Flower – A Fascinating Organ Of Angiosperms

Flowers are the primary sites of sexual reproduction in flowering plants (angiosperms). They are not just beautiful for human enjoyment but are integral to the plant's reproductive cycle, showcasing a remarkable diversity in their structures and adaptations. These adaptations are crucial for the successful formation of fruits and seeds, which are the end products of sexual reproduction. Flowers also hold significant aesthetic, ornamental, social, religious, and cultural value for humans, often symbolizing various emotions.

Pre-Fertilisation: Structures And Events

Stamen, Microsporangium And Pollen Grain

The process of sexual reproduction begins long before the flower appears, with hormonal and structural changes leading to the development of floral buds. The stamen, the male reproductive organ, consists of a stalk-like filament and a terminal anther. The anther is typically bilobed, with each lobe containing two pollen sacs (thecae), making it dithecous. A transverse section of a young anther reveals four wall layers: the outermost epidermis, the endothecium, middle layers, and the innermost tapetum. The epidermis, endothecium, and middle layers provide protection and aid in the dehiscence of the anther, while the tapetum nourishes the developing pollen grains. Within each microsporangium, a group of compactly arranged cells called sporogenous tissue undergoes meiosis to form microspore tetrads. Each cell in the sporogenous tissue is a potential microspore or pollen mother cell (PMC).

The process of microspore formation from a PMC through meiosis is called microsporogenesis. As the anther matures and dehydrates, these microspores separate and develop into pollen grains. Each microsporangium can produce thousands of pollen grains.

A pollen grain is the male gametophyte. It is generally spherical and 25-50 micrometers in diameter, enclosed by a two-layered wall. The outer layer, the exine, is made of sporopollenin, a highly resistant organic substance that protects the pollen and aids in its fossilization. The exine has prominent germ pores where sporopollenin is absent. The inner wall, the intine, is thin and composed of cellulose and pectin. A mature pollen grain contains two cells: a larger vegetative cell with abundant food reserves and a prominent nucleus, and a smaller generative cell that floats in the vegetative cell's cytoplasm. In about 60% of angiosperms, pollen is shed at this two-celled stage. In the remaining species, the generative cell divides mitotically before shedding, forming two male gametes, resulting in a three-celled pollen grain.

Pollen grains are rich in nutrients and are sometimes used as food supplements. However, pollen from certain plants like Parthenium can cause severe allergies and respiratory problems.

Pollen viability, the period during which a pollen grain can germinate, varies significantly. In cereals like rice and wheat, it's as short as 30 minutes, while in some Rosaceae, Leguminoseae, and Solanaceae families, it can last for months. Pollen can be preserved for years in liquid nitrogen (-196°C) for use in crop breeding programs, similar to seed banks.

The Pistil, Megasporangium (Ovule) And Embryo Sac

The gynoecium, the female reproductive part of a flower, can consist of one or more pistils. A pistil typically comprises three parts: the stigma, style, and ovary. The stigma acts as the receptive surface for pollen grains. The style is a stalk connecting the stigma to the ovary, which is the swollen basal part containing the ovarian cavity. Inside the ovary, attached to the placenta, are the megasporangia, commonly known as ovules. The number of ovules can range from one (e.g., wheat, mango) to many (e.g., papaya, orchids).

A typical angiosperm ovule is a small structure attached to the placenta by a stalk called the funicle. The point of attachment is the hilum. The ovule is usually surrounded by one or two protective layers called integuments, which leave a small opening at the tip called the micropyle. Opposite the micropyle is the chalaza, the basal part of the ovule.

Within the integuments is a mass of cells called the nucellus, which contains stored food. Embedded within the nucellus is the embryo sac, or female gametophyte. Usually, a single embryo sac develops from a megaspore.

The process of megaspore formation from the megaspore mother cell (MMC) through meiosis is called megasporogenesis. The MMC, typically located in the micropylar region of the nucellus, undergoes meiosis to produce four megaspores. In most flowering plants, three of these megaspores degenerate, and one functional megaspore develops into the embryo sac. This type of development is known as monosporic development.

