Flower – A Fascinating Organ Of Angiosperms

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Introduction To The Flower

Flowers are the reproductive structures of angiosperms (flowering plants). They are not only important for plant reproduction but also hold significant aesthetic, cultural, social, and religious value for humans, often used to express emotions.

From a biological perspective, flowers are sites of sexual reproduction in angiosperms, exhibiting remarkable diversity in morphology and structure to ensure successful formation of seeds and fruits, the end products of sexual reproduction.

The primary reproductive units in a flower are the androecium (male) and the gynoecium (female).

Pre-Fertilisation: Structures And Events

Before the flower fully blooms, hormonal and structural changes within the plant initiate the development of the floral primordium. This leads to the formation of inflorescences (clusters of flowers) and individual flower buds.

Within the developing flower bud, the male and female reproductive whorls, the androecium and gynoecium, differentiate and mature.

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Stamen, Microsporangium And Pollen Grain

The stamen is the male reproductive organ of a flower, collectively forming the androecium. A typical stamen consists of two parts:

• Filament: A long, slender stalk, usually attached to the thalamus or a petal at its base.
• Anther: A terminal structure where pollen is produced. Anthers are generally bilobed, meaning they have two lobes.

A transverse section of a typical angiosperm anther reveals its bilobed nature, with each lobe containing two chambers called theca. Thus, the anther is often dithecous. A groove usually separates the two theca lengthwise.

Inside the anther, at the corners of the tetragonal structure, are four microsporangia. These microsporangia develop into pollen sacs which extend along the length of the anther and are filled with pollen grains.

Structure of Microsporangium

Each microsporangium is roughly circular in outline and is surrounded by four wall layers:

1. Epidermis: The outermost protective layer.
2. Endothecium: Located below the epidermis, it helps in the dehiscence (opening) of the anther.
3. Middle Layers: One to three layers situated below the endothecium.
4. Tapetum: The innermost layer, which is crucial for nourishing the developing pollen grains. Tapetal cells are characterised by dense cytoplasm and often contain more than one nucleus (multinucleate).

The outer three layers (epidermis, endothecium, middle layers) provide protection and aid in the release of pollen by the anther's dehiscence.

In a young anther, the central region of each microsporangium is occupied by a mass of compactly arranged, homogenous cells called the sporogenous tissue.

Microsporogenesis

This is the process of forming microspores from the sporogenous tissue within the microsporangium.

• As the anther matures, the cells of the sporogenous tissue function as Pollen Mother Cells (PMCs) or Microspore Mother Cells.
• Each PMC (which is diploid, 2n) undergoes meiotic division.
• Meiosis in a PMC produces a cluster of four haploid cells called a microspore tetrad (n).
• Each cell within the sporogenous tissue has the potential to become a PMC and produce a microspore tetrad.

As the anthers dry out, the microspores in a tetrad separate from each other and develop into individual pollen grains.

Thousands of microspores/pollen grains are formed inside each microsporangium and are released when the anther dehisces.

Pollen Grain

The pollen grain represents the male gametophyte in angiosperms. Pollen grains from different species show considerable variation in size, shape, color, and surface design.

A typical pollen grain is spherical and has a diameter of approximately $25-50$ micrometers. It is surrounded by a two-layered wall:

• Exine: The tough outer layer, made of sporopollenin. Sporopollenin is highly resistant to high temperatures, strong acids, alkalis, and enzymatic degradation, making pollen grains well-preserved as fossils. The exine is patterned and has openings called germ pores where sporopollenin is absent.
• Intine: The inner layer, which is thin, continuous, and composed of cellulose and pectin.

Inside the intine, the pollen grain cytoplasm is enclosed by a plasma membrane.

At maturity, the pollen grain typically contains two cells:

• Vegetative cell: Larger cell with abundant food reserves and a large, irregularly shaped nucleus.
• Generative cell: Smaller cell, spindle-shaped with dense cytoplasm and a nucleus, floats in the cytoplasm of the vegetative cell.

In over 60% of angiosperms, pollen is shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically before shedding to form two male gametes, resulting in a 3-celled stage (vegetative cell + two male gametes).

Pollen grains of some plants, like Parthenium (carrot grass), can cause severe allergies and respiratory problems such as asthma and bronchitis in some individuals.

Pollen grains are rich in nutrients and are increasingly used as food supplements in the form of tablets and syrups, sometimes marketed to enhance performance in athletes.

The viability period of pollen grains (how long they remain functional) varies greatly depending on the species and environmental conditions (temperature, humidity). It can range from about 30 minutes (in cereals like rice and wheat) to several months (in plants like those in Rosaceae, Leguminosae, and Solanaceae).

