PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 7. THE FLOWER AS A SPECIAL REPRODUCTIVE ORGAN OF ANGIOSPERMS

7.6. Ontogenetic Development of the Stamen. Characteristics of Processes Occurring in the Anther

7.6.1. Stamen Formation, Anther Cytology/cytology/67.html">Development and Structure

As noted earlier, stamens originate on the receptacle after The formation of the perianth elements. In most angiosperms, if stamens are arranged in several whorls or spirally, their development proceeds from the periphery to the center; however, in Brassicaceae, stamens form in a centrifugal direction—from the center to the periphery.

Initially, the meristematic protuberance of the initiating anther grows uniformly; the Cells of the surface meristematic layer divide anticlinally (perpendicular to the surface) to give rise to the epidermis. The epidermal cells exhibit a structure typical of this tissue. The outer walls of the epidermal cells are strongly thickened, and a cuticle layer, and sometimes a wax layer, forms on their surface. The Cell walls of the epidermis thicken particularly strongly at the anther dehiscence sites. All other cells retain their meristematic function for a prolonged period, leading to the enlargement of the meristematic protuberance and, subsequently, to its differentiation. During differentiation, the protuberance acquires a four-lobed shape. Eventually, one microsporangium (pollen chamber) will form in each lobe (Fig. 165).

Class="center">Fig. 165. Initiation and Development of the stamen anther: A—meristematic protuberance with a differentiated subepidermal cell; B—appearance of the primary archesporium; C—Formation of the secondary archesporium and parietal cells; D—onset of microsporangium formation with sporogenous cells and Differentiation of the anther wall; E—formed microsporangia with sporogenous tissue: 1—epidermis; 2—subepidermal cell; 3—primary archesporium; 4—secondary archesporium; 5—parietal cells; 6—developing sporogenous tissue; 7—cells forming the anther wall; 8—vascular bundle; 9—microsporangium with sporogenous tissue; 10—wall of the young anther

In the early Selection/3.html">Stages of development, in each of the anther lobes beneath the epidermis, single cells begin to differentiate, representing the primary archesporial cells. They differ from the remaining meristematic cells by their larger size, larger Nucleus, and, undoubtedly, biochemical features. In particular, a high content of RNA, basic and acidic Proteins, and certain Enzymes (peroxidases, Phosphatases) has been detected in the primary archesporial cells; however, from a biochemical standpoint, the archesporium remains very poorly studied. This fully applies to most embryological structures.

Primary archesporial cells divide tangentially (periclinally — parallel to the organ surface) via mitosis, giving rise to two cell types: parietal cells and secondary archesporial cells. The anther wall is formed from the parietal cells As a result of a series of mitotic divisions and subsequent differentiation. In various species of angiosperms, it varies in thickness and exhibits certain specific features, but its General structural plan is fundamentally uniform (Fig. 166).

Fig. 166. Fragment of a developing anther. Differentiation of the anther wall and microsporangium (pollen chamber): 1—epidermis; 2—endothecium (fibrous layer); 3—middle layer cells; 4—tapetum (lining layer); 5—microsporangium with sporogenous cells (microsporocytes)

Beneath the epidermis lies the endothecium, or fibrous layer. In terrestrial plants, this layer develops distinctive (fibrous) thickenings on the inner side of The Cell wall and takes an active part in anther dehiscence. During anther maturation, which is accompanied by intensive Water loss in all cells and particularly in surface cells, the walls of the fibrous layer cells begin to collapse at thinner points, leading to active anther dehiscence through the rupture of its wall. In aquatic plants, fibrous thickenings do not form in endothecium cells, and anther dehiscence occurs passively via cell degradation.

The middle layer is located behind the endothecium, characterizing its position within the anther wall. The term was introduced in 1898 by J. M. Coulter. The cells of the middle layer are small, and their thin walls possess a Primary Structure. The middle layer can vary in thickness and represents a short-lived anther layer. According to available data, it is characterized by the presence of starch, carotene, and lipid globules, indicating that this layer performs a trophic function. It is believed that the middle layer cells break down early and their contents are utilized to nourish the developing microsporocytes and microspores.