The nucleus of the functional megaspore divides mitotically, leading to a 2-nucleate, then 4-nucleate, and finally an 8-nucleate stage. These nuclear divisions are free nuclear (without cell wall formation). Subsequently, cell walls are formed, organizing the typical 7-celled, 8-nucleate embryo sac. This embryo sac has a characteristic structure: three cells at the micropylar end forming the egg apparatus (two synergids and one egg cell), three cells at the chalazal end called antipodals, and a large central cell containing two polar nuclei. The synergids possess a special thickening at the micropylar tip called the filiform apparatus, which guides the pollen tube.

Pollination

Since the male and female gametes in flowering plants are non-motile, pollination is essential to bring them together for fertilization. Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

There are three types of pollination:

1. Autogamy: Transfer of pollen grains within the same flower. This is common in cleistogamous flowers, which remain closed, ensuring self-pollination.
2. Geitonogamy: Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant. Genetically, it's similar to autogamy.
3. Xenogamy: Transfer of pollen grains from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species. This is the only type of pollination that brings genetically different pollen to the stigma.

Plants utilize two abiotic agents (wind and water) and one biotic agent (animals) for pollination. Wind pollination is common, and flowers adapted for it are typically light, non-sticky, have well-exposed stamens, and large, feathery stigmas. Water pollination is rare, observed mainly in some aquatic plants like Vallisneria and seagrasses, where pollen grains are adapted to water currents and are often protected by a mucilaginous covering. Wind and water-pollinated flowers are usually inconspicuous, lacking bright colors and nectar.

The majority of flowering plants rely on animals, especially insects (like bees), birds, and bats, for pollination. These flowers are often large, colorful, fragrant, and rich in nectar or pollen as rewards. Flies and beetles are attracted by foul odors. The pollinating animals come into contact with the anthers and stigma while collecting rewards, facilitating pollen transfer. In some cases, like the relationship between the Yucca plant and its pollinator moth, there's a strong mutual dependence, with the moth laying eggs in the ovary while pollinating the flower.

To prevent self-pollination and inbreeding depression, flowering plants have developed several outbreeding devices:

• Pre-synchronization of pollen release and stigma receptivity: Pollen may be released before or after the stigma becomes receptive.
• Spatial separation of anthers and stigma: Anthers and stigma are positioned differently to prevent self-pollination.
• Self-incompatibility: A genetic mechanism preventing self-pollen germination or pollen tube growth.
• Unisexual flowers: Producing either male or female flowers, which can prevent autogamy and geitonogamy if they occur on separate plants (dioecy).

Pollen-pistil interaction is a crucial process where the pistil recognizes compatible pollen. If the pollen is compatible, it germinates on the stigma, and the pollen tube grows through the style to reach the ovule. This interaction involves chemical communication between the pollen and pistil. The pollen tube carries the male gametes, which are released into a synergid within the ovule. Special techniques like emasculation (removal of anthers before dehiscence) and bagging are used in plant breeding to ensure controlled pollination and produce desired hybrids.

Double Fertilisation

After a compatible pollen grain penetrates a synergid, it releases two male gametes. One male gamete fuses with the egg cell, a process called syngamy, forming a diploid zygote. The other male gamete fuses with the two polar nuclei in the central cell, a process termed triple fusion, producing a triploid primary endosperm nucleus (PEN). The occurrence of both syngamy and triple fusion within the embryo sac is known as double fertilisation, a unique characteristic of angiosperms. The central cell develops into the primary endosperm cell (PEC), which later forms the endosperm, while the zygote develops into the embryo.

