Reproduction

  • Process by which new organisms of the same species are produced
  • Essential for the survival and continuation of a species
  • Two types: asexual and sexual reproduction

Asexual Reproduction

  • Involves only one parent
  • Offspring are genetically identical to the parent
  • Methods include binary fission, budding, and vegetative propagation

Sexual Reproduction

  • Involves two parents
  • Offspring inherit traits from both parents
  • Essential for genetic variation and adaptation to changing environments

Sexual Reproduction in Flowering Plants

  • Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs
  • Fertilization is the fusion of male and female gametes to form a zygote
  • Development of seed is a result of fertilization and leads to the formation of fruit

Development of Seed

  • After fertilization, the zygote undergoes divisions and develops into an embryo
  • The embryo is protected by the seed coat, which provides a protective covering
  • The seed contains the embryo, food reserves, and a protective seed coat

Parts of a Seed

  • Seed coat: Outer protective layer of the seed
  • Embryo: Developing plant inside the seed
  • Cotyledons: Structures that store food for the developing plant
  • Radicle: Embryonic root
  • Plumule: Embryonic shoot

Germination of Seeds

  • Germination is the process by which a seed grows into a new plant
  • Requires favorable conditions such as water, oxygen, and suitable temperature
  • During germination, the radicle emerges first, followed by the plumule

Types of Pollination

  • Self-pollination: Pollen from the anther is transferred to the stigma of the same flower or another flower on the same plant
  • Cross-pollination: Pollen is transferred from the anther of one flower to the stigma of another flower on a different plant

Agents of Pollination

  • Wind: Pollen is light and can be carried by wind to reach the stigma of other flowers
  • Insects: Bees, butterflies, and other insects transfer pollen as they visit flowers in search of nectar
  • Birds: Certain birds, such as hummingbirds, feed on nectar and transfer pollen while doing so

Structure of a Flower

  • Sepals: Outermost part of the flower, protects the flower bud
  • Petals: Attract pollinators with their color and fragrance
  • Stamen: Male reproductive organ consisting of anther and filament
  • Pistil: Female reproductive organ consisting of stigma, style, and ovary

Modes of Sexual Reproduction

  • Majority of flowering plants reproduce sexually
  • Self-fertilization: Transfer of pollen from the anther to the stigma of the same flower
    • Example: Pea plants
  • Cross-fertilization: Transfer of pollen from the anther of one flower to the stigma of another flower on a different plant
    • Example: Apple trees

Process of Fertilization

  • Pollen grain lands on the stigma of a flower
  • Pollen tube grows from the pollen grain down through the style into the ovary
  • Male gametes travel through the pollen tube to reach the female gametes in the ovule
  • Fusion of male and female gametes results in fertilization and formation of a zygote

Development of Embryo

  • After fertilization, the zygote undergoes cell divisions to form an embryo
  • The embryo consists of an embryonic root (radicle), an embryonic shoot (plumule), and one or two cotyledons
  • Cotyledons provide nourishment to the developing embryo
  • Different angiosperm species may have different numbers and arrangements of cotyledons

Development of Endosperm

  • Endosperm is a nutritive tissue that surrounds and nourishes the embryo in the seed
  • In some plants, endosperm is formed as a result of fusion of the male and female nuclei without fertilization
  • In other plants, endosperm is formed as a result of fertilization
  • Examples: Maize, coconut

Types of Endosperm

  • Nuclear endosperm: Nucleus divides repeatedly without cell division, resulting in a multinucleate cell
  • Cellular endosperm: Nucleus divides and cell walls form around each nucleus, resulting in a multicellular endosperm
  • Examples: Wheat, coconut

Components of Fruit

  • Ovary wall develops into the fruit after fertilization
  • Fruit protects the enclosed seeds and aids in seed dispersal
  • Pericarp: The wall of the fruit, consists of three layers: exocarp, mesocarp, and endocarp
  • Examples: Apple, mango, tomato

Types of Fruits

  • Simple fruit: Develops from a single ovary of a single flower
    • Examples: Tomato, pea, cherry
  • Aggregate fruit: Develops from many ovaries of a single flower
    • Examples: Raspberry, blackberry
  • Multiple fruit: Develops from many ovaries of many flowers in an inflorescence
    • Examples: Pineapple, fig

