Flower – A Fascinating Organ of Angiosperms
Flowers have aesthetic and cultural significance, symbolizing emotions like love and grief. For biologists, they are sites of sexual reproduction, showcasing intricate processes. Ornamental and culturally significant flowers highlight their importance.
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| L.S. of Flower |
Pre-Fertilization: Structures and Events
Before blooming, plants undergo changes leading to the development of floral primordia. Inflorescences form, carrying buds that become flowers. Inside, the male (androecium) and female (gynoecium) reproductive structures develop, preparing for sexual reproduction.
Understanding these processes enhances our appreciation of plant life and biodiversity.
Stamen, Microsporangium, and Pollen Grain
Structure of a Stamen
A typical stamen consists of two main parts:
- Filament: A long, slender stalk.
- Anther: A generally bilobed terminal structure.
The filament's proximal end is attached to the thalamus or the petal of the flower. Stamens vary in number and length across different species. Observing and comparing stamens from various flowers under a dissecting microscope can reveal a significant variation in size, shape, and anther attachment.
Anatomy of an Anther
A typical angiosperm anther is bilobed, with each lobe containing two theca, making it dithecous. Often, a longitudinal groove separates the theca. In a transverse section, an anther appears tetragonal with four microsporangia located at the corners, two in each lobe. These microsporangia develop into pollen sacs, which extend through the length of the anther and contain pollen grains.
Structure of Microsporangium
In a transverse section, a typical microsporangium appears nearly circular and is surrounded by four wall layers:
- Epidermis
- Endothecium
- Middle Layers
- Tapetum
The outer three layers provide protection and assist in the dehiscence of the anther to release pollen. The innermost layer, the tapetum, nourishes developing pollen grains with its dense cytoplasm and multinucleate cells.
Development of Microsporangium
When the anther is young, a central group of compactly arranged, homogeneous cells called sporogenous tissue occupies each microsporangium.
Microsporogenesis: The Formation of Pollen Grains
Understanding Microsporogenesis
Microsporogenesis is the process by which the cells of the sporogenous tissue in an anther undergo meiotic divisions to form microspore tetrads. Each cell within the sporogenous tissue can develop into a microspore tetrad, making them potential pollen or microspore mother cells.
The Process of Microsporogenesis
During microsporogenesis, a pollen mother cell (PMC) undergoes meiosis to produce four microspores, which are grouped together in a structure known as a microspore tetrad. These microspores have a haploid (n) ploidy level, resulting from the reduction division of meiosis.
Maturation and Development
As the anther matures, it dehydrates, causing the microspores to dissociate from the tetrad structure and develop into individual pollen grains. Each microsporangium within the anther can produce thousands of microspores or pollen grains.
Release of Pollen Grains
Once fully developed, the pollen grains are released through the dehiscence of the anther, ready to participate in the fertilization process. This intricate process ensures the continuation of plant species through sexual reproduction.
Pollen Grain: The Male Gametophytes
Introduction to Pollen Grains
Pollen grains represent the male gametophytes in flowering plants. If you touch the opened anthers of flowers like Hibiscus, you will notice yellowish, powdery pollen grains on your fingers. Observing these grains under a microscope reveals a fascinating variety of sizes, shapes, colors, and designs unique to different species.
Structure and Composition
- Size and Shape: Pollen grains are generally spherical, measuring about 25-50 micrometers in diameter.
- Wall Layers: Pollen grains have a prominent two-layered wall.
- Exine: The hard outer layer made of sporopollenin, an extremely resistant organic material. Sporopollenin can withstand high temperatures, strong acids, and alkalis. It is so durable that no enzyme capable of degrading sporopollenin has been discovered yet. The exine has prominent apertures called germ pores where sporopollenin is absent. These pores facilitate the growth of the pollen tube during fertilization. The exine's durability also makes pollen grains well-preserved as fossils.
- Intine: The inner wall is thin and continuous, composed of cellulose and pectin.
Internal Structure
The cytoplasm of the pollen grain is surrounded by a plasma membrane. When mature, the pollen grain contains two cells:
- Vegetative Cell: This larger cell has abundant food reserves and a large, irregularly shaped nucleus.
