Notes on "Sexual Reproduction In Flowering Plants" Part-1

 Notes on "Sexual Reproduction In Flowering Plants" Part-1



by:-Nagraj Sir


  • Flowering plants show the sexual mode of reproduction, and flowers are sites of sexual reproduction.
  • Flowers function as a reproductive organ of plants, and the end product of sexual reproduction is fruits and seeds.
  • All flowering plants show sexual reproduction.
Flowers are the site of sexual reproduction in flowering plants.

(i) A flower has following parts arranged in four whorls, i.e. calyx (sepals), corolla (petals), androecium and gynoecium. 
(iii) Flowers may contain both male (stamens) and female (carpels or pistils) reproductive parts androecium (male reproductive organ) and gynoecium (female reproductive organ)

Pre-fertilisation: Structures and Events

• Several structural and hormonal changes lead to formation and development of the floral primordium. Inflorescence is formed that bears floral buds and then flower.

• In flowers, male (androecium) and female (gynoecium) differentiate and develops in which male and female gametes are produced.

Stamen, Microsporangium and Pollen Grain :

• Stamen consists of long and slender stalk called filament and generally bilobed anthers. Each lobe contains two theca (dithecious).

• The anther is four-sided structure consisting of four microsporangia, two in each lobes.

• Microsporangia develop further and become pollen sacs which contain pollen grains.• Microsporangium is generally surrounded by four layered walls- the epidermis, endothecium, middle layer and tapetum. Innermost layer tapetum nourishes the developing pollen grains.

• Sporogenous tissues- It is compactly arranged homogenous cells which are present at centre of each microsporangium when the anther is young.

Microsporogenesis- The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis.

• The cells of sporogenous tissues undergo meiotic division to form microspore tetrad. As the anther mature and dehydrate, the microspore dissociate and develops into pollen grains.

Pollen grain represents the male gametophytes. Pollen grains are made of 2 layered Wall,

1. Exine (outer layer):- Made of sporopollenin- most resistant organic matter known.It can withstand high temperatures and strong acids and alkali. No enzyme can degrade sporopollenin

2. Intine (inner layer):--Thin and continuous layer, Made of cellulose and pectin

MATURE POLLEN 
— A mature pollen consist of 2 cells with nucleus (Vegetative and Generative)

VEGETATIVE CELL

  • Bigger
  • Abundant food reserve
  • Large irregular nucleus
  • Responsible for the development of pollen grain

GENERATIVE CELL

  • Small
  • Involves in syngamy (fuse with an egg)
  • Dense cytoplasm and nucleus

The Pistil, Megasporangium (Ovule) and Embryo sac

• Gynoecium may consists of single pistil (monocarpellary) or more than one pistil (polycarpellary) which may be fused (syncarpous) or free (apocarpous).

• Each pistil has three parts the stigma, style and ovary. Inside the ovary is ovarian cavity (locule). The placenta is located inside the ovarian cavity. Megasporangia (ovules) arise from placenta.

Megasporangium (ovule)

  • Ovule is a small structure attached to placenta.
  • Funicle – stalk by which ovule is attached to placenta
  • Hilum- junction between ovule and funicle
  • Integuments- protective envelops
  • Micropyle- small opening at the tip of ovule into where pollen tube enters
  • Chalaza- basal part of ovule
  • Nucellus (2n)-mass of cells enclosed in integuments. Has abundant food reserve.


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