Chapter 2: Sexual Reproduction in Flowering Plants
Introduction
Sexual reproduction in flowering plants involves the production of male and female gametes, transfer of the male gamete to the female ovule in a process called pollination, and fertilization leading to the formation of seeds and fruit. This chapter delves into the intricate details of these processes.
Flower: A Fascinating Organ of Angiosperms
Structure of a Flower
The flower is the reproductive structure of angiosperms. It typically consists of four main whorls:- Calyx: The outermost whorl, consisting of sepals, usually green and leaf-like, which protect the flower bud.
- Corolla: Composed of petals, usually brightly colored to attract pollinators.
- Androecium: The male reproductive part, consisting of stamens. Each stamen has a filament and an anther, where pollen grains are produced.
- Gynoecium: The female reproductive part, composed of one or more carpels. Each carpel consists of an ovary, style, and stigma. The ovary contains ovules which develop into seeds after fertilization.
Function of a Flower
The primary function of a flower is reproduction. Flowers facilitate the union of male and female gametes through the process of pollination, followed by fertilization, seed formation, and fruit development.
Pre-Fertilization: Structures and Events
Development of Male Gametophyte
The male gametophyte develops within the pollen sacs of the anther. Each microspore mother cell undergoes meiosis to produce four haploid microspores, which develop into pollen grains. Each pollen grain contains two cells - the generative cell and the tube cell.
Development of Female Gametophyte
The female gametophyte, or embryo sac, develops within the ovule. A megaspore mother cell undergoes meiosis to produce four haploid megaspores, three of which degenerate, leaving one to develop into the embryo sac. The embryo sac typically contains seven cells and eight nuclei, including the egg cell and central cell with two polar nuclei.
Pollination
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. It can be of different types:- Self-Pollination: Transfer of pollen within the same flower or to another flower on the same plant.
- Cross-Pollination: Transfer of pollen from one flower to another flower on a different plant of the same species. It can be facilitated by various agents like wind, water, insects, birds, and animals.
Kinds of Pollination
Introduction
Pollination is the process of transferring pollen grains from the male anther of a flower to the female stigma. This crucial step in the reproductive cycle of flowering plants can occur through various mechanisms. Pollination can be broadly categorized into two main types: Self-Pollination and Cross-Pollination.
Self-Pollination
Self-pollination occurs when pollen grains from the anther are transferred to the stigma of the same flower or another flower on the same plant. It can be further divided into:
- Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower. This type of self-pollination ensures that the plant can reproduce in the absence of pollinators. Examples include peas and tomatoes.
- Geitonogamy: Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant. Though it involves two different flowers, it is still considered self-pollination as the genetic material comes from the same plant. Examples include maize and cucurbits.
Cross-Pollination
Cross-pollination, or allogamy, occurs when pollen grains are transferred from the anther of one flower to the stigma of a flower on a different plant of the same species. This type of pollination promotes genetic diversity and can be facilitated by various agents. Cross-pollination can be further classified based on the pollinating agents:
1. Abiotic Agents
- Anemophily (Wind Pollination): Pollen grains are carried by the wind to the stigma of another flower. Plants adapted to wind pollination produce large quantities of light, dry pollen grains. Examples include grasses, wheat, and corn.
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Hydrophily (Water Pollination): Pollen grains are transferred through water. This type of pollination is common in aquatic plants. There are two types:
- Surface Hydrophily: Pollen grains float on the water surface and reach the stigma. Example: Vallisneria.
- Submerged Hydrophily: Pollen grains are carried under the water. Example: Zostera.
2. Biotic Agents
- Entomophily (Insect Pollination): Insects such as bees, butterflies, and beetles transfer pollen grains from one flower to another. Flowers adapted to insect pollination are usually brightly colored and have nectar to attract insects. Examples include sunflowers, roses, and orchids.
- Ornithophily (Bird Pollination): Birds, especially hummingbirds and sunbirds, transfer pollen grains. Flowers adapted to bird pollination are often brightly colored, tubular, and produce abundant nectar. Examples include hibiscus and fuchsia.
- Chiropterophily (Bat Pollination): Bats transfer pollen grains while feeding on the nectar of flowers. Flowers adapted to bat pollination are usually large, nocturnal, and have a strong fragrance. Examples include baobab and agave.
