PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 8. POLLINATION AND FERTILIZATION IN ANGIOSPERMS. EMBRYO AND ENDOSPERM DEVELOPMENT

8.1. Pollination in Angiosperms

Despite the fact that The formation of the female gametophyte begins considerably later than that of the male, both gametophytes mature almost simultaneously. By the time the male gametophyte is mature, the female gametophyte is fully prepared for Fertilization.

In most angiosperms, The process of fertilization is preceded by pollination—The transfer of pollen from the stamens to the stigma of the pistil. Over the course of evolution, two MAIN TYPES OF pollination have emerged: autogamy (self-pollination) and allogamy, or xenogamy (cross-pollination). In self-pollination, pollen is transferred from the stamens to the pistil within the same flower. In cross-pollination, pollen from one flower is transferred to the stigma of a pistil in another flower. Self-pollination can occur in both open (chasmogamous) flowers (such as pea — Pisum) and closed (cleistogamous) flowers (such as violet — Viola, wood sorrel — Oxalis) (Fig. 188). In some species of violets and common wood sorrel, chasmogamous flowers develop early in spring; however, due to the scarcity of pollinators at that time, these flowers generally fail to pollinate and remain barren. Later in the season, alongside the chasmogamous flowers, inconspicuous cleistogamous flowers develop at the Base of the shoots, where Pollination and Fertilization take place, subsequently producing fruits and seeds.

Class="center">Fig. 188. Chasmogamous (1) and cleistogamous (2) flowers of hairy violet (Viola hirta)

A special type of pollination is geitonogamy, which involves the transfer of pollen from one flower to another located on the same plant. Some scientists view this type of pollination as an extended form of self-pollination, whereas others consider it a restricted form of cross-pollination.

Cross-pollination can be either abiotic or biotic. The predominant type of abiotic pollination is wind pollination (anemophily). Plants have evolved a range of adaptations for anemophily, including: the flowering of woody plants prior to leaf emergence (alder, hazel); long, wind-swayed catkin inflorescences (alder, hazel) or long stamens with large anthers (grasses); inconspicuous, unisexual flowers lacking a perianth (sedges) or possessing a heavily reduced perianth (rushes); and abundant, lightweight pollen with a smooth exine (hazel, birch), among others.

An ancient type of abiotic pollination is hydrophily, or pollination via Water. Typical hydrophily is characteristic of aquatic plants (such as water starwort — Callitriche, naiad — Najas, hornwort — Ceratophyllum), which exhibit specific pollen adaptations. In certain hydrophilic plants, the pollen grains are extremely elongated (reaching 2,500 µm in Zostera), which is of great adaptive significance because such pollen is highly mobile and capable of rapid dispersal in water. Crucially, the pollen of all hydrophilic plants does not burst upon contact with water (unlike the pollen of terrestrial plants) and remains viable. Compared to anemophily, hydrophily is a relatively rare type of abiotic cross-pollination.

Biotic pollination can be carried out by various insects (entomophily)—such as bees, bumblebees, butterflies, moths, wasps, beetles, hymenopterans, and ants—as well as by vertebrate animals, including birds, bats, and small primates. Through long-term evolution, plants have developed specific adaptations to attract invertebrate and vertebrate pollinators. These adaptations include flowers with prominent perianths, specific corolla coloration, distinct floral scents, characteristic floral shapes (often mimicking the body form of female pollinating insects), abundant pollen, and the presence of nectar. The visits of pollinators to various types of flowers are driven by their nutritional requirements during different stages of their life cycle.

Among many researchers, the misconception that self-pollination represents an evolutionary dead end is unfortunately widespread. This view was shaped largely by the ideas of Charles Darwin, who demonstrated the adaptations of angiosperm flowers for cross-pollination and described the mechanisms that prevent self-pollination.

Cross-pollinating plants indeed possess specialized mechanisms to prevent self-pollination, most notably dichogamy and heterostyly. Dichogamy involves the asynchronous maturation of stamens and pistils, manifesting either as protandry (earlier maturation of the stamens) (Fig. 189) or protogyny (earlier maturation of the pistils) (Fig. 190).

Fig. 189. Protandry in yellow mountain saxifrage (Saxifraga aizoides): A—flower with well-developed stamens and underdeveloped stylodia; B—flower with well-developed stylodia and withered stamen anthers

Fig. 190. Protogyny in figwort (Scrophularia nodosa) (Cytology/practical/54.html">Longitudinal section of the flower): A—flower with a well-developed style and underdeveloped stamens; B—flower with well-developed stamens and a withering style

Protandry occurs more frequently than protogyny and is characteristic of members of the Apiaceae, Campanulaceae, Onagraceae, Geraniaceae, and several other families. Protogyny is observed in many Brassicaceae, Plantaginaceae, and certain Asteraceae (such as cocklebur — Xanthium). Under dichogamy, the time gap between stamen and pistil maturation ranges from several hours to several days depending on the species.

