Sexual Reproduction in Flowering Plants

 A. Parts of Flowers and it's functions :

1. Calyx (Sepals)

• Sepal :

Green leaf-like structures

Protect flower in bud stage

🌸 2. Corolla (Petals).                               

Petal :

Colored part of flower

Attracts insects for pollination

🌼 3. Androecium (Male Reproductive Part)

👉 Called Stamen

Parts of Stamen:

a) Anther

Produces pollen grains (male gametes)

b) Filament

Supports anther

🌺 4. Gynoecium (Female Reproductive Part)

👉 Called Pistil / Carpel

Parts of Pistil:

a) Stigma

Receives pollen

b) Style

Connects stigma to ovary

c) Ovary

Contains ovules

d) Ovule

Develops into seed after fertilization

🌱 Supporting Parts of Flower

a) Thalamus (Receptacle)

Base where all parts are attached

b) Pedicel

Stalk of the flower

B. Pre - fertilisation : Structures and Events 

1. From Shoot Apex to Flower Bud (Vegetative → Reproductive)

🌿 Shoot Apical Meristem (SAM)

Initially produces leaves and stem (vegetative phase).

Under certain conditions (light, temperature), it transforms into a floral meristem.

🌼 Transition to Flowering

This change is called floral induction.

🌞 Key Hormones Involved

Florigen (flowering hormone; produced in leaves, moves to SAM)

Gibberellins (GA) → promote flowering in some plants

Cytokinins → help cell division in buds

Auxin → organ development

2. Formation of Flower Buds

🌸 Structural Changes

SAM becomes dome-shaped (active growth)

Produces floral primordia instead of leaf primordia

Bud develops into a flower

🌺 Parts Formed

Sepals

Petals

Stamens (male)

Carpels (female)

C. Stamen, Microsporangium and Pollen Grain :

🌼 1. Stamen (Male Reproductive Part)

The stamen is the male reproductive organ of a flower.

✳️ Structure:

It has two main parts:

(a) Filament

• A long, thin stalk
• Supports the anther
• Attaches the stamen to the flower (thalamus or petal)

(b) Anther

• The terminal (top) part
• Responsible for producing pollen grains
• Usually bilobed (has two lobes)

🌼 2. Anther (Detailed)

The anther is the most important part of the stamen because it produces pollen.

✳️ Key Features:

• It is usually bilobed (2 lobes)
• Each lobe contains 2 theca
• Therefore, it is called dithecous
• 👉 A longitudinal groove often separates the two lobes.

✳️ Internal Structure:

• The anther is four-sided (tetragonal)
• It contains 4 microsporangia (two in each lobe)

🌼 3. Microsporangium

A microsporangium is a small chamber inside the anther where pollen develops.

✳️ Important Points:

• There are 4 microsporangia in one anther
• Located at the corners of the anther
• They later develop into pollen sacs

🌼 4. Pollen Sac

• A microsporangium develops into a pollen sac
• It contains many pollen grains
• These sacs extend along the length of the anther

🌼 5. Structure of Microsporangium

In cross-section, a microsporangium appears circular.

✳️ It has 4 wall layers:

1. Epidermis

• Outermost layer
• Protects the structure

2. Endothecium

• Helps in dehiscence (opening of anther)
• Assists in releasing pollen

3. Middle Layers

• Located between endothecium and tapetum
• Provide support
• Usually degenerate later

4. Tapetum (Most Important)

• Innermost layer
• Provides nutrition to developing pollen grains

🌼 6. Tapetum (Detailed)

The tapetum plays a crucial role in pollen development.

✳️ Functions:

• Supplies nutrients to pollen grains
• Helps in pollen wall formation

✳️ Special Characteristics:

• Cells have dense cytoplasm
• Often contain more than one nucleus (binucleate or multinucleate)

❓ Why binucleate?

Because the nucleus divides but the cell does not divide completely

🌼 7. Sporogenous Tissue

Located in the center of microsporangium

Made up of homogeneous, compact cells

✳️ Function:

• These cells act as mother cells
• They will form microspores (pollen grains)

🌼 8. Microsporogenesis

This is the process of formation of microspores (pollen grains).

✳️ Steps:

• Sporogenous cells develop into microspore mother cells (MMC)
• Each MMC undergoes meiosis (reduction division)
• This produces 4 haploid microspores
• These 4 cells remain together and form a tetrad

❓ Important Concept (Exam Question)

👉 What is the ploidy of tetrad cells?

Answer: Haploid (n)

Because they are formed by meiosis

🌼 9. Pollen Grains

• Microspores develop into pollen grains
• These are the male gametophytes

✳️ Function:

• Carry male gametes for fertilization

🎯 Final Summary (Very Important)

• Stamen = male part of flower
• Anther = bilobed, dithecous
• 4 microsporangia present
• Microsporangia → pollen sacs
• Tapetum = nutrition provider
• Sporogenous tissue → forms pollen
• Meiosis → 4 haploid microspores (tetrad)