Pollen grains can be stored for long periods (years) in liquid nitrogen at $-196^\circ \textsf{C}$. These "pollen banks" are useful in plant breeding programs, similar to seed banks.

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The Pistil, Megasporangium (Ovule) And Embryo Sac

The pistil (or carpel) is the female reproductive part of a flower, forming the gynoecium. A gynoecium can have one pistil (monocarpellary) or more than one (multicarpellary).

If there are multiple pistils, they can be fused together (syncarpous) or remain free (apocarpous).

Each pistil typically has three parts:

• Stigma: The receptive tip that serves as the landing platform for pollen grains.
• Style: An elongated, slender structure connecting the stigma to the ovary.
• Ovary: The basal, swollen part containing the ovarian cavity or locule.

Inside the ovarian cavity is the placenta, from which the megasporangia, commonly called ovules, arise. The number of ovules per ovary can vary from one (e.g., wheat, paddy, mango) to many (e.g., papaya, watermelon, orchids).

The Megasporangium (Ovule)

The ovule is a small structure attached to the placenta by a stalk called the funicle. The point where the funicle fuses with the ovule body is called the hilum.

The ovule is protected by one or two layers called integuments, which enclose the central tissue (nucellus) except for a small opening at one end called the micropyle. The opposite end of the micropyle is the chalaza, representing the basal part of the ovule.

Inside the integuments is a mass of parenchymatous cells called the nucellus, which contains abundant reserve food material.

Located within the nucellus is the embryo sac or female gametophyte. A typical ovule usually contains a single embryo sac, formed from a megaspore.

Megasporogenesis

This is the process of forming megaspores from the Megaspore Mother Cell (MMC) within the ovule.

• In the micropylar region of the nucellus, a single cell differentiates as the Megaspore Mother Cell (MMC). The MMC is a large diploid cell (2n) with dense cytoplasm and a prominent nucleus.
• The MMC undergoes meiotic division. This is essential because the gametes (derived from the megaspore) must be haploid.
• Meiosis of the MMC produces four megaspores (n).

Female Gametophyte (Embryo Sac)

In most flowering plants, only one of the four megaspores produced by meiosis is functional, while the other three degenerate. The functional megaspore develops into the embryo sac.

This method of embryo sac formation from a single megaspore is called monosporic development.

The ploidy levels are as follows:

• Nucellus cells: Diploid (2n)
• Megaspore Mother Cell (MMC): Diploid (2n)
• Functional megaspore: Haploid (n)
• Cells of the female gametophyte (embryo sac): Haploid (n)

Development of the embryo sac from the functional megaspore involves mitotic divisions of the megaspore nucleus:

• The nucleus of the functional megaspore undergoes mitosis to form two nuclei, which move to opposite poles, forming a 2-nucleate embryo sac.
• Two more sequential mitotic divisions occur, resulting in 4-nucleate and then 8-nucleate stages. These divisions are often "free nuclear," meaning cell wall formation does not immediately follow nuclear division.

After the 8-nucleate stage, cell walls are formed, organising the nuclei into the typical 7-celled, 8-nucleate embryo sac (female gametophyte).

The arrangement of cells within the mature embryo sac is characteristic:

• At the micropylar end: The egg apparatus, consisting of one egg cell and two synergids. The synergids have special cellular thickenings called the filiform apparatus at the micropylar tip, which guides the pollen tube entry.
• At the chalazal end: Three antipodal cells.
• In the center: A large central cell containing two polar nuclei.

Thus, a mature embryo sac contains 8 nuclei but is organised into 7 cells (1 egg cell, 2 synergids, 3 antipodals, and 1 large central cell).

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Pollination

Since both the male gametes (within pollen grains) and the female gamete (egg cell within the embryo sac) are non-motile in flowering plants, a mechanism is needed to bring them together for fertilisation. This mechanism is called pollination.

Pollination is defined as the transfer of pollen grains from the anther to the stigma of a pistil.

Flowering plants have developed various adaptations and often rely on external agents for pollination.

Kinds of Pollination

Based on the source of pollen, pollination is classified into three types:

1. Autogamy (Self-pollination): Transfer of pollen from the anther to the stigma of the same flower. Complete autogamy is rare in flowers that open and expose their reproductive parts. It requires synchrony in pollen release and stigma receptivity, and the anther and stigma must be positioned close together.
2. Geitonogamy: Transfer of pollen from the anther of one flower to the stigma of another flower on the same plant. Although it involves a pollinating agent like cross-pollination, it is genetically similar to autogamy because the pollen comes from the same genetic individual.
3. Xenogamy (Cross-pollination): Transfer of pollen 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 introduces genetically different pollen to the stigma, leading to genetic variation in offspring.

