STRUCTURE & TYPES OF OVULE

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University: Rabindranath Tagore University (RTU), Hojai, Assam

Course: B.Sc. Botany (Honours)

Subject: Morphology, Embryology & Anatomy of Angiosperms (BOT-MAJOR-2)

🔵 UNIT III: Ovule, Fertilization & Embryology
STRUCTURE & TYPES OF OVULE

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1️⃣ INTRODUCTION — What Exactly Is an Ovule?

The ovule is an integumented megasporangium present inside the ovary of angiosperms.

After fertilization → Ovule transforms into Seed.

🧠 How to Think About It:

Ovule = Future Seed
Ovary = Future Fruit

Always connect structure with destiny.

🌱 "Inside the ovary sleeps the seed of tomorrow."

2️⃣ STRUCTURE OF A TYPICAL ANGIOSPERM OVULE

A typical ovule consists of:

Funiculus + Hilum + Integuments + Nucellus + Embryo Sac + Micropyle + Chalaza

🧠 Visualization Strategy:

Imagine the ovule like a small protected chamber with an entry gate.
Outside → protection Inside → reproduction Apex → entry Base → nutrient supply

🔶 1. FUNICULUS — The Lifeline

• Stalk attaching ovule to placenta • Conducts nutrients • When fused with body → forms Raphe (in anatropous ovule)

🧠 Think:
Funiculus = Umbilical cord of the ovule

📌 No funiculus → No nutrient flow.

🔶 2. HILUM — The Scar of Attachment

• Point where funiculus attaches to ovule • Visible as a scar in seed

🧠 Hilum = "Healing Mark"
After detachment, it leaves a visible scar.

🔶 3. INTEGUMENTS — Protective Coverings

• Surround nucellus • Usually two → Bitegmic • Sometimes one → Unitegmic • Leave small opening called Micropyle

🧠 Think of integuments as:
"Security layers around a biological treasure."

🔐 Two coats = Bitegmic 🔐 One coat = Unitegmic

🔶 4. MICROPYLE — The Gateway

• Small opening at apex • Entry point of pollen tube • Essential for fertilization

🧠 Micropyle = Door for male gamete

No micropyle → No entry → No fertilization

🚪 "Life enters through a tiny door."

🔶 5. NUCELLUS — The Nutrient Core

• Central parenchymatous tissue • Contains Megaspore Mother Cell (MMC) • Provides nourishment

🧠 Nucellus = Biological kitchen
It feeds and houses the developing female gametophyte.

🔶 6. CHALAZA — The Opposite Pole

• Located opposite micropyle • Region where integuments & nucellus meet • Vascular supply enters here

🧠 Micropyle = Entry Chalaza = Supply base

Always think in terms of polarity.

🔶 7. EMBRYO SAC — The Female Gametophyte

Develops inside nucellus.

Contains:

1. Egg cell
2. Synergids
3. Antipodals
4. Polar nuclei (Central cell)

🧠 Standard Angiosperm Pattern:
7 cells, 8 nuclei

📌 3 + 2 + 2 = 7 cells 8 nuclei rule — Always remember for exams.

3️⃣ TYPES OF OVULE (Based on Orientation)

Ovules are classified based on the position of micropyle in relation to funiculus.

Always focus on alignment:
➤ Is it straight? ➤ Is it inverted? ➤ Is it curved?

🧠 Core Thinking Strategy:

Imagine a vertical axis passing through:
Micropyle → Embryo sac → Chalaza → Funiculus

Now ask:
Are they in one line? Are they inverted? Are they bent?

📌 Orientation = Spatial Biology.
See the structure in 3D inside your mind.

🔷 1. ORTHOTROPOUS OVULE (Atropous)

Straight Ovule

• Micropyle, chalaza & funiculus lie in one straight line • No curvature • Body remains upright

🧠 Think:
Orthotropous = Original straight position

"Ortho" means straight.

Occurrence:
• Primitive angiosperms • Some gymnosperms

📏 Ortho = Straight line.
Micropyle ↑ Chalaza ↑ Funiculus (All aligned)

🔷 2. ANATROPOUS OVULE (Most Common Type)

Inverted Ovule

• Ovule turns 180° during development • Micropyle lies close to hilum • Funiculus fuses with body → forms Raphe

🧠 Think:
"Ana" = Upward / Backward turn.

It flips over during development.

This is the MOST COMMON ovule type in angiosperms.

Found in majority of flowering plants.

🔄 Anatropous = Turned ovule.
Most common → Always mention in exams.

🔷 3. CAMPYLOTROPOUS OVULE

Slightly Curved Ovule

• Ovule is curved • Embryo sac also becomes curved • Micropyle & chalaza not in straight line

🧠 Think:
"Campylo" = Bent / Curved.

Not fully inverted, just bent.

Occurrence:
• Found in some dicot families

🌙 Campylotropous = Crescent-shaped.
Slight bend, not full flip.

