Reproduction - Sexual Reproduction In Flowering Plants

Male Sex Organ (Androecium)

  • Androecium is the male reproductive organ in flowering plants
  • It consists of stamens that produce pollen grains
  • Each stamen is made up of two parts:
    • Filament: A slender stalk supporting the anther
    • Anther: The structure that contains the pollen sacs
  • The number of stamens varies among different species of flowering plants
  • The arrangement of stamens can be:
    • Monadelphous: Filaments are fused into a single group
    • Diadelphous: Filaments are fused into two groups
    • Polyadelphous: Filaments are fused into more than two groups
    • Stamens can also be free, where the filaments are not fused

Sexual Reproduction In Flowering Plants

Structure of the Stamen

  • The filament is a long, slender structure
  • It holds the anther at the top
  • The anther is divided into two pollen sacs
  • The pollen sacs contain pollen grains
  • Each pollen grain contains male gametes (sperm cells)

Sexual Reproduction In Flowering Plants

Development of Pollen Grains

  • The development of pollen grains is called microsporogenesis
  • It occurs inside the anther
  • Microsporocytes (2n) undergo meiosis to produce microspores (n)
  • Each microspore develops into a pollen grain
  • The pollen grain is covered by a protective wall called the exine
  • The exine is made of a complex chemical substance called sporopollenin

Sexual Reproduction In Flowering Plants

Structure of a Pollen Grain

  • A pollen grain consists of:
    • Exine: The outer wall made of sporopollenin
    • Intine: The inner wall made of cellulose
    • Generative cell: Divides to form two male gametes
    • Tube cell: Forms a pollen tube during pollination

Sexual Reproduction In Flowering Plants

Pollination

  • Pollination is the transfer of pollen from the anther to the stigma
  • It can be of two types:
    • Self-pollination: Pollen is transferred from the anther of a flower to the stigma of the same flower, or to another flower of the same plant
    • Cross-pollination: Pollen is transferred from the anther of a flower to the stigma of a flower on a different plant of the same species

Sexual Reproduction In Flowering Plants

Agents of Pollination

  • Pollination can be done by:
    • Wind (anemophily): Pollen grains are small, light, and produced in large numbers. Ex: Grasses and conifers
    • Water (hydrophily): Pollen grains are released into the water for transport. Ex: Vallisneria and Hydrilla
    • Animals (zoophily): Pollen grains are sticky or carried by animals. Ex: Bees, birds, and bats

Sexual Reproduction In Flowering Plants

Structure of a Flower

  • A flower is the reproductive structure of a flowering plant
  • It consists of four main parts:
    • Calyx: The outermost whorl, made of sepals
    • Corolla: The next whorl, made of petals
    • Androecium: The male reproductive organ, made of stamens
    • Gynoecium: The female reproductive organ, made of carpels

Sexual Reproduction In Flowering Plants

Structure of a Carpels

  • The carpel is the female reproductive structure in flowering plants
  • It consists of three parts:
    • Stigma: The sticky, receptive portion where pollen grains land
    • Style: A slender tube that connects the stigma to the ovary
    • Ovary: The enlarged basal part that contains ovules