Some plants, like Viola (pansy), Oxalis, and Commelina, produce two types of flowers:

• Chasmogamous flowers: These flowers open normally, exposing the anthers and stigma.
• Cleistogamous flowers: These flowers do not open at all. The anthers and stigma lie close together. When anthers dehisce within the closed flower, pollen directly contacts the stigma, ensuring pollination. Cleistogamous flowers are invariably autogamous and guarantee seed set even without pollinators.

Agents of Pollination

Plants use abiotic (non-living) or biotic (living) agents for pollination. The majority of plants use biotic agents.

Abiotic Agents:

• Wind (Anemophily): This is the most common abiotic agent. Wind-pollinated flowers often have:
  • Light, non-sticky pollen grains for easy transport by wind.
  • Well-exposed stamens to release pollen into the air currents.
  • Large, feathery stigmas to efficiently trap airborne pollen.
  • Often single ovule per ovary and numerous flowers clustered in inflorescences (e.g., Corn cob, Grasses). Wind pollination is a chance event, so large amounts of pollen are produced.
• Water (Hydrophily): Relatively rare in flowering plants, limited to about 30 genera (mostly monocots). More common for gamete transport in lower plants (algae, bryophytes, pteridophytes). Examples include Vallisneria and Hydrilla (freshwater) and Zostera (seagrasses).
  • In Vallisneria, female flowers reach the water surface, and male flowers/pollen float passively to them.
  • In seagrasses, female flowers are submerged, and pollen (often long, ribbon-like) is released and carried underwater.
  • Pollen in water-pollinated species is often protected from wetting by a mucilaginous covering.

Wind and water-pollinated flowers generally lack bright colors, fragrance, and nectar, as these are adaptations to attract animals.

Biotic Agents:

• A wide variety of animals are used, including insects (bees, butterflies, flies, beetles, wasps, ants, moths), birds (sunbirds, hummingbirds), bats, and sometimes larger animals like primates, rodents, and reptiles.
• Insects, especially bees, are the dominant biotic pollinators.
• Animal-pollinated flowers are often specifically adapted to their particular pollinator species.
• These flowers are typically large, colorful, fragrant, and produce rewards like nectar and pollen to attract visitors. If flowers are small, they are grouped into conspicuous inflorescences.
• Flowers pollinated by flies and beetles may emit foul odors to attract these animals.
• Floral rewards ensure repeated visits from animals. When animals collect rewards, their bodies come into contact with the anthers and stigma, picking up and transferring pollen (which is often sticky).
• Some plants offer safe places for laying eggs as a reward, like the large flower of Amorphophallus.
• There are also mutualistic relationships, like between the moth and Yucca plant, where both rely on each other for completing their life cycles. The moth pollinates the Yucca flower and lays eggs in its ovary; the moth larvae feed on developing seeds.

Animals that visit flowers but consume pollen or nectar without facilitating pollination are called pollen/nectar robbers.

Outbreeding Devices

To avoid the negative effects of continuous self-pollination (inbreeding depression), flowering plants have evolved mechanisms to discourage autogamy and promote cross-pollination. These include:

• Lack of synchrony: Pollen release and stigma receptivity occur at different times (either pollen is released before stigma is receptive or vice versa).
• Anther and stigma position: The spatial arrangement of anthers and stigma is such that pollen cannot easily reach the stigma of the same flower.
• Self-incompatibility: A genetic mechanism that prevents self-pollen (from the same flower or same plant) from germinating on the stigma or inhibiting pollen tube growth in the pistil, thereby preventing self-fertilisation.
• Production of unisexual flowers:
  • If both male and female flowers are on the same plant (monoecious, e.g., Castor, Maize), it prevents autogamy but not geitonogamy.
  • If male and female flowers are on different plants (dioecious, e.g., Papaya, Date palm), it prevents both autogamy and geitonogamy, ensuring only xenogamy occurs.

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Pollen-Pistil Interaction

Pollination is not just pollen landing on the stigma; it is a dynamic process involving recognition of pollen by the pistil, followed by acceptance or rejection.

The pistil can distinguish between compatible pollen (of the same species) and incompatible pollen (of a different species or self-pollen in self-incompatible plants).

This recognition is mediated by a chemical dialogue between components of the pollen and the pistil.

