function. Cambium cells are tabular in shape and elongated parallel to The surface of axial organs. The cambium is a vascular meristem; its cells divide periclinally (parallel to the surface) and give rise to secondary conducting tissues. The presence of initials is characteristic of the cambium. Initial cells vary somewhat in shape and give rise to different elements of secondary conducting tissues: parenchymatous initial cells produce ray parenchyma elements, while prosenchymatous (fusiform) initial cells produce conducting elements.

The phellogen, or cork cambium, is a specialized secondary meristem that develops from epidermal cells, subepidermal parenchymal cells of the primary cortex, and the pericycle. The phellogen gives rise to the periderm—a secondary protective tissue.

Based on their position within the plant body, the procambium, cambium, and pericycle are lateral meristems.

Secondary meristems also include wound meristems, which arise when Plant Tissues and organs are injured. As a result, parenchymal cells surrounding the wound begin to actively divide in various directions, forming an outgrowth known as a callus (from Latin callus—thick Skin, hard callus). Sometimes, a phellogen forms from parenchymal cells, sealing the wound with cork.

Cells of various meristems can divide in different directions. If The Cell plate is laid down at a right angle to the organ's surface, such division is called anticlinal. Single-layered protective tissues are formed through such divisions. Divisions in which newly formed cell walls are parallel to the nearest surface of the organ are called periclinal. They are particularly characteristic of lateral meristems; thickening of axial organs typically occurs through such divisions. In cases where the partition is laid down tangentially to the circumference, the division is termed tangential.

During the differentiation of meristematic cells into permanent tissues, two Types of Growth can be observed: symplastic and intrusive (interpositional). In symplastic growth, the cell walls of adjacent cells grow in a coordinated manner, without disrupting their plasmodesmatal connections. In intrusive growth, such coordination is absent, and certain cells wedge themselves into the spaces formed between other cells.