Beyond the middle layer, the tapetum, or nutritive layer, is formed. It bounds (lines) the microsporangium and is directly adjacent to the microsporocytes. The tapetum is a multifunctional tissue that ensures the normal progression of Meiosis during microspore formation, participates in The Development of the male gametophyte (pollen), and contributes to the formation of the pollen grain wall. The performance of these diverse Functions is facilitated by both the MORPHOLOGICAL STRUCTURE OF the tapetal cells and their biochemical characteristics. Tapetal cells are large, with large nuclei containing multiple nucleoli. Quite often, alongside uninucleate cells, multinucleate tapetal cells are observed. It has also been noted that some tapetal cells possess polyploid nuclei. Nucleic Acids, a high concentration of enzymes, heteroauxin, ascorbic acid, and other components have been detected in tapetal cells. Nevertheless, biochemical studies of the tapetum remain fragmentary and have been performed on various cultures. Despite this, embryologists believe that the aforementioned features indicate a high physiological activity of tapetal cells. This is also supported by Electron Microscopy data. A large number of Ribosomes, Mitochondria, and a well-developed Endoplasmic reticulum have been found in tapetal cells. Special structures—«Ubisch bodies» or orbicules—have been identified on the inner side of the tapetal cell walls. Orbicules contain sporopollenin, a specialized substance that is a component of the outer pollen wall. Two Types of tapetum are distinguished: secretory (glandular) and amoeboid. Cells of the glandular tapetum do not break down for a long time and function as a secretory tissue. In the amoeboid tapetum, cell walls break down by the onset of male gametophyte formation, and the cell contents fuse into a common mass.

Whether one type of tapetum has any advantages over the other, what their evolutionary relationship is (which type is primitive), and what their taxonomic significance is remain unclear and continue to be a subject of debate and scientific Discussion.

Simultaneously and in parallel with the formation of the anther wall, secondary archesporial cells are formed from primary archesporial cells through mitotic division. These cells either directly become microspore mother cells or undergo a series of mitotic divisions to give rise to microspore mother cells, or microsporocytes. Numerous microsporocytes fill the microsporangia, which are located in pairs in each half of the anther (see Fig. 166).

At a specific stage of development, reproductive processes commence within the microsporangia: microsporogenesis, development of the male gametophyte, and formation of male sex cells (Gametes)—sperm cells.

7.6.2. Structure of Microsporocytes. Microsporogenesis

Microsporocytes are diploid cells possessing a callose wall, non-vacuolated Cytoplasm, and large nuclei. Young microsporocytes fit tightly against one another, but as they develop and increase in size, the microsporocyte cells separate and come to lie freely within the microsporangium cavity (Fig. 167). The isolation of microsporocytes leads to their Separation and signifies preparation for microsporogenesis—The process of microspore formation.

Fig. 167. Microsporocytes of broad beans (Vicia faba) in the anther microsporangia prior to the onset of reduction division: 1—nucleus; 2—nucleolus; 3—cytoplasm; 4—microsporocyte wall

The process of microsporogenesis begins with the reductional (meiotic) division of the microsporocyte nucleus (Fig. 168), followed by cytokinesis, i.e., the formation of cell walls. As a result of reductional division, a tetrad of haploid microspores is formed from each diploid microsporocyte.

The deposition of cell walls during microspore formation can be successive or simultaneous.

Fig. 168. Meiosis in microsporocyte nuclei of broad beans (Vicia faba): A, B—prophase I stages; C—metaphase I; D—metaphase II

In the successive type of development, cell plate formation during meiosis occurs in two stages. At the end of meiosis I, a cell plate forms between the daughter nuclei of the divided microsporocyte nucleus, resulting in a dyad. In meiosis II, the nuclei of the dyad divide, after which a wall forms between the newly created nuclei, producing a typical tetrad.

In the simultaneous type of microspore development, cytokinesis occurs at the end of meiosis II. The formation of cell walls in this case proceeds from the periphery to the center, and four cells are formed simultaneously. Thus, each microspore within the tetrad is enclosed in its own wall, while the entire tetrad is surrounded by a callose wall derived from the mother cell (Fig. 169). Microspores are arranged variously within the tetrad, which is determined by the orientation of the division spindle (achromatic spindle) and dictates the tetrad type. Tetrahedral and isobilateral tetrads are most frequently formed during microsporogenesis, whereas decussate tetrads are less common (Fig. 170). T-shaped and linear tetrads occur very rarely.