Seed Dispersal

  • Seed dispersal is the process by which seeds are spread away from the parent plant
  • Important for preventing competition for resources and for colonization of new areas
  • Methods of seed dispersal include wind, water, animals, and self-dispersal

Methods of Seed Dispersal

  • Wind dispersal: Seeds have adaptations such as wings or parachutes to be carried by wind
  • Water dispersal: Seeds have adaptations to float in water and be carried by currents
  • Animal dispersal: Seeds have adaptations to be carried by animals through ingestion or attachment
  • Self-dispersal: Seeds have mechanisms to forcibly discharge or jump away from the parent plant

Germination Requirements

  • For germination to occur, seeds require suitable conditions:
    • Adequate moisture for hydration
    • Proper temperature for enzyme activity
    • Sufficient oxygen for respiration
    • Suitable light conditions for certain seeds
  • Factors affecting seed germination include seed dormancy, soil composition, and pH levels

Seed Dormancy

  • Seed dormancy is a condition in which a viable seed fails to germinate even when provided with favorable conditions
  • It helps the plant avoid germination during unfavorable seasons or conditions
  • Examples of seed dormancy mechanisms:
    • Hard seed coat that requires scarification (physical or chemical damage) for water absorption
    • Chemical inhibitors that prevent germination until specific conditions are met

Scarification

  • Scarification is the process of breaking or weakening the seed coat to improve water absorption and break seed dormancy
  • Methods of scarification include:
    • Physical scarification: Abrasion of the seed coat through rubbing or sanding
    • Chemical scarification: Treating the seed coat with acid or other chemicals to soften or thin it
  • Examples of seeds that require scarification for germination: Mimosa pudica, lupin seeds

Seed Germination and Factors Affecting It

  • Seed germination is the resumption of growth of the embryo after a period of dormancy
  • Factors affecting seed germination:
    • Water availability and hydration of the seed
    • Temperature: Optimal temperature ranges vary for different species
    • Oxygen availability for cellular respiration
    • Light requirements: Some seeds require light for germination, while others need darkness

Photoblastic Seeds

  • Photoblastic seeds exhibit different responses to light during germination
  • Positive photoblastic seeds require light for germination
    • Examples: Lettuce, petunia
  • Negative photoblastic seeds require darkness for germination
    • Examples: Tomato, potato

Germination Process

  1. Imbibition: Absorption of water by the seed, causing it to swell and activate enzymes
  1. Activation of enzymes: Enzymes break down stored food reserves in the cotyledons or endosperm
  1. Growth of radicle: Emergence of the radicle, which develops into the root
  1. Growth of plumule: Emergence of the plumule, which develops into the shoot
  1. Development of true leaves: The plumule develops into true leaves, and the plant starts photosynthesis

Germination in Monocots and Dicots

  • Monocots: Have one cotyledon (e.g., maize, wheat)
    • The cotyledon remains underground during germination
    • The first leaf emerges directly from the coleoptile
  • Dicots: Have two cotyledons (e.g., beans, peas)
    • The cotyledons emerge above ground during germination
    • The first leaf emerges directly from between the cotyledons

Significance of Seed Germination

  • Seed germination is essential for the survival and reproduction of plants
  • It allows the plant to resume growth and develop into a mature plant
  • Seed germination contributes to the overall biodiversity and ecological balance of an ecosystem
  • It plays a crucial role in agricultural practices and crop production

Seedling Establishment

  • After seed germination, the seedling undergoes further growth and development to establish itself as a mature plant
  • Factors affecting seedling establishment:
    • Availability of light, water, and nutrients
    • Competition with other plants for resources
    • Predation or herbivory by animals
    • Environmental factors such as temperature and humidity

Growth and Development of Plants

  • Plants exhibit indeterminate growth, meaning they continue to grow throughout their lifespan
  • Factors influencing plant growth and development:
    • Environmental factors: Light, temperature, water, nutrients
    • Hormones: Plant hormones regulate growth, development, and responses to stimuli
    • Genetic factors: Genetic makeup determines growth patterns, flowering, and fruiting

Key Terms

  • Reproduction
  • Asexual reproduction
  • Sexual reproduction
  • Pollination
  • Fertilization
  • Zygote
  • Embryo
  • Seed coat
  • Cotyledons
  • Radicle
  • Plumule
  • Germination
  • Photoblastic
  • Monocots
  • Dicots
  • Seed dormancy
  • Scarification
  • Seedling establishment
  • Indeterminate growth