If the pollen is compatible:

• The pistil accepts the pollen and promotes post-pollination events.
• The pollen grain germinates on the stigma, forming a pollen tube through one of the germ pores.
• The contents of the pollen grain move into the pollen tube.
• The pollen tube grows through the tissues of the stigma and style towards the ovary.
• If the pollen was shed at the 2-celled stage (vegetative + generative), the generative cell divides mitotically during pollen tube growth to form two male gametes. If shed at the 3-celled stage (vegetative + two male gametes), the pollen tube already carries the two male gametes.
• The pollen tube enters the ovule, usually through the micropyle.
• It then enters one of the synergids via the filiform apparatus, which guides its entry.

If the pollen is incompatible, the pistil rejects it by preventing pollen germination on the stigma or inhibiting pollen tube growth in the style.

The entire sequence of events from pollen deposition to pollen tube entry into the ovule is collectively termed pollen-pistil interaction.

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Artificial Hybridisation

This is a technique used by plant breeders to cross different plant varieties or species to combine desirable traits, aiming to produce superior hybrids.

It involves controlled pollination to ensure that only the desired pollen is used and the stigma is protected from unwanted pollen.

Key techniques include:

• Emasculation: If the female parent plant has bisexual flowers, the anthers are removed from the flower bud using forceps before they dehisce (release pollen). This prevents self-pollination.
• Bagging: The emasculated flower (or the female flower bud in unisexual flowers) is covered with a bag (often butter paper) to prevent contamination by unwanted foreign pollen.
• Once the stigma of the bagged flower becomes receptive, desired mature pollen from the male parent is collected and dusted onto the stigma.
• The flower is then rebagged to prevent further contamination, and the fruit is allowed to develop.

If the female parent plant produces unisexual flowers, emasculation is not necessary. The female flower buds are simply bagged before opening, and pollination is performed with desired pollen when the stigma is receptive, followed by rebagging.

Double Fertilisation

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Syngamy And Triple Fusion

After the pollen tube enters the synergid, it releases the two male gametes into the cytoplasm of the synergid.

Within the embryo sac, two fusion events occur:

1. Syngamy (Generative Fertilisation): One male gamete moves towards the egg cell and fuses with its nucleus. This results in the formation of a diploid (2n) zygote.
2. Triple Fusion: The second male gamete moves towards the large central cell and fuses with the two polar nuclei (or their fusion product, the diploid secondary nucleus). This fusion involves three haploid nuclei (one male gamete nucleus + two polar nuclei), resulting in the formation of a triploid (3n) Primary Endosperm Nucleus (PEN).

Since two fusion events (syngamy and triple fusion) occur within the same embryo sac, the phenomenon is called double fertilisation. This is a unique event characteristic of flowering plants (angiosperms).

The central cell, after triple fusion, becomes the Primary Endosperm Cell (PEC), which develops into the endosperm. The zygote develops into the embryo.

Post-Fertilisation: Structures And Events

These are the events that follow double fertilisation in a flower, leading to the formation of the seed and fruit.

Key post-fertilisation events include:

• Development of the endosperm from the PEC.
• Development of the embryo from the zygote.
• Maturation of the ovule into a seed.
• Development of the ovary into a fruit.

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Endosperm

The endosperm is a nutritive tissue that provides nourishment to the developing embryo. Its development usually precedes embryo development, ensuring a food supply is available.

The Primary Endosperm Cell (PEC), formed by triple fusion, divides repeatedly to form the triploid endosperm tissue.

The most common type of endosperm development is free-nuclear endosperm, where the PEN undergoes successive nuclear divisions without immediate cell wall formation, creating a multi-nucleate condition. This is followed by cellularisation, where cell walls are laid down, making the endosperm cellular.

The liquid "coconut water" inside a tender coconut is an example of free-nuclear endosperm, containing thousands of nuclei. The white kernel surrounding it is the cellular endosperm.

The endosperm may either be completely consumed by the developing embryo before the seed matures (e.g., in non-albuminous seeds like pea, groundnut, beans) or it may persist in the mature seed (e.g., in albuminous seeds like castor, coconut, wheat, maize) and be used during seed germination.

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Embryo

The embryo develops from the zygote, located at the micropylar end of the embryo sac. Zygote division often occurs only after some endosperm is formed to provide nourishment.

The early stages of embryo development (embryogeny) are similar in both monocotyledonous and dicotyledonous plants.

The zygote initially develops into a proembryo, which then progresses through distinct stages such as globular, heart-shaped, and finally the 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 (shoot tip).
• The cylindrical part below the cotyledons is the hypocotyl, terminating in the radicle (root tip).
• The root tip is covered by a protective layer called the root cap.

Monocotyledonous embryos possess only one cotyledon.