Fig. 169. Tetrahedral microspore tetrad of the broad bean (Vicia faba): 1—microspore; 2—microspore nucleus with nucleolus; 3—microspore wall; 4—tapetal cell; 5—tapetal Cell Nucleus; 6—microspore tetrad wall

Fig. 170. Types of microspore tetrads: A—decussate tetrad of heather (Calluna vulgaris); B—tetrahedral tetrad of cowberry (Vaccinium vitis-idaea); C—isobilateral tetrad of garden pea (Pisum sativum)

In most plants, tetrads are short-lived. Soon after formation, the microsporocyte (microspore mother cell) wall enclosing the tetrad swells, undergoes mucilaginous degeneration, and breaks down, allowing the tetrad to separate into individual microspores.

Each microspore is a haploid cell exhibiting typical cellular architecture. The microspore is rounded in shape, enveloped by a primary cell wall, and contains dense, non-vacuolated cytoplasm, a centrally located nucleus, and all cellular Organelles (Fig. 171). With the Isolation of the microspores, the process of microsporogenesis concludes, and right there within the microsporangium, a new process begins—the germination of microspores and the formation of the male gametophyte, namely pollen grains (pollen).

Fig. 171. Bean microspores after tetrad separation: 1—microspore wall; 2—nucleus; 3—cytoplasm

Each microspore has the potential to develop into a male gametophyte only if it has resulted from a normally completed reduction division process. However, various types of irregularities are occasionally observed during meiosis. Most frequently, these anomalies are associated with improper chromosome segregation in metaphase and anaphase of meiosis I and meiosis II (Fig. 172). Meiotic irregularities lead to the formation of microspores with more or less pronounced defects. Such microspores are either completely incapable of further development, or they give rise to deformed and sterile pollen.

Fig. 172. Meiotic abnormalities during bean microsporogenesis: A—chromosome elimination from the metaphase plate; B—chromosome leading; C—chromosome lagging; D—rupture of a chromosomal bridge and fragment formation; E, F—formation of chromosomal bridges

7.6.3. Formation of the Male Gametophyte (Pollen). Morphological and Physiological-Biochemical Features of Pollen

The initial sign of the onset of male gametophyte development is A change in the shape of the microspore.

Initially spherical, the microspore becomes elliptical, while its nucleus remains centrally positioned as before. As the microspore develops, it gradually increases in size, and a large vacuole appears within its cytoplasm, causing The Nucleus to shift toward one of the poles.

The male gametophyte is formed through the mitotic division of the microspore nucleus followed by a unique process of cellularization. The mitotic division of the developing microspore nucleus proceeds typically through all phases (Fig. 173); however, the achromatic spindle is atypical in being asymmetrical. The spindle fibers directed toward the equatorial zone of the developing pollen grain are significantly longer than those directed toward the pole. The consequence of this asymmetrical achromatic spindle formation is the generation of two functionally and morphologically distinct nuclei. The nucleus located in the central part of the enlarged cell is termed the vegetative nucleus, while the one formed near the cell wall is the generative nucleus. Despite being formed via mitotic division and seemingly expected to possess identical properties, they differ in morphological, physiological, and functional characteristics. The vegetative nucleus is larger than the generative one and stains less intensely with the Feulgen reagent (a specific test for DNA), indicating a difference in their nucleic acid content. The generative nucleus is denser than the vegetative one.

Fig. 173. Formation of the male gametophyte (pollen grain, pollen) in the broad bean (Vicia faba): A–D — successive stages of mitosis during microspore nuclear division: A—microspore before the onset of mitosis (equatorial view); B—prophase; C—metaphase (polar view); D—anaphase; E—telophase; F—formation of vegetative and generative nuclei; G—formation of vegetative and generative cells; H—bicellular bean pollen: 1—vegetative nucleus; 2—generative nucleus; 3—vegetative cell; 4—generative cell

Soon after the formation of the vegetative and generative nuclei, cytoplasmic Cleavage occurs, or—as some embryologists suggest—a callose wall is formed, resulting in two unequal cells: a larger vegetative cell and a smaller generative cell.

The first Description of the two unequal cells of a pollen grain was provided by the German botanist C. Nägeli (1818–1891) in 1842. The smaller cell was named the "generative cell" by E. Strasburger, who also defined its function.