- Generative Cell: This smaller cell floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
Maturity and Shedding
In over 60% of angiosperms, pollen grains are shed at the 2-celled stage. In the remaining species, the generative cell divides mitotically before the pollen grains are shed, resulting in a 3-celled stage with two male gametes.
Nutritional and Functional Benefits of Pollen Grains
Pollen Grains as Nutritional Supplements
Pollen grains are nutrient-rich and have recently gained popularity as food supplements. In Western countries, various pollen products are available in the form of tablets and syrups. These supplements are marketed with claims of enhancing the performance of athletes and racehorses due to their nutritional benefits.
Viability of Pollen Grains
For successful fertilization, pollen grains must land on the stigma before losing viability. The viability period of pollen grains varies significantly based on temperature and humidity. For example:
- In cereals like rice and wheat, pollen grains lose viability within 30 minutes of release.
- In some species of the Rosaceae, Leguminoseae, and Solanaceae families, pollen grains can remain viable for months.
Long-term Storage and Use
Just as semen or sperm can be stored for artificial insemination, pollen grains can also be preserved. Pollen from various species can be stored for years in liquid nitrogen at -196°C. These stored pollen grains serve as pollen banks, similar to seed banks, and are valuable in crop breeding programs.
The Pistil, Megasporangium (Ovule), and Embryo Sac
The Gynoecium: Female Reproductive Part of the Flower
The gynoecium is the female reproductive structure of a flower. It can consist of a single pistil (monocarpellary) or multiple pistils (multicarpellary). When multiple pistils are present, they can be either fused together (syncarpous) or free (apocarpous).
Each pistil has three main parts:
- Stigma: Serves as the landing platform for pollen grains.
- Style: The elongated, slender part beneath the stigma.
- Ovary: The basal, bulged part of the pistil housing the ovarian cavity (locule), where the placenta is located. From the placenta arise the megasporangia (ovules). The number of ovules can vary from one (as in wheat, paddy, mango) to many (as in papaya, watermelon, orchids).
Structure of the Megasporangium (Ovule)
A typical angiosperm ovule consists of the following parts:
- Funicle: The stalk that attaches the ovule to the placenta.
- Hilum: The region where the body of the ovule fuses with the funicle, representing the junction between the two.
- Integuments: Protective envelopes that encircle the nucellus, except at the tip where a small opening called the micropyle is located.
- Chalaza: The basal part of the ovule opposite the micropyle.
- Nucellus: A mass of cells inside the integuments with abundant reserve food materials.
- Embryo Sac: The female gametophyte located within the nucellus. Each ovule typically contains a single embryo sac formed from a megaspore.
Megasporogenesis and Female Gametophyte Development
Megasporogenesis: Formation of Megaspores
Megasporogenesis is the process through which megaspores form from the megaspore mother cell (MMC). Typically, a single MMC differentiates in the micropylar region of the nucellus within an ovule. This cell is characterized by its large size, dense cytoplasm, and prominent nucleus. The MMC undergoes meiosis, resulting in the production of four megaspores. This meiotic division is crucial as it reduces the chromosome number by half, preparing the cells for fertilization.
Development of the Female Gametophyte
In most flowering plants, only one of the four megaspores remains functional, while the other three degenerate. The functional megaspore develops into the female gametophyte, also known as the embryo sac, through a process termed monosporic development.
The formation of the embryo sac involves several stages:
- Mitotic Division: The nucleus of the functional megaspore divides mitotically to produce two nuclei, which move to opposite poles, forming a 2-nucleate embryo sac.
- Sequential Divisions: Two more mitotic divisions result in a 4-nucleate and then an 8-nucleate embryo sac. These divisions are free nuclear, meaning they are not immediately followed by cell wall formation.
- Cell Wall Formation: After the 8-nucleate stage, cell walls form, organizing the nuclei into a typical female gametophyte or embryo sac.