Heterostyly is a specific type of adaptation found in Representatives of the Primulaceae, Polygonaceae, Caryophyllaceae, and Boraginaceae families. In heterostylous plants, different flowers of the same species produce stamens and pistil styles of varying lengths. Flowers with long stamens feature short styles (or short stylodia), whereas flowers with short stamens feature long styles (or long stylodia). Fertilization occurs successfully only when pollen from long-stamened flowers is deposited on the stigma of a long-styled pistil, or when pollen from short-stamened flowers reaches the stigma of a short-styled pistil. In other combinations, the pollen fails to germinate due to self-incompatibility between the pollen and the stigma of the same flower type (Fig. 191).

Fig. 191. Heterostyly in buckwheat (Fagopyrum sagittatum): A—flower with long stylodia and short stamens; B—flower with short stylodia and long stamens: 1—stylodia; 2—stamens; 3—nectaries

It is now firmly established that angiosperms have evolved two modes of pollination: cross-pollination and self-pollination. Despite their determination, these pollination modes are highly labile and dynamic, capable of shifting in response to climatic conditions during the flowering period; thus, self-pollinators may occasionally undergo cross-pollination, and cross-pollinating plants may exhibit self-pollination. This behavioral plasticity carries immense evolutionary significance. Free cross-pollination within large populations makes it extremely difficult to fix novel breeding combinations, as any successful genetic combination tends to break down and cannot be permanently retained in the progeny. Conversely, self-pollination in cross-pollinators facilitates the isolation of new forms and their fixation across subsequent generations, thereby driving speciation. Meanwhile, occasional cross-pollination in self-pollinating species enhances the heterozygosity of the offspring and, consequently, the viability of the species.

8.1.1. Pollen Germination on the Pistil Stigma

Once pollen arrives on the pistil stigma through various vectors, it adheres and initiates germination. Pollen germination on the stigma is preceded by a series of physiological, biochemical, and cytological processes: the secretion of lipid droplets on the pollen grain surface, the uptake of water from the pistil Tissues into the pollen, the activation of Enzymes, and the redistribution of all cytoplasmic components within the pollen grain. The initial sign of germination—the Formation of the pollen tube—is cyclosis, a rotational streaming movement of the pollen grain Cytoplasm. As germination proceeds, the intine bulges outward through one of the apertures to form the pollen tube, into which the entire Contents of the pollen grain gradually migrate: the cytoplasm of the vegetative Cell containing its Nucleus, all Organelles, and the generative cell (in bicellular pollen) or the two sperm Cells (in tricellular pollen) (Fig. 192).

Fig. 192. Germinating pollen grain (pollen) of broad bean (Vicia faba): 1—pollen grain; 2—pollen tube; 3—vegetative Cell Nucleus; 4—generative cell

The pollen tube penetrates the stigma, then enters the style, and begins to grow through it toward the Ovary. The style varies in Structure and may be open, closed, or semi-open. Depending on this structure, the pollen tube advances through the style in different ways. In an open style, it glides along the canal wall. In closed and semi-open styles, the pollen tube makes its way by breaking down the glandular cells lining the style, and then passes onto the ovary wall. Further on, the pollen tube moves along the ovary wall until it approaches the Location OF THE ovule.

The pollen tube grows along a concentration gradient of growth substances and Calcium Ions. However, it is quite likely that other chemotropic factors also facilitate the movement of the pollen tube toward the ovule. Since the pollen tube remains in close contact with maternal tissues as it advances through the style and ovary walls, it is evident that some specific physiological interaction is established between them, which remains insufficiently studied.

In dicotyledonous plants characterized by bicellular pollen, the generative cell divides mitotically within the pollen tube to form two sperm cells (Fig. 193). If the pollen tube is wide, mitosis proceeds typically, with all stages clearly visible during the mitotic division of the generative cell nucleus: prophase, metaphase, anaphase, and telophase. In narrow pollen tubes, a typical metaphase plate is not formed. Mitotic nuclear division is followed by cytokinesis. Thus, the sperm cells are haploid cells possessing cell walls In addition to the Plasmalemma. The sperm cytoplasm occupies a peripheral position and is also concentrated at the poles. The Nucleus lacks a nucleolus, and its Chromatin is in a condensed state. Typically, the sperm cytoplasm contains a set of organelles characteristic of plant cells: Mitochondria, dictyosomes, granular Endoplasmic reticulum, Ribosomes, and microtubules. Small vacuoles and Plastids are also present. In some cereals and Asteraceae, sperm dimorphism has been observed, manifesting not only in differences in shape and size, but also in organelle content.

Fig. 193. Sperm cell in the pollen tube of broad bean (Vicia faba)

The question of how the generative cell and sperm cells move within the pollen tube remains problematic, and various authors' views on their mechanisms of movement are contradictory. Some researchers believe that the generative cell and sperm cells move actively, regardless of where they were formed, whereas others indicate that they are transported passively, carried along by cytoplasmic streaming. The function of the vegetative nucleus is also unclear. Based on the fact that it is always located at the growing tip of the pollen tube, some authors suggest that the vegetative nucleus regulates the growth of the pollen tube, which exhibits specific polarization. As the pollen tube elongates, the pollen grain and the base of the pollen tube become depleted, and the living contents are separated from the empty space by a callose plug. In this way, the integrity of the male gametophyte is maintained. The pollen tube is characterized exclusively by apical growth, with the growing zone being separated from the rest of the tube by successive callose plugs.