• In grass family embryos, the single cotyledon is called the scutellum and is positioned laterally on the embryonal axis.
• The lower end of the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called the coleorrhiza.
• The portion of the embryonal axis above the scutellum attachment is the epicotyl. It contains the shoot apex and leaf primordia enclosed in a hollow, protective structure called the coleoptile.

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Seed

In angiosperms, the seed is the final product of sexual reproduction. It is essentially a fertilised ovule that has matured.

Seeds are formed inside the fruit. A seed typically comprises:

• Seed coat(s): Developed from the ovule's integument(s), providing protection. The micropyle often remains as a small pore in the seed coat, facilitating water and oxygen entry during germination.
• Cotyledon(s): Seed leaves of the embryo. They can be simple structures or thick and swollen due to stored food reserves (e.g., legumes).
• Embryo axis: Contains the plumule (future shoot) and radicle (future root).

Mature seeds are classified based on the presence or absence of residual endosperm:

• Non-albuminous (Ex-albuminous) seeds: The endosperm is completely consumed by the embryo during development (e.g., pea, groundnut, beans).
• Albuminous seeds: The endosperm persists in the mature seed and is used during germination (e.g., wheat, maize, barley, castor, coconut).

Occasionally, remnants of the nucellus may also persist in some seeds (e.g., black pepper, beet). This residual nucellus is called perisperm.

As seeds mature, their water content decreases (typically to $10-15\%$ moisture). The embryo's metabolic activity slows down, and it may enter a state of inactivity called dormancy.

Seed dormancy allows seeds to survive unfavorable conditions and germinate only when conditions (moisture, oxygen, suitable temperature) are favorable.

Seed viability (the ability to germinate) varies greatly, from a few months to hundreds or even thousands of years under suitable storage conditions. Remarkable examples of long viability include the Lupinus arcticus seed (germinated after $\sim$10,000 years) and Phoenix dactylifera (date palm) seed ($\sim$2000 years old).

Seeds offer several advantages to angiosperms:

• Reproduction is independent of water.
• Better adaptation for dispersal to new habitats, aiding colonization.
• Contain food reserves that nourish the young seedling.
• Hard seed coat protects the embryo.
• Products of sexual reproduction, introducing genetic variation.

The ability of seeds to undergo dehydration and dormancy is crucial for agriculture, allowing storage of food crops and planting in the next season.

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Fruit

Simultaneously with the maturation of ovules into seeds, the ovary develops into the fruit.

The wall of the ovary develops into the wall of the fruit, called the pericarp. The pericarp can be fleshy (e.g., guava, mango) or dry (e.g., groundnut, mustard).

In most plants, other floral parts (sepals, petals, stamens) wither and fall off as the fruit develops. However, in some cases (e.g., apple, strawberry, cashew), the thalamus also contributes to fruit formation. Such fruits are called false fruits.

Fruits that develop solely from the ovary are called true fruits.

Normally, fruits are formed as a result of fertilisation. However, in some species, fruits develop without fertilisation. These are called parthenocarpic fruits, e.g., Banana. Parthenocarpy can sometimes be induced artificially using growth hormones, resulting in seedless fruits.

Fruits often evolve mechanisms for dispersal of seeds.

Apomixis And Polyembryony

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Apomixis

Apomixis is a special mechanism in some flowering plants (like certain species of Asteraceae and grasses) where seeds are produced without fertilisation. Essentially, it is a form of asexual reproduction that mimics sexual reproduction.

Different methods of apomixis exist:

• In some species, a diploid egg cell is formed without undergoing reductional division (meiosis) and develops directly into the embryo without fertilisation.
• In others, like many Citrus and Mango varieties, some nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into embryos (nucellar embryony).

Apomictic embryos are genetically identical to the parent plant (clones) because they are formed without meiosis and fertilisation.

Apomixis is significant in the hybrid seed industry. Hybrid varieties offer increased productivity, but hybrid seeds usually lose their superior traits in the next generation due to segregation. Producing new hybrid seeds each year is costly. If hybrid seeds could be made apomictic, farmers could save seeds from their crop and plant them the following year without losing the hybrid vigor, making hybrid technology more affordable. Research is ongoing to understand the genetics of apomixis and transfer this trait to hybrid varieties.

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Polyembryony

Polyembryony is the occurrence of more than one embryo in a single seed. It is often observed in species exhibiting apomixis (like Citrus and Mango), where multiple embryos can arise from different sources (e.g., zygote and nucellar cells) within the same ovule.

The embryos formed through nucellar embryony are apomictic and genetically identical to the parent plant, while the embryo formed from the zygote (if syngamy also occurred) would be sexual and genetically different.

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