Structure of the Mature Embryo Sac
A mature angiosperm embryo sac is typically 7-celled but 8-nucleate. The distribution of cells within the embryo sac is characteristic:
- Egg Apparatus: Located at the micropylar end, consisting of one egg cell and two synergids. The synergids have filiform apparatus, which guide the pollen tube into the synergid.
- Central Cell: Contains two polar nuclei situated below the egg apparatus in a large central cell.
- Antipodals: Three cells located at the chalazal end.
Pollination: Bringing Gametes Together in Flowering Plants
Pollination is a crucial process in flowering plants where pollen grains from the anther are transferred to the stigma of a pistil, enabling fertilization. Since both male and female gametes are non-motile, pollination ensures they come together for reproduction. Flowering plants have developed various adaptations to facilitate pollination using external agents.
Types of Pollination
Pollination can be categorized into three types based on the pollen source:
Autogamy
- Definition: Pollination within the same flower.
- Process: Pollen grains transfer from the anther to the stigma of the same flower.
- Requirements: Synchrony in pollen release and stigma receptivity, and proximity of anthers and stigma.
- Example: Plants like Viola, Oxalis, and Commelina produce cleistogamous flowers that remain closed, ensuring self-pollination.
- Advantages/Disadvantages: Cleistogamy guarantees seed production without pollinators but may reduce genetic diversity.
Geitonogamy
- Definition: Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
- Nature: Functionally cross-pollination but genetically similar to autogamy.
- Example: Common in plants where multiple flowers bloom simultaneously.
Xenogamy
- Definition: Transfer of pollen grains from the anther of one plant to the stigma of a different plant.
- Significance: This is true cross-pollination, introducing genetic diversity by bringing different genetic material to the stigma.
Agents of Pollination: Wind, Water, and Animals
Plants employ various agents for pollination, including two abiotic agents, wind and water, and one biotic agent, animals. While most plants rely on biotic agents for pollination, a small proportion utilizes abiotic agents.
Wind Pollination
- Characteristics: Wind-pollinated flowers produce copious amounts of lightweight, non-sticky pollen to be dispersed by wind currents.
- Structural Adaptations: These flowers often have exposed stamens and large, feathery stigmas to capture airborne pollen grains.
- Examples: Common in grasses, with the corn cob being a familiar example where tassels function as stigmas and styles to trap pollen grains.
Water Pollination
- Occurrence: Water pollination is rare in flowering plants, limited to around 30 genera, primarily monocotyledons.
- Structural Features: Flowers of water-pollinated plants are typically submerged or emerge above the water surface, facilitating passive transport of pollen grains by water currents.
- Examples: Vallisneria, Hydrilla, and marine sea-grasses like Zostera utilize water for pollination.
- Adaptations: Pollen grains in water-pollinated species are often long and ribbon-like, protected by a mucilaginous covering to prevent wetting.
Comparisons and Adaptations
- Color and Nectar: Wind and water-pollinated flowers are usually less colorful and do not produce nectar. This is because they do not rely on attracting pollinators for transfer, instead relying on chance encounters for successful pollination.
Animal Pollination in Flowering Plants
Common Pollinating Agents
- Diverse Range: Flowering plants utilize various animals for pollination, including bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds), and bats.
- Dominant Agents: Among animals, insects, especially bees, are the primary pollinators. However, larger animals such as primates, rodents, and reptiles have also been reported as pollinators in certain species.
Floral Adaptations
- Specialized Flowers: Animal-pollinated flowers are often adapted to attract specific pollinators. They may be large, colorful, fragrant, and rich in nectar.
- Attraction Mechanisms: Flowers attract animals through color, fragrance, or secretion of foul odors. Flowers pollinated by flies and beetles may emit foul odors to attract these insects.
- Floral Rewards: Animals are enticed by rewards like nectar and pollen grains, which they harvest from flowers. During this process, animals come in contact with both the anthers and stigma, facilitating pollination.
Mutualistic Relationships
- Co-evolution: Some plants and animals have co-evolved in mutualistic relationships. For example, certain moths and plants like Yucca are interdependent for reproduction.