The vegetative and generative cells differ not only in size. According to available data, the vegetative cell functions as a storage unit with high metabolic activity. It contains large amounts of Lipids, starch, and proteins, which are essential for the subsequent development of the pollen, primarily for the growth of the pollen tube. In terms of protein content, the vegetative cell is richer than the generative cell, and its proteins are characterized by a higher acidity, with Arginine predominating. DNA Synthesis in the Nucleus of the vegetative cell occurs much later than in the generative nucleus, most frequently just before pollen maturation. Intensive cytoplasmic synthesis is observed within the vegetative cell, leading to the disappearance of the vacuole. Prior to maturation, the vegetative cell undergoes dehydration.

The generative cell contains a small amount of cytoplasm but possesses all cellular organelles.

Reserve substances, particularly starch, are absent from the cytoplasm of the generative cell. DNA content in the generative cell nucleus doubles rapidly, whereas protein accumulation proceeds slowly. During the formation of the generative cell, RNA content initially drops sharply, followed by its intensive synthesis in both the cytoplasm and the nucleus, accompanied by a simultaneous decrease in RNA within the nucleolus.

The ultrastructure of vegetative and generative cells began to be studied in the 1960s. Research has shown that both cells contain all standard cell organelles, though certain differences exist. The cytoplasm of the vegetative cell is particularly rich in pectin vesicles, which play a key role in forming the pollen tube wall. Plastids are occasionally absent from the cytoplasm of the generative cell.

Shortly after the formation of the vegetative and generative cells, the nucleus of the generative cell undergoes severe compression, making its internal structure indistinguishable; its shape becomes elliptical, and the cytoplasm surrounds it in a thin layer. As the generative cell compresses, the vegetative cell increases in size. However, the formation of the male gametophyte does not end with the creation of these two cells. The generative cell shifts toward the equatorial region of the developing male gametophyte, penetrates the cytoplasm of the vegetative cell, and positions itself close to the vegetative nucleus, taking on a crescent or lenticular shape (see Fig. 173, 3).

Simultaneously with the transformation of the Internal Structure of the microspore as it develops into the male gametophyte, its wall also undergoes modification. Gradually, a cellulosic wall impregnated with sporopollenin forms over the cytoplasmic membrane. Consequently, the pollen grain becomes encased in two layers: an outer layer, the exine, and an inner layer, the intine. The exine is not a continuous layer; it features apertures, which are breaks in the exine structure (Fig. 174). Being extremely durable, the exine enables the pollen not only to maintain its constant shape but also to retain its viability (ability to germinate) over extended periods.

Fig. 174. Diagram of The structure of a pollen grain: A—three-pored pollen of peach-leaved bellflower (Campanula persicifolia); B—three-furrowed pollen of swede (Brassica napus) (a — equatorial view; b — polar view): 1—exine; 2—exine structure; 3—pore-like apertures; 4—furrow-like apertures

Nevertheless, irregularities in the mitotic division of the microspore nucleus occasionally occur during pollen development. Anaphase represents the most vulnerable stage of mitosis. Due to aberrant chromosome segregation during anaphase, so-called chromosomal bridges may form (Fig. 175). If these bridges break, micronuclei can develop within the pollen instead of two normally formed nuclei. Pollen containing micronuclei is typically non-viable or exhibits significantly reduced viability.

Fig. 175. Mitotic irregularity during the formation of the male gametophyte in the broad bean (Vicia faba): 1—formation of a chromosomal bridge in mitotic anaphase

In dicotyledonous plants, pollen development within the anther concludes at the two-celled stage, consisting of a vegetative and a generative cell. Pollen comprising these two cells is termed bicellular pollen, or a bicellular male gametophyte. The formation of gametes—sperm cells—in such pollen occurs only after it lands on the stigma of the pistil and begins to germinate. At that point, the nucleus of the generative cell divides mitotically, followed by cytokinesis, resulting in the formation of two sperm cells.

In monocotyledonous plants, the generative cell undergoes mitotic division while the pollen is still inside the pollen chamber. As a result, such pollen contains a vegetative cell and two sperm cells. This type of pollen is referred to as tricellular pollen, or a tricellular male gametophyte.