Post-Fertilisation : Structures And Events

Endosperm

Endosperm development usually precedes embryo development, as the endosperm provides nourishment to the developing embryo. The primary endosperm nucleus (PEN) undergoes successive nuclear divisions to form free nuclei, creating a stage called free-nuclear endosperm. Later, cell walls form, making the endosperm cellular. The coconut water from tender coconuts is a familiar example of free-nuclear endosperm, while the white kernel is the cellular endosperm. The endosperm can be completely consumed by the embryo before seed maturation (e.g., pea, groundnut) or persist in the mature seed (e.g., castor, wheat, maize) to be utilized during germination.

Embryo

The embryo develops from the zygote, typically located at the micropylar end of the embryo sac. Embryo development (embryogeny) shows similarities in both monocotyledons and dicotyledons. The zygote transforms into a proembryo, then a globular stage, followed by a heart-shaped stage, and finally a mature embryo.

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The part of the embryonal axis above the cotyledons is the epicotyl, which terminates in the plumule (stem tip). The cylindrical part below the cotyledons is the hypocotyl, which ends in the radicle (root tip), usually protected by a root cap.

Monocotyledonous embryos possess only one cotyledon. In grasses, this cotyledon is called a scutellum, situated laterally to the embryonal axis. The lower end of the embryonal axis has the radical and root cap, enclosed in a protective sheath called the coleorrhiza. The part above the scutellum is the epicotyl, which includes the shoot apex and leaf primordia, enclosed in a protective sheath called the coleoptile.

Seed

The seed, the final product of sexual reproduction in angiosperms, is essentially a fertilised ovule and is typically found within a fruit. A seed usually comprises seed coat(s), cotyledon(s), and an embryo axis. Cotyledons are often thick and store food.

Mature seeds can be:

• Non-albuminous (ex-albuminous): These seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).
• Albuminous: These seeds retain a portion of the endosperm (e.g., wheat, maize, castor).

In some seeds like black pepper and beet, remnants of the nucellus, called perisperm, may persist.

The hardened integuments of the ovule form the protective seed coat. The micropyle persists as a small pore in the seed coat, facilitating the entry of water and oxygen during germination. As seeds mature, their water content decreases, metabolic activity slows down, and they may enter a state of dormancy. Under favourable conditions (moisture, oxygen, suitable temperature), they germinate.

Simultaneously with ovule maturation into seeds, the ovary develops into a fruit. The ovary wall forms the fruit wall, known as the pericarp. Fruits can be fleshy (e.g., guava, mango) or dry (e.g., groundnut, mustard). Some fruits, like apple and strawberry, develop from floral parts other than the ovary, termed false fruits. Fruits developing solely from the ovary are called true fruits. Some fruits, like bananas, develop without fertilization, a phenomenon known as parthenocarpy, which can be induced using growth hormones and results in seedless fruits.

Seeds offer significant advantages, including independence from water for reproduction, better dispersal strategies, and nourishment for seedlings through stored food reserves. The hard seed coat protects the embryo. Sexual reproduction leads to new genetic combinations and variations.

Seed dormancy is crucial for storage and annual crop cycles. The viability of seeds varies greatly, from a few months to hundreds of years. For instance, seeds of Lupinus arcticus and Phoenix dactylifera have shown remarkable long-term viability.

Apomixis And Polyembryony

Apomixis is a special mechanism found in some flowering plants (e.g., Asteraceae, grasses) where seeds are produced without fertilization. It is essentially a form of asexual reproduction that mimics sexual reproduction. This can occur through the development of a diploid egg cell without reduction division or, more commonly, through the development of embryos from nucellar cells surrounding the embryo sac, as seen in citrus and mango. Apomictic embryos are genetically identical to the maternal plant and can be considered clones.

Polyembryony is the occurrence of more than one embryo in a seed. This often happens when nucellar cells also develop into embryos alongside the zygotic embryo. The cultivation of hybrids is important for increasing crop productivity, but hybrid seeds need to be produced annually. Apomicts offer a solution by maintaining hybrid characteristics across generations, reducing the need for continuous hybrid seed production and making them valuable for agriculture and horticulture.

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