- Myrmecophily (Ant Pollination): Ants transfer pollen grains while moving from one flower to another. Examples include certain species of orchids.
- Malacophily (Snail Pollination): Snails and slugs transfer pollen grains. This is rare and occurs in some tropical plants.
Significance of Cross-Pollination
Cross-pollination has several advantages:
- Promotes genetic diversity, which enhances the adaptability and survival of species.
- Produces more vigorous and healthy offspring compared to self-pollination.
- Reduces the chances of genetic defects due to inbreeding.
Double Fertilization
Double fertilization is a unique feature of angiosperms where two fertilization events occur. One sperm cell fuses with the egg cell to form the zygote (which develops into the embryo), and the other sperm cell fuses with the two polar nuclei to form the triploid primary endosperm nucleus (which develops into the endosperm, providing nourishment to the developing embryo).
Post-Fertilization: Structures and Events
Endosperm
The endosperm provides nourishment to the developing embryo. It can be of different types:- Nuclear Endosperm: The most common type, where free-nuclear divisions occur without cell wall formation initially.
- Cellular Endosperm: The endosperm cells form immediately after fertilization.
- Helobial Endosperm: An intermediate type between nuclear and cellular endosperm.
Embryo
The embryo develops from the zygote and consists of the following parts:- Radicle: The embryonic root.
- Plumule: The embryonic shoot.
- Cotyledons: The seed leaves that provide nourishment to the developing seedling.
Seed
A seed is a mature ovule that contains the embryo and stored food, enclosed within a protective seed coat. Seeds are classified based on the number of cotyledons:- Monocotyledons: Seeds with one cotyledon (e.g., wheat, maize).
- Dicotyledons: Seeds with two cotyledons (e.g., pea, bean).
Fruit
A fruit develops from the ovary after fertilization. It encloses and protects the seeds. Fruits can be classified into:- Simple Fruits: Develop from a single ovary of a single flower.
- Aggregate Fruits: Develop from multiple ovaries of a single flower.
- Multiple Fruits: Develop from the ovaries of multiple flowers growing in a cluster.
Apomixis and Polyembryony
Apomixis
Apomixis is a form of asexual reproduction that mimics sexual reproduction but does not involve fertilization. It leads to the formation of seeds without fertilization, resulting in offspring that are genetically identical to the parent plant.
Polyembryony
Polyembryony is the phenomenon where multiple embryos develop within a single seed. This can occur due to the fertilization of multiple egg cells or through the development of additional embryos from other cells of the ovule.
Practice Questions and Answers
Microsporogenesis: The process of formation of microspores from microspore mother cells (MMC) through meiosis. It occurs in the anther. The microspores develop into pollen grains.
Megasporogenesis: The process of formation of megaspores from megaspore mother cells (MMC) through meiosis. It occurs in the ovule. One of the megaspores develops into the female gametophyte (embryo sac).
The type of cell division that occurs during both these events is meiosis. The structures formed at the end of microsporogenesis are microspores, and at the end of megasporogenesis, megaspores are formed.
The correct developmental sequence is:
- Sporogenous tissue
- Pollen mother cell
- Microspore tetrad
- Pollen grain
- Male gametes
A typical angiosperm ovule consists of the following parts:
- Funicle: The stalk that attaches the ovule to the placenta.
- Hilum: The region where the funicle is attached to the ovule body.
- Integuments: Protective layers surrounding the ovule, leaving an opening called the micropyle.
- Micropyle: The opening through which the pollen tube enters the ovule.
- Nucellus: The mass of parenchymatous cells inside the integuments, providing nourishment to the developing embryo sac.
- Embryo Sac: The female gametophyte present within the nucellus, containing the egg apparatus, central cell, and antipodal cells.
The mature female gametophyte (embryo sac) is typically 7-celled and 8-nucleate, and consists of:
- One egg cell: Located at the micropylar end, forming part of the egg apparatus.
- Two synergids: Also located at the micropylar end, flanking the egg cell, and forming part of the egg apparatus.
- Three antipodal cells: Located at the chalazal end.
- One central cell: Located in the center with two polar nuclei.