As early as the 1940s, Professor V. V. Finn (1878–1957) of Kyiv University established that sperm cells function as true cells and, according to his terminology, can be classified as either "rich in cytoplasm" (polyplasmic) or "poor in cytoplasm" (oligoplasmic). Modern research reveals that the cytoplasm in sperm cells is distributed in a thin layer along the lateral walls and concentrates at the poles. The nuclear Chromatin exists in a condensed state, and a nucleolus is typically absent. Sperm cells contain all typical cellular organelles except for plastids. In terms of shape, sperm cells may be dimorphic (differing in morphological features) or isomorphic (having identical structures). According to S. G. Navashin’s concepts, both sperm cells are enantiomorphic (mirror-images of each other).

Pollen of angiosperms varies widely in size, shape, number of apertures, and exine structure (Fig. 176). Pollen dimensions range from 10 µm (genus Ficus) to 400 µm (families Cucurbitaceae and Malvaceae).

Fig. 176. Pollen structure in various angiosperm representatives: A—pedunculate oak (Quercus robur); B—Amur maple (Acer ginnala); C—fen bedstraw (Galium mollugo); D—ox-eye daisy (Leucanthemum vulgare); E—white campion (Melandrium album); F—least yellow water-lily (Nuphar pumilum); G—greater stitchwort (Stellaria holostea); H—fireweed (Chamaenerion angustifolium); I—silver birch (Betula pendula); K—black alder (Alnus glutinosa): 1—polar view; 2—equatorial view

Pollen may be spherical (families Caryophyllaceae, Chenopodiaceae), triangular (common hazel — Corylus avellana), or elliptical (family Fabaceae). In aquatic plants, pollen grains are elongated and thread-like.

The number of apertures in the pollen grain wall varies across a wide range. Among angiosperms, monoaperturate (families Liliaceae, Gramineae) and triaperturate (family Fabaceae) pollen types predominate. In Caryophyllaceae and Chenopodiaceae, pollen grains are panaperturate (or periaperturate), meaning apertures are distributed across the entire surface of the pollen grain (see Fig. 176). Aperture outlines may be rounded or elongated, allowing botanists to distinguish between porate pollen grains (with rounded apertures) and colpate pollen grains (with elongated apertures).

Pollen germination occurs through the apertures, which is associated with the formation of a pollen tube. Typically, the pollen tube emerges through a single aperture, though polysiphony can occasionally occur. However, even if multiple pollen tubes form, the Contents of the pollen grain migrate into only one of them (Fig. 177).

Fig. 177. The phenomenon of polysiphony: A—in the broad bean (Vicia faba); B—in the garden pea (Pisum sativum): 1—pollen tubes; 2—apertures in the exine

The STRUCTURE OF THE exine varies depending on the plant's pollination mechanism. In wind-pollinated plants, the exine is smooth, whereas in insect-pollinated plants, it is sculpted (see Fig. 174). Various outgrowths, spines, and furrows form on the exine surface of cross-pollinated plants, facilitating the attachment of pollen grains to insect or avian vectors.

Pollen is characterized by a fairly high degree of morphological species-Specificity, which finds Practical Application in geology, paleobotany (via spore-pollen analysis), the food industry (determining honey composition), and medicine (allergy Diagnostics).

In recent years, researchers have begun studying the pollen of various plants (such as maize, tomato, rapeseed, bluegrass, ragweed, petunia, and tobacco) to identify specific pollen genes and determine their putative functions. Studies have established that the mature pollen of various species contains between 20,000 and 24,000 genes, which accounts for approximately 60% of the genes found in a SHOOT.

Earlier studies demonstrated that pollen contains a variety of inorganic and organic substances. Among Inorganic Components, Na, K, Ca, P, Fe, Mg, Cu, Al, S, and Mn have been identified. Furthermore, pollen contains various sugars (sucrose, glucose, maltose, fructose), lipids, carotenoids, and about 20 Amino Acids. In terms of relative vitamin content, pollen surpasses all other plant parts. It is rich in Vitamin C, B-group Vitamins, E, PP, and others. The high physiological activity of pollen is evidenced by the presence of 32 enzymes, heteroauxin, and compounds containing SH-groups.

Its high vitamin content has allowed plant pollen to be utilized in certain cases as a vitamin Supplement. At the same time, the pollen of many plants (such as grasses and ragweed) triggers allergic reactions, resulting in dermatitis, lacrimation, rhinitis, breathing difficulties, and other symptoms requiring medical Treatment.