Chapter:2

What are Sexual Reproduction In Flowering Plants?

Flower

It is the reproductive organ of plants. It consists of 4 whorls:

  • Calyx (outermost)
  • Corolla (next to calyx)
  • Androecium (next to corolla)
  • Gynoecium (innermost)

Androecium (Male Reproductive Part)

  • Stamen consist of anther and filament.
  • Anther has two lobes (bilobed). Each lobe consists of two theca (dithecous) and filament (long slender stalk). Proximal end of the filament is attached to the thalamus or petal.
  • Sporogenous tissue is compactly arranged homogenous cells present at center of each microsporangium when the anther is young.

A typical microsporangium is surrounded by 4 walls

  • Epidermis: Protects and help in dehiscence of anther.
  • Endothecium: Layer below epidermis.
  • Middle layers: Layers below endothecium.
  • Tapetum: Nourishes the developing pollen grain.

Pollen Grains

Pollen grains/male gametophyte - spherical in shape and made up of 2 layers+ 1 Germ Pore+ 2 Cells

  • Exine
  • Intine
  • Germ Pores
  • Cells
  • Vegetative Cell: Bigger and have abundant food reserve which is responsible for the development of pollen grain.
  • Generative Cell: Small and involves in syngamy (fuse with an egg). Its function is formation of male gametes.

Gynoecium:(Female Reproductive Part Of Flower)

  • Gynoecium with one is pistil – Monocarpellary, Gynoecium with more than one pistil is – Multicarpellary, Fused pistil – Syncarpous , Free pistil – Apocarpous
  • Pistil (Female Reproductive Part) consist of: Stigma, Style, Ovary.
  • Ovule is a small structure attached to placenta. The parts of the ovule are: Funicle, Hilum, Integuments, Micropyle, Chalaza, Nucellus.
  • Process of formation of megaspores from megaspore mother cells is called megasporogenesis.
  • Megaspore mother cells (2n) meiotically
  • ↓
  • 4 megaspores (haploid)
  • Only 1 megaspore is functional and forms gametophyte and the rest 3 degenerates.
  • The embryo sac develops from the functional megaspore (n).
  • Egg appartus-present at the micropylar end and consists of 2 synergids and 1 egg cell.

Pollination

The transfer of pollen grains from anther to stigma of a pistil is called pollination.

Based on the Source of Pollen, Pollination is of 3 types

  • Autogamy: Transfer of pollen grains from anther to stigma of the same flower.
  • Geitonogamy: Transfer of pollen grains from anther to stigma of another flower of the same plant, Genetically similar. Ex: cucurbits
  • Xenogamy: Transfer of pollen grains from anther to stigma of another flower of different plant.

Agents Of Pollination

  • Abiotic agents: Wind and Water.
  • Biotic agents: Insects, Birds, Bats, Reptiles and Mammals

Ways to avoid Self-pollination

  • Release of pollen grains and receptivity of stigma are not synchronized.
  • Stigma and anther are placed at different positions.
  • Self-incompatibility.
  • Production of unisexual flowers (prevents autogamy but not geitonogamy). e.g. castor, maize.
  • Male and Female flowers are produced on different flowers (prevents autogamy & geitonogamy). e.g. papaya.

Steps Of Pollen: Pistil Interaction

Recognition of compatible pollen→ Germination of pollen →grains Development of Male Gametophyte

Major Post- Fertilization Events

  • Development of embryo
  • Development of endosperm
  • Maturation of ovule into seed
  • Ripening of ovary into fruit

Two Types of Endosperm Development

  • Free Nuclear Type: Nuclear division is not followed by cell wall formation and nuclei remain free e.g., coconut water.
  • Cellular Type: Division of nucleus is immediately followed by cell wall formation.

Embryo

Zygote → A Proembryo → A Mature embryo (heart-shaped)

It consists of: Embryonal Axis, Cotyledons, Plumule and Radicle.

Seed

Fertilized and mature ovule develops into seed.

  • Seed consists of: Cotyledon(s), Embryonal Axis, Seed coat, Testa (outer coat), Tegmen (inner coat).
  • Seed are of two types: Albuminous and Non-Albuminous.
  • Perisperm : remnants of nucellus that is persistent. Ex: Black pepper.
  • Dormancy: state of inactivity.

Fruit

  • True Fruit: Developed from ripened ovary.
  • Parthenogenesis: Ovary gets transformed to fruit without fertilization. Ex: Banana.
  • Parthenocarpy: Seedless fruit which is formed due to gibberellins & auxins.
  • False Fruit: Any part of flower other than ovary forms the fruit. Ex: Apple.

Other Modes of Reproduction

  • Apomixis: Apomixis: Form of asexual reproduction where seed is formed without fertilization. Ex. Citrus and Mango.
  • Polyembryony: Occurrence of more than one embryo in a seed. Ex: Citrus, groundnut.