Sexual Reproduction In Flowering Plants

Structure of an Ovary

  • The ovary is the part of the carpel where ovules are produced
  • It can be positioned in different ways:
    • Superior ovary: The ovary is situated above the attachment of other floral parts
    • Inferior ovary: The ovary is situated below the attachment of other floral parts
  • The number of carpels and the arrangement of ovaries vary among different species of flowering plants
  1. Structure of a Pollen Grain:
  • A pollen grain consists of:
    • Exine: The outer wall made of sporopollenin
    • Intine: The inner wall made of cellulose
    • Generative cell: Divides to form two male gametes
    • Tube cell: Forms a pollen tube during pollination
  • The exine protects the pollen grain from desiccation and external factors
  • The intine provides mechanical support to the pollen grain
  • The generative cell undergoes mitosis to produce two male gametes
  • The tube cell elongates and forms a pollen tube during pollination
  1. Pollination:
  • Pollination is the transfer of pollen from the anther to the stigma
  • It can be of two types:
    • Self-pollination: Pollen is transferred from the anther of a flower to the stigma of the same flower, or to another flower of the same plant
    • Cross-pollination: Pollen is transferred from the anther of a flower to the stigma of a flower on a different plant of the same species
  • Pollination ensures the transfer of male gametes to the female reproductive structure
  • It is a crucial step in the sexual reproduction of flowering plants
  • The success of pollination determines the formation of seeds and fruits
  1. Agents of Pollination:
  • Pollination can be done by different agents:
    • Wind (anemophily): Pollen grains are small, light, and produced in large numbers. Ex: Grasses and conifers
    • Water (hydrophily): Pollen grains are released into the water for transport. Ex: Vallisneria and Hydrilla
    • Animals (zoophily): Pollen grains are sticky or carried by animals. Ex: Bees, birds, and bats
  • Different agents of pollination have different adaptations for pollen transfer
  • Examples of co-evolution between plants and their pollinators can be observed in the structure and colors of flowers
  1. Structure of a Flower:
  • A flower is the reproductive structure of a flowering plant
  • It consists of four main parts:
    • Calyx: The outermost whorl, made of sepals
    • Corolla: The next whorl, made of petals
    • Androecium: The male reproductive organ, made of stamens
    • Gynoecium: The female reproductive organ, made of carpels
  • The arrangement and number of these parts may vary among different species of flowering plants
  • The flower plays a crucial role in attracting pollinators and facilitating the process of sexual reproduction
  1. Structure of a Carpels:
  • The carpel is the female reproductive structure in flowering plants
  • It consists of three parts:
    • Stigma: The sticky, receptive portion where pollen grains land
    • Style: A slender tube that connects the stigma to the ovary
    • Ovary: The enlarged basal part that contains ovules
  • The stigma and style aid in the capture and transportation of pollen grains
  • The ovary is the base of the carpel where ovules are produced and fertilization takes place
  1. Structure of an Ovary:
  • The ovary is the part of the carpel where ovules are produced
  • It can be positioned in different ways:
    • Superior ovary: The ovary is situated above the attachment of other floral parts
    • Inferior ovary: The ovary is situated below the attachment of other floral parts
  • The ovary protects and nourishes the developing ovules
  • It serves as the site of fertilization and later transforms into a fruit after fertilization
  1. The Process of Double Fertilization:
  • Double fertilization is a unique process in flowering plants
  • It involves the fertilization of two different structures in the female reproductive organ
  • The pollen tube enters the ovary through the style and reaches the embryo sac
  • One male gamete fuses with the egg cell to form the zygote (2n)
  • The other male gamete fuses with the two polar nuclei to form the endosperm (3n)
  • This process ensures the formation of both the embryo and the nutritive tissue
  1. Development of Ovules:
  • Ovules are the structures inside the ovary that develop into seeds after fertilization
  • Ovules contain the female gametophyte, which produces the female gametes
  • The development of ovules is called megasporogenesis
  • It involves the formation and development of megaspores (n) inside the ovule
  • Only one megaspore survives and develops into the female gametophyte or embryo sac
  1. Structure of the Embryo Sac:
  • The embryo sac is the female gametophyte in flowering plants
  • It is located inside the ovule and contains the female gametes
  • The embryo sac consists of several cells arranged in three main regions:
    • The egg apparatus: Contains the egg cell and two synergids
    • The central cell: Contains two polar nuclei
    • Antipodal cells: Three cells opposite to the micropylar end of the embryo sac
  • The female gametes are present within the egg apparatus and are involved in fertilization
  1. Fertilization and Seed Formation:
  • After pollen grains reach the stigma, they germinate and form a pollen tube
  • The pollen tube grows through the style and reaches the ovule
  • Double fertilization occurs inside the embryo sac:
    • One male gamete fuses with the egg cell to form the zygote (2n)
    • The other male gamete fuses with the two polar nuclei to form the endosperm (3n)
  • After fertilization, the zygote develops into an embryo, and the endosperm develops into a nutritive tissue
  • Eventually, a seed is formed, which contains the embryo, endosperm, and protective seed coat Here are slides 21 to 30 for the lecture:
  1. Development of Fruit:
  • After fertilization, the ovary develops into a fruit