- Specialized Roles: In these relationships, animals may use flowers not only for nectar but also as safe places to lay eggs, ensuring the continuation of both species.
Outbreeding Devices
- Preventing Inbreeding: To avoid inbreeding depression, flowering plants have developed mechanisms to discourage self-pollination and encourage cross-pollination.
- Synchronizing Pollination: Some species have evolved asynchronous pollen release and stigma receptivity to prevent self-pollination.
- Physical Separation: Others have positioned anthers and stigmas differently within the flower, ensuring pollen cannot reach the stigma of the same flower.
- Genetic Mechanisms: Self-incompatibility mechanisms inhibit self-pollen germination or pollen tube growth, preventing fertilization.
- Unisexual Flowers: Plants produce separate male and female flowers or are dioecious, with male and female flowers on different plants, effectively preventing both self-pollination and geitonogamy.
Understanding Pollen-Pistil Interaction
Importance of Pollen Recognition
- Selective Process: Pollination doesn't always ensure the transfer of compatible pollen. The pistil has the ability to recognize pollen, distinguishing between compatible and incompatible types.
- Recognition Mechanism: Chemical components of both pollen and pistil mediate this recognition process, leading to either acceptance or rejection of pollen grains.
- Continuous Dialogue: The interaction between pollen grains and the pistil is a continuous dialogue, crucial for successful fertilization.
Post-Pollination Events
- Germination Process: After compatible pollination, pollen germinates on the stigma, forming a pollen tube through which its contents move.
- Pollen Tube Growth: The pollen tube extends through the stigma and style, reaching the ovary. In some plants, the generative cell divides during this process to form male gametes.
- Entry into Ovule: Upon reaching the ovary, the pollen tube enters the ovule through the micropyle, guided by the filiform apparatus present in the synergids.
Manipulating Pollen-Pistil Interaction
- Breeding Applications: Understanding pollen-pistil interaction provides insights for plant breeders to manipulate the process, even in incompatible pollinations, to produce desired hybrids.
- Potential Benefits: This knowledge offers opportunities to enhance crop breeding programs by facilitating controlled pollination and hybridization techniques.
Pollen-pistil interaction is a dynamic process essential for successful fertilization in flowering plants. Advances in understanding this interaction hold promise for improving breeding strategies and developing new plant varieties with desirable traits.
Artificial Hybridisation Techniques in Crop Improvement
Importance of Controlled Pollination
- Breeder's Objective: Breeders aim to create superior varieties by crossing different species and genera to combine desirable traits.
- Controlled Pollination: In crossing experiments, it's crucial to ensure that only desired pollen grains fertilize the stigma, avoiding contamination from unwanted pollen.
Emasculation and Bagging Techniques
- Emasculation Process: In flowers with bisexual characteristics, anthers are removed from the flower bud before they release pollen. This step, called emasculation, prevents self-pollination and ensures controlled fertilization.
- Bagging Procedure: Emasculated flowers are covered with bags made of suitable materials like butter paper to protect the stigma from contamination. This process, known as bagging, maintains the purity of pollination.
- Pollination and Rebagging: Once the stigma is receptive, mature pollen grains from the male parent are applied to the stigma. After pollination, flowers are rebagged to prevent further contamination, allowing fruits to develop without interference.
Unisexual Flower Handling
- Bagging Unisexual Flowers: If the female parent bears unisexual flowers, emasculation is unnecessary. Instead, female flower buds are bagged before opening to protect the stigma.
- Pollination Procedure: Once the stigma is receptive, desired pollen is applied, and the flower is rebagged to ensure controlled fertilization.
Advantages of Artificial Hybridisation
- Precision in Trait Selection: Artificial hybridisation allows breeders to precisely select desired traits for the development of commercially superior varieties.
- Prevention of Unwanted Crosses: Emasculation and bagging techniques minimize the risk of unwanted crosses, ensuring the genetic purity of the resulting hybrids.
Artificial hybridisation techniques play a vital role in crop improvement programmes, enabling breeders to create new varieties with desired characteristics while maintaining genetic purity and preventing unwanted crosses.
