Detailed Notes -1 – : Chapter 1 : Sexual Reproduction in Flowering Plants

Chapter 1 : Sexual Reproduction in Flowering Plants

Introduction

1. Significance of Sexual Reproduction in Plants:

   • Sexual reproduction in plants serves a crucial role beyond human enjoyment.
   • It facilitates the creation of diverse flowers with various scents, colors, and forms.
   • These attributes aid in attracting pollinators and ensuring successful reproduction.

2. Purpose of Flowers:

   • Flowers are not solely for human aesthetic pleasure; they primarily exist to aid in sexual reproduction.
   • Their beauty, scents, and colors evolved to attract pollinators, such as insects or birds, essential for fertilization.

3. Universal Presence of Sexual Reproduction:

   • All flowering plants (angiosperms) engage in sexual reproduction.
   • This process is essential for the perpetuation and diversification of plant species.

4. Adaptations for Reproduction:

   • The morphology and structure of flowers display a wide range of adaptations.
   • These adaptations ensure successful formation of fruits and seeds, the end products of sexual reproduction.
   • Examples of adaptations include diverse inflorescence structures and specialized floral parts.
5.  

6. Understanding Sexual Reproduction in Angiosperms:

   • To comprehend sexual reproduction in flowering plants, it’s crucial to study their morphology, structure, and reproductive processes.
   • By understanding these aspects, we gain insight into the intricate mechanisms that drive plant reproduction and species survival.

1.1 FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS 

1. Significance of Flowers in Human Culture:

   • Flowers hold immense cultural, social, and religious importance.
   • They serve as symbols for expressing human emotions such as love, joy, grief, and celebration.

2. Common Ornamental Flowers Cultivated in Gardens:

   • Rose (Rosa spp.)
   • Tulip (Tulipa spp.)
   • Daisy (Bellis perennis)
   • Orchid (Orchidaceae family)
   • Sunflower (Helianthus annuus)

3. Flowers Used in Social and Cultural Celebrations:

   • Marigold (Tagetes spp.): Used in Indian festivals and weddings.
   • Jasmine (Jasminum spp.): Utilized in traditional ceremonies in South Asian cultures.
   • Lotus (Nelumbo spp.): Holds cultural and religious significance in Asia.
   • Chrysanthemum (Chrysanthemum spp.): Commonly used in East Asian celebrations.
   • Carnation (Dianthus spp.): Found in bouquets for special occasions like Mother’s Day.

4. Definition of Floriculture:

   • Floriculture refers to the cultivation and management of flowering plants.
   • It encompasses the production of flowers for aesthetic, commercial, and cultural purposes.

5. Parts of a Flower Involved in Sexual Reproduction:

   • Stamen: Male reproductive organ comprising the anther and filament; produces pollen grains containing sperm cells.
   • Pistil (or Carpel): Female reproductive organ including stigma, style, and ovary; ovary houses ovules containing egg cells for fertilization.

1.2 PRE-FERTILISATION: STRUCTURES AND EVENTS

1. Pre-Fertilization Process:

   • The decision for a plant to flower occurs before the visible emergence of the flower itself.
   • This decision triggers a series of hormonal and structural changes within the plant.
   • These changes lead to the differentiation and subsequent development of floral primordia.

2. Formation of Inflorescences:

   • Inflorescences are structures that bear floral buds and eventually give rise to flowers.
   • They serve as the framework for the development and arrangement of flowers on the plant.

3. Differentiation of Reproductive Structures:

   • Within the flower, the male and female reproductive structures, known as the androecium and gynoecium, respectively, differentiate and develop.
   • The androecium consists of a whorl of stamens, which represent the male reproductive organs responsible for producing pollen.
   • The gynoecium represents the female reproductive organ and typically consists of one or more carpels, each containing one or more ovules where female gametes are produced.

1.2.1 Stamen, Microsporangium and Pollen Grain  

1. Structure of a Stamen:

   • A stamen consists of two main parts: the filament and the anther.
   • The filament is a long, slender stalk attached to the thalamus or petal of the flower.
   • The anther is a terminal structure at the end of the filament, typically bilobed, and contains pollen sacs.

2. Variation in Stamens Across Species:

   • The number and length of stamens vary among different flower species, leading to a wide range of sizes and shapes.
   • Observation of stamens from various species under a dissecting microscope reveals diversity in shape and attachment of anthers.

3. Microsporangium and Pollen Sac Development:

   • The anther of a typical angiosperm is bilobed, with each lobe containing two microsporangia or pollen sacs.
   • Microsporangia develop further to become pollen sacs, which extend longitudinally throughout the length of the anther and are packed with pollen grains.

4. Structure of Microsporangium:

   • A typical microsporangium appears near-circular in outline in a transverse section.
   • It is surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum.
   • The tapetum nourishes developing pollen grains and often contains multiple nuclei per cell.

5. Microsporogenesis:

   • Microsporogenesis involves meiotic divisions of cells within the sporogenous tissue of the anther.
   • These divisions give rise to microspore tetrads, with each cell having haploid ploidy.
   • Each cell of the sporogenous tissue has the potential to become a microspore mother cell, initiating the process of microsporogenesis.

6. Pollen Grain Structure:

   • Pollen grains serve as male gametophytes.
   • They typically measure 25-50 micrometers in diameter and have a prominent two-layered wall.
   • The outer layer, called the exine, is made of sporopollenin, a highly resistant organic material.
   • The exine exhibits various patterns and designs and contains germ pores.
   • The inner wall, called the intine, is a thin layer made of cellulose and pectin.
   • Mature pollen grains contain two cells: the vegetative cell and the generative cell.