Chasmogamous flowers: These are flowers that open at maturity, exposing their reproductive parts to facilitate cross-pollination. They have well-developed petals and are often fragrant to attract pollinators.
Cross-pollination in cleistogamous flowers: Cleistogamous flowers do not open and remain closed. As a result, cross-pollination cannot occur in these flowers because the pollen is transferred directly from the anther to the stigma within the same flower, ensuring self-pollination. This guarantees seed production even in the absence of pollinators.
Two strategies evolved to prevent self-pollination in flowers are:
- Dichogamy: Temporal separation of male and female reproductive organ maturation. In protandry, anthers mature before stigmas, while in protogyny, stigmas mature before anthers.
- Self-incompatibility: A genetic mechanism in which pollen from the same flower or plant is unable to fertilize the ovules, promoting cross-pollination.
Self-incompatibility is a genetic mechanism that prevents self-pollination and promotes cross-pollination by inhibiting the growth of pollen tubes from the same plant. In self-incompatible species, self-pollination does not lead to seed formation because the pollen is either rejected or fails to germinate on the stigma, or the pollen tube growth is arrested before reaching the ovule.
The bagging technique involves covering the flowers or inflorescences with a bag (usually made of paper or plastic) to prevent unwanted pollen from contaminating the flowers. This technique is useful in a plant breeding program to ensure controlled pollination, prevent cross-pollination, and maintain the purity of the desired cross by allowing only selected pollen to fertilize the flowers.
Triple fusion is the fusion of the sperm cell with the two polar nuclei in the central cell of the embryo sac. This process occurs in the ovule. During fertilization, one of the two sperm cells released from the pollen tube fuses with the egg cell to form the zygote (syngamy), while the other sperm cell fuses with the two polar nuclei to form a triploid primary endosperm nucleus. The nuclei involved in triple fusion are one male gamete (sperm cell) and two polar nuclei.
(a) hypocotyl and epicotyl;
(b) coleoptile and coleorhiza;
(c) integument and testa;
(d) perisperm and pericarp.
(a) Hypocotyl and Epicotyl:
Hypocotyl: The part of the embryonic axis below the cotyledons that develops into the root system.
Epicotyl: The part of the embryonic axis above the cotyledons that develops into the shoot system.
(b) Coleoptile and Coleorhiza:
Coleoptile: A protective sheath covering the young shoot in monocots.
Coleorhiza: A protective sheath covering the young root in monocots.
(c) Integument and Testa:
Integument: The outer protective layer of the ovule.
Testa: The seed coat formed from the integuments after fertilization.
(d) Perisperm and Pericarp:
Perisperm: The persistent nucellus in some seeds, providing nourishment to the embryo.
Pericarp: The fruit wall developed from the ovary wall after fertilization.
An apple is called a false fruit because it develops from the ovary as well as other parts of the flower, such as the thalamus (receptacle). The thalamus swells and becomes fleshy, forming the main edible part of the fruit, while the true fruit (derived from the ovary) is the core that contains the seeds.
Emasculation is the removal of anthers from a flower to prevent self-pollination. A plant breeder employs this technique when performing controlled cross-pollination to ensure that the flower is pollinated only by the desired pollen from another plant. Emasculation is done before the anthers mature and release pollen.
Parthenocarpy can be induced in fruits where seedlessness is a desirable trait, such as bananas, grapes, oranges, and cucumbers. These fruits are selected because seedless varieties are preferred for consumption and have higher commercial value. Additionally, inducing parthenocarpy ensures consistent fruit production even in the absence of pollination.
The tapetum is the innermost layer of the anther wall, providing nourishment to the developing pollen grains. It plays a crucial role in the formation of the pollen-grain wall by secreting enzymes, hormones, and nutrients. The tapetum also contributes sporopollenin, which forms the tough outer layer (exine) of the pollen grain, protecting it from physical and chemical damage.
Apomixis is a form of asexual reproduction that mimics sexual reproduction but does not involve fertilization. In apomixis, seeds are formed without the fusion of gametes, resulting in offspring that are genetically identical to the parent plant. The importance of apomixis lies in its potential for plant breeding, as it allows the propagation of hybrid plants with desirable traits without genetic segregation, ensuring uniformity and stability in crop production.