Sexual Reproduction in Flowering Plants

Notes

Sexual Reproduction in Flowering Plants

1. Introduction

The organs specialised to perform sexual reproduction in angiosperms are called flowers.

• Flowers are basically modified condensed shoots.
• A typical flower has four whorls of floral leaves.

The four whorls are:

1. Sepals (Calyx)
2. Petals (Corolla)
3. Stamens (Androecium) – Male reproductive part
4. Carpels (Gynoecium) – Female reproductive part

Petals and sepals together form the floral envelope, while stamens and carpels are reproductive parts.

2. Pre-Fertilisation: Structures and Events

3. Male Reproductive Organ – Stamen

Stamen is the male reproductive unit of an angiosperm flower.

It consists of two main parts:

1. A long and slender stalk called filament.
2. A bilobed structure called anther.

The length of stamens is variable in different flowers.

The proximal end of the filament is attached to the thalamus or petal of the flower.

The anther is usually bilobed, with each lobe having two theca; therefore, it is called dithecous.

The anther is four-lobed or tetragonal.

It consists of four microsporangia located at the corners, two in each lobe.

4. Structure of Microsporangium

The microsporangium develops into a pollen sac. It runs longitudinally through the length of an anther.

A microsporangium appears approximately circular in outline and is surrounded by four wall layers:

1. Epidermis
2. Endothecium
3. Middle layers
4. Tapetum

Epidermis

• It is one cell thick.
• It performs a protective function.

Endothecium

• It is a single-layered structure.
• Its cells have fibrous thickening.
• It helps in anther dehiscence at maturity.

Middle Layers

• There are generally 1–5 middle layers.
• They degenerate during the maturity of the anther.

Tapetum

• Tapetal cells are nutritive.
• They provide nourishment to the developing pollen grains.
• The cells of tapetum possess dense cytoplasm.
• They generally have more than one nucleus.

When the anther is young, a group of compactly arranged homogeneous cells called sporogenous tissue occupies the centre of each microsporangium.

5. Microsporogenesis

The cells of the sporogenous tissue undergo meiotic division and form microspore tetrads.

Each cell of the sporogenous tissue that is capable of giving rise to a microspore tetrad is known as a Pollen Mother Cell (PMC) or Microspore Mother Cell.