🔎 COMPARATIVE THINKING TABLE

Feature	Orthotropous	Anatropous	Campylotropous
Shape	Straight	Inverted	Curved
Alignment	All in one line	Micropyle near hilum	Not in straight line
Curvature	Absent	180° inversion	Slight bend
Occurrence	Primitive plants	Most angiosperms	Some dicots

📌 Straight → Orthotropous 🔄 Turned → Anatropous 🌙 Bent → Campylotropous

4️⃣ COMPARATIVE TABLE OF OVULE TYPES

Classification is based on orientation of the ovule and the relative position of micropyle and funiculus.

Feature	Orthotropous	Anatropous	Campylotropous
Orientation	Straight	Completely inverted	Slightly curved
Micropyle Position	Opposite funiculus	Near hilum	Slightly displaced
Raphe	Absent	Present	May be present
Occurrence	Rare	Most common	Moderate

🧠 Pattern Recognition:

Straight → No raphe → Rare Inverted → Raphe present → Most common Curved → Partial modification → Moderate occurrence

5️⃣ TYPES OF OVULE (Based on Number of Integuments)

Classification here is based on the number of protective coverings (integuments) surrounding the nucellus.

Ask yourself:
How many protective layers surround the megasporangium?

🧠 Thinking Framework:

Integuments = Protective coats.
More coats → More protection.
Fewer coats → Primitive or modified condition.

🔹 1. UNITEGMIC OVULE

• Single integument surrounds the nucellus.

Occurrence:
• Common in some dicots.

🧠 "Uni" means one.
One protective coat.

1 coat → Unitegmic.

🔹 2. BITEGMIC OVULE

• Two integuments surround the nucellus.

This is the most common condition in angiosperms.

🧠 "Bi" means two.
Double protection around the female gametophyte.

2 coats → Bitegmic → Most angiosperms.

🔹 3. ATEGMIC OVULE

• No integument present.

Occurrence:
• Very rare condition.

🧠 "A-" means absence.
No protective covering.

A = Absent → Ategmic.

6️⃣ FUNCTIONS OF OVULE PARTS

Every structural part has a precise physiological role.

Structure always connects to function.

Part	Function
Funiculus	Nutrient conduction
Integuments	Protection
Micropyle	Pollen tube entry
Nucellus	Nutrition & megaspore development
Embryo sac	Fertilization
Chalaza	Vascular supply

🧠 Functional Flow:

Chalaza → Nutrients enter Funiculus → Nutrients conducted Nucellus → Nourishment Micropyle → Entry of pollen tube Embryo sac → Site of fertilization Integuments → Protection throughout

📌 Protection + Nutrition + Entry + Fertilization
These four ideas summarize ovule function.

7️⃣ DEVELOPMENTAL SIGNIFICANCE OF OVULE

Ovule formation is not just structural development — it marks the beginning of the female reproductive phase in angiosperms.

🧠 Deep Conceptual Thinking:

When ovule forms, three major biological events are initiated:

1️⃣ Beginning of female gametophyte development
2️⃣ Site of megasporogenesis
3️⃣ Preparation for double fertilization

✔ Ovule = Structural foundation ✔ Embryo sac = Functional reproductive unit ✔ Fertilization = Future seed formation

📌 No ovule → No megaspore → No embryo sac → No seed.

Ovule is the biological starting point of seed life.

🧠 Always think in developmental sequence:

Structure appears → Cells differentiate → Meiosis occurs → Gametophyte forms → Fertilization happens.

8️⃣ DIAGRAMMATIC DEVELOPMENTAL FLOW OF OVULE

Ovule development follows a precise chronological order.

🧠 Visual Flow (Step-by-Step Biological Sequence):

Placenta ↓ Funiculus develops ↓ Integuments form ↓ Nucellus differentiates ↓ Megaspore Mother Cell (MMC) forms ↓ Meiosis → Megaspore formation ↓ Embryo sac formation ↓ Ovule ready for fertilization

Think of it like constructing a biological house:

Foundation (Placenta) → Support (Funiculus) → Walls (Integuments) → Central chamber (Nucellus) → Reproductive cell (MMC) → Gametophyte (Embryo sac)

📌 Order is everything.
Development always moves from outer structure → inner specialization → reproductive readiness.

Final Insight:
Ovule is not a static structure — it is a dynamic developmental system preparing for double fertilization.

9️⃣ EXAM-ORIENTED IMPORTANT POINTS

These are high-yield facts frequently asked in university exams.
Revise them as rapid-fire recall points.

🧠 How to Study This Section:

Do not just read — convert each line into a quick mental question.
Example: "What is an ovule?" → Immediate answer.

✔ Ovule = Integumented megasporangium

✔ Anatropous ovule is most common

✔ Raphe present only in anatropous ovule

✔ Micropyle is entry point of pollen tube

✔ Bitegmic ovules common in angiosperms

✔ Chalaza is opposite to micropyle

📌 Rapid Recall Pattern:

Structure → Orientation → Entry → Protection → Position.

If you can recall these in 20 seconds, you are exam-ready.

Final Tip:
These are definition-level statements — write them exactly and precisely in exams.

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