7. Viability and Storage of Pollen Grains:

   • Pollen grains must land on the stigma within a certain timeframe to remain viable for fertilization.
   • Viability of pollen grains varies depending on temperature and humidity, ranging from minutes to months.
   • Pollen grains can be stored for extended periods, even years, in liquid nitrogen, enabling their use in breeding programs.

1.2.2 The Pistil, Megasporangium (ovule) and Embryo sac

1. Structure of the Pistil:

   • The pistil is the female reproductive part of the flower.
   • It may be monocarpellary (consisting of a single pistil) or multicarpellary (with more than one pistil).
   • Each pistil comprises three parts: stigma, style, and ovary.
   • The stigma is the landing platform for pollen grains, the style is the elongated slender part beneath the stigma, and the ovary is the basal bulged part.

2. Structure of the Ovule (Megasporangium):

   • The ovule is attached to the placenta via a stalk called the funicle.
   • It consists of one or two protective integuments surrounding the nucellus, except at the micropyle.
   • The micropyle is a small opening at the tip of the ovule.
   • Within the nucellus is the embryo sac or female gametophyte, derived from a single megaspore.

3. Megasporogenesis:

   • Megasporogenesis involves the formation of megaspores from the megaspore mother cell (MMC).
   • The MMC undergoes meiotic division, resulting in the production of four megaspores.

4. Formation of Female Gametophyte (Embryo Sac):

   • In most flowering plants, only one megaspore becomes functional, while the others degenerate.
   • The functional megaspore develops into the female gametophyte through a series of mitotic divisions.
   • The nucleus of the functional megaspore divides to form two nuclei, leading to the formation of the 2-nucleate embryo sac.
   • Subsequent mitotic divisions result in the formation of the 4-nucleate and 8-nucleate stages of the embryo sac.
   • Cell walls are then laid down, organizing the embryo sac into its typical structure.

5. Structure of the Mature Embryo Sac:

   • The mature embryo sac is 7-celled and 8-nucleate.
   • It contains three cells at the micropylar end called the egg apparatus, consisting of two synergids and one egg cell.
   • At the chalazal end are three antipodal cells, and in the central cell are two polar nuclei.

1.2.3 Pollination

1. Definition and Importance of Pollination:

   • Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.
   • It is essential for bringing together the non-motile male and female gametes for fertilization.

2. External Agents of Pollination:

   • External agents of pollination include:
     • Biotic agents: animals such as bees, butterflies, flies, beetles, wasps, ants, moths, birds, and bats.
     • Abiotic agents: wind and water.

3. Types of Pollination:

   • Autogamy: Pollination occurs within the same flower. Rare in flowers with exposed anthers and stigma.
   • Geitonogamy: Pollen transfer from one flower to another flower on the same plant.
   • Xenogamy: Pollen transfer from one flower to another flower on a different plant, ensuring genetic diversity.

4. Pollination by Wind:

   • Common in grasses and certain other plants.
   • Characteristics include lightweight, non-sticky pollen grains, well-exposed stamens, and feathery stigmas.
   • Wind-pollinated flowers are often not colorful, lack nectar, and produce large quantities of pollen.

5. Pollination by Water:

   • Occurs in about 30 genera, mostly monocotyledons.
   • Pollen grains are carried passively by water currents.
   • Examples include Vallisneria and Hydrilla in freshwater and Zostera in marine environments.

6. Pollination by Animals:

   • Majority of flowering plants use animals as pollinating agents.
   • Common animal pollinators include bees, butterflies, flies, beetles, birds, and bats.
   • Flowers adapted for animal pollination are typically large, colorful, fragrant, and produce nectar.
   • Animal-pollinated flowers often have specific adaptations to attract particular pollinators.

7. Specialized Pollination Relationships:

   • Some plants have evolved specialized relationships with certain pollinators.
   • Examples include mutualistic relationships between yucca plants and yucca moths.
   • Floral rewards may include nectar, pollen, or safe places for egg-laying.

8. Observation and Identification of Pollinators:

   • Observing flowers over time and at different times of the day can help identify potential pollinators.
   • Characteristics of flowers may correlate with the types of animals that visit them.
   • Only visitors that come in contact with the anthers and stigma can bring about pollination, distinguishing them from pollen/nectar robbers.

1. Outbreeding Devices:

   • Majority of flowering plants produce hermaphrodite flowers, leading to the possibility of self-pollination and inbreeding depression.
   • To prevent self-pollination and encourage cross-pollination, plants have developed various mechanisms:
     • Desynchronization of pollen release and stigma receptivity.
     • Physical separation of anther and stigma positions within the flower.
     • Self-incompatibility mechanisms that prevent self-pollen from fertilizing ovules.
     • Production of unisexual flowers, either on separate plants (dioecy) or on the same plant (monoecy).

2. Pollen-Pistil Interaction:

   • Pollination does not always guarantee the transfer of compatible pollen to the stigma.
   • The pistil has the ability to recognize and accept compatible pollen while rejecting incompatible pollen.
   • Chemical components of pollen and pistil mediate this recognition and response.
   • Following compatible pollination, the pollen grain germinates on the stigma, forming a pollen tube that grows through the style to reach the ovary and enter the ovule.

3. Artificial Hybridization:

   • Breeders often cross different species and genera to combine desirable traits.
   • Emasculation involves the removal of anthers from bisexual flower buds before anther dehiscence to prevent unwanted self-pollination.
   • Bagging protects emasculated flowers from contamination with unwanted pollen.
   • Pollination with desired pollen is carried out once the stigma becomes receptive, and the flowers are rebagged to prevent contamination.