Double Fertilisation in Flowering Plants
Unique Process in Flowering Plants
- Double Fertilisation: A distinctive reproductive mechanism exclusive to flowering plants, involving two significant fusions.
- Syngamy: One of the male gametes combines with the egg cell nucleus, forming a diploid zygote.
- Triple Fusion: The other male gamete merges with two polar nuclei, generating a triploid primary endosperm nucleus (PEN), crucial for endosperm development.
Sequence of Events
- Pollen Tube Entry: The pollen tube penetrates one of the synergids, delivering the male gametes.
- Syngamy: One male gamete merges with the egg cell, leading to the formation of the diploid zygote, initiating embryo development.
- Triple Fusion: Simultaneously, the second male gamete combines with the two polar nuclei, resulting in the formation of the triploid primary endosperm nucleus (PEN), essential for endosperm formation.
- Endosperm Development: The primary endosperm nucleus (PEN) develops into endosperm, providing essential nutrients for the growing embryo.
Significance of Double Fertilisation
- Nutrient Provision: The endosperm formed through double fertilisation serves as a nutrient-rich tissue vital for the sustenance of the developing embryo.
- Genetic Variation: Double fertilisation promotes genetic diversity by combining genetic material from both parents, contributing to the evolutionary success of flowering plants.
- Reproductive Efficiency: This process ensures efficient resource allocation, optimizing the chances of successful seed development and plant propagation.
Double fertilisation is a remarkable reproductive strategy in flowering plants, ensuring both embryonic and endosperm development, essential for seed viability and plant reproduction. Understanding this intricate process provides valuable insights into the reproductive biology of flowering plants.
Post-Fertilisation Events: Endosperm Development
Significance of Endosperm Development
- Nutrient Provision: Endosperm serves as a rich source of reserve food materials, crucial for nourishing the developing embryo.
- Sequential Development: Endosperm development precedes embryo development, ensuring that the embryo receives adequate nutrition for its growth and development.
Free-Nuclear Endosperm
- Process: Initially, the primary endosperm nucleus (PEN) undergoes successive nuclear divisions without cell wall formation, resulting in the formation of numerous free nuclei.
- Cellularisation: Subsequently, cell wall formation occurs, converting the free-nuclear endosperm into cellular endosperm.
- Variability: The number of free nuclei formed before cellularisation varies across plant species.
Persistence and Utilisation
- Seed Maturation: Endosperm may either be completely consumed by the developing embryo before seed maturation or persist in the mature seed.
- Examples: In some plants like peas, groundnuts, and beans, the endosperm is consumed during embryo development, while in others like castor and coconut, it persists in the mature seed.
Practical Observations
- Exploration: Split open seeds of various plants like castor, peas, beans, groundnuts, and coconuts to observe the presence and persistence of endosperm.
- Comparison: Investigate the presence of endosperm in cereals such as wheat, rice, and maize to understand its role in different plant species.
Embryo Development
Embryo Formation and Adaptations
- Zygote Location: The embryo develops at the micropylar end of the embryo sac where the zygote resides.
- Assured Nutrition: Most zygotes divide only after sufficient endosperm is formed, ensuring assured nutrition for the developing embryo.
Embryogeny in Dicotyledons
- Stages: Embryogeny in dicotyledons progresses through stages including proembryo, globular, heart-shaped, and mature embryo.
- Embryo Structure: A typical dicotyledonous embryo consists of an embryonal axis, two cotyledons, an epicotyl terminating with the plumule, and a hypocotyl terminating with the radicle.
Embryos of Monocotyledons
- Structure: Monocotyledonous embryos have only one cotyledon, known as the scutellum in the grass family.
- Parts: The embryonal axis in monocots includes the radical, root cap enclosed in a coleorrhiza, and the epicotyl terminating with the coleoptile, which encloses the shoot apex and leaf primordia.
Practical Observation
- Seed Soaking: Soak seeds of various plants like wheat, maize, peas, chickpeas, and groundnuts overnight in water.
- Observation: Split open the soaked seeds to observe the different parts of the embryo and seed structure.
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