  • The fruit protects the developing seeds and aids in their dispersal
  • Fruits can be classified into different types based on their origin:
    • Simple fruits: Develop from a single ovary of a single flower. Ex: Mango, Tomato
    • Aggregate fruits: Develop from several ovaries of a single flower. Ex: Raspberry, Strawberry
    • Multiple fruits: Develop from the ovaries of several flowers in an inflorescence. Ex: Pineapple, Mulberry
  1. Structure of a Fruit:
  • A fruit consists of three main parts:
    • Exocarp: The outermost layer of the fruit
    • Mesocarp: The middle, fleshy layer of the fruit
    • Endocarp: The innermost layer surrounding the seed(s)
  • The fruit may also contain other structures like the receptacle or calyx
  • The fruit’s structure varies depending on its type, such as dry or fleshy, and the number of seeds it contains
  1. Seed Dispersal:
  • Seed dispersal is the process by which seeds are spread away from the parent plant
  • It helps in avoiding competition and colonization of new habitats
  • Seeds can be dispersed in various ways:
    • Wind: Seeds with wing-like structures or lightweight seeds are dispersed by wind. Ex: Dandelion, Maple
    • Water: Buoyant seeds are dispersed by water. Ex: Coconut, Water lily
    • Animals: Seeds with adaptations to attach to fur, be ingested by animals, or get dispersed in animal droppings. Ex: Burdock, Tomato
    • Gravity: Heavy seeds fall directly beneath the parent plant. Ex: Acorn, Mango
    • Self-dispersal: Seeds that have explosive mechanisms or mechanisms to disperse on their own. Ex: Pea, Balsam
  1. Germination:
  • Germination is the process by which a seed grows into a new plant
  • It involves the activation of a dormant seed and the emergence of a radicle (embryonic root) and plumule (embryonic shoot)
  • Conditions required for germination include:
    • Water: Water activates enzymes and softens the seed coat
    • Oxygen: Oxygen is necessary for respiration
    • Proper temperature: Optimum temperature is required for enzyme activity
    • Suitable light or darkness: Some seeds require light, while others germinate in darkness
    • Hormones: Plant hormones like gibberellins stimulate germination
  1. Parts of a Germinating Seed:
  • The germinating seed consists of the following parts:
    • Seed coat: Protects the seed and is shed during germination
    • Embryo: The young plant in the seed
      • Radicle: Embryonic root
      • Plumule: Embryonic shoot
      • Cotyledons: Seed leaves that store nutrients
    • Endosperm: Nutritive tissue in some seeds that provides energy and nutrients during germination
    • Hypocotyl: Region between the radicle and cotyledons
    • Epicotyl: Region between the cotyledons and plumule tip
  1. Conditions for Seed Germination:
  • Seeds require specific conditions for germination, including:
    • Adequate water: Activation of enzymes and softening of the seed coat
    • Sufficient oxygen: Required for cellular respiration
    • Proper temperature: Optimum temperature range for enzyme activity
    • Suitable light or darkness: Photoperiod requirements vary among different plant species
    • Hormonal balance: Plant hormones like gibberellins play a role in germination
  1. Factors Affecting Seed Germination:
  • Several factors can affect seed germination, including:
    • Temperature: Optimum temperature is required for enzyme activity. Low or high temperatures can inhibit germination
    • Light exposure: Some seeds require light for germination (photoblastic), while others require darkness (scotoblastic)
    • Moisture availability: Adequate moisture is essential for enzymatic reactions and seed coat softening
    • Seed dormancy: Some seeds have physiological or physical dormancy, requiring specific treatments to break dormancy
    • Nutrient availability: Seeds rely on stored nutrients within the endosperm or cotyledons until they can photosynthesize
  1. Importance of Sexual Reproduction:
  • Sexual reproduction has several advantages for plants, such as:
    • Genetic variation: Allows for the creation of new combinations of traits, increasing adaptability to changing environments
    • Evolutionary adaptation: Natural selection acts on genetic variation, leading to the survival of more fit individuals and populations
    • Enhanced genetic diversity: Increases resistance to diseases and pests, and facilitates the colonization of new habitats
    • Strengthens species survival: Sexual reproduction enables the formation of seeds and fruits, enhancing dispersal and survival rates
  1. Human Intervention in Sexual Reproduction:
  • Humans play a role in aiding sexual reproduction in plants through various methods, such as:
    • Artificial pollination: Controlled transfer of pollen between flowers to produce desired traits or hybrids
    • Tissue culture: In vitro cultivation of plant cells, tissues, or organs for mass production of plants with desirable traits
    • Seed banks: Conservation of plant genetic diversity through the collection and storage of seeds from different plant species
    • Genetic engineering: Manipulation of an organism’s genetic material to introduce desired traits or enhance resistance to diseases and pests
  1. Conclusion:
  • Sexual reproduction in flowering plants involves the fusion of male and female gametes to form seeds and fruits
  • The male reproductive organ, the androecium, produces pollen grains containing male gametes
  • Pollination is the transfer of pollen from the anther to the stigma, which can occur through various agents
  • After pollination, fertilization occurs inside the ovule, leading to the development of seeds and fruits
  • Seed dispersal allows for the colonization of new habitats and avoids competition between parent and offspring plants
  • Germination is the process by which a seed grows into a new plant under suitable conditions
  • Human intervention in sexual reproduction can aid plant breeding and conservation efforts