ICSE 9 Biology Seed Short

⚡ Quick Revision : Classification & External Features

1. Classification Based on Size & Cotyledons

Seeds vary significantly in size, from tiny dust-like seeds of Orchids to the massive seeds of Double Coconut.

• ✔ Monocotyledonous: Contain one cotyledon (Maize, Grass, Wheat).
• ✔ Dicotyledonous: Contain two cotyledons (Bean, Gram, Pea).

2. Classification Based on Endosperm (Food Storage)

This is a high-frequency exam topic. The location of stored food determines the category:

• A. Albuminous (Endospermic) Seeds: The cotyledons are thin and papery; food is stored in the Endosperm.
  Examples: Monocots (Maize, Rice) and some Dicots (Castor, Poppy).
• B. Exalbuminous (Non-endospermic) Seeds: The endosperm is consumed during development; food is stored in the thick Cotyledons.
  Examples: Most Dicots (Gram, Pea, Mango) and a few Monocots (Amorphophallus).

3. External Structure of a Bean Seed

Feature	Description & Importance
Testa	The outermost hard, brownish seed coat. Protects from insects/bacteria.
Tegmen	A thin, white inner layer next to the testa.
Hilum	Distinct whitish scar where the seed was attached to the pod (ovary wall).
Micropyle	Small pore near the hilum. 1. Absorbs water. 2. Provides $O_2$ for respiration.

External Morphology: Bean Seed

❌ Don't Confuse:

The Micropyle in the ovule is for the entry of the pollen tube; in the seed, it is for the entry of water and air.

⚡ Quick Revision : Internal Structure of Dicot Seed (Bean)

1. The Embryo: The Heart of the Seed

When you remove the seed coat of a soaked bean, the seed easily splits into two fleshy Cotyledons. Attached to these is the Embryonic Axis.

• ✔ Radicle: The lower, pointed end of the axis that grows downward to form the Root System.
• ✔ Plumule: The upper, leafy end of the axis that grows upward to form the Shoot System.

Internal Anatomy: Dicot Embryo

2. Defining the Axis: Epicotyl & Hypocotyl

ICSE exams frequently ask for the definitions of these two regions based on their position relative to the cotyledons:

• A. Epicotyl: The region of the embryonic axis above the point of attachment of cotyledons. It terminates in the plumule.
• B. Hypocotyl: The region of the embryonic axis below the point of attachment of cotyledons. It terminates in the radicle.

Exam Insight

In the bean seed, the cotyledons perform two roles:
1. They store food for the embryo.
2. They protect the delicate plumule and radicle during the early stages of life.

❌ Don't Confuse:

The Radicle always emerges first during germination to anchor the plant and absorb water. The Plumule emerges later.

⚡ Quick Revision : Structure of Monocot Seed (Maize Grain)

1. Why is it called a "Grain" and not a "Seed"?

In maize, the fruit wall (pericarp) and the seed coat (testa) are fused together to form a single protective layer. Therefore, technically, a maize grain is a one-seeded fruit.

L.S. of Maize Grain

2. Major Regions of the Maize Grain

Region	Key Details & Function
Endosperm	The largest part; stores starch. The outer layer is the protein-rich Aleurone layer.
Scutellum	The single, shield-shaped cotyledon. It absorbs food from the endosperm.
Coleoptile	A protective sheath covering the Plumule.
Coleorhiza	A protective sheath covering the Radicle.

3. Epithelial Layer

A thin layer separates the endosperm from the embryo. This layer secretes enzymes to convert stored starch into soluble sugars for the growing embryo.

Exam Tip: Technical Terms

Remember that Coleoptile and Coleorhiza are features unique to monocots like maize. They are rarely present in dicots.

❌ Don't Confuse:

The Aleurone layer is for protein storage, while the main Endosperm is for starch storage. Both are triploid ($3n$) tissues.

⚡ Quick Revision : Germination – Definition & Conditions

1. What is Germination?

Germination is the process by which the dormant embryo wakes up, resumes growth, and develops into a seedling. It is essentially the "re-start" of the plant's life cycle after a period of rest.

2. Three Essential Conditions for Germination

For a seed to germinate, three external factors must be present simultaneously. ICSE often asks for the biological reason behind each:

• A. Water (Moisture):
  • Function: Softens the seed coat (making it burst) and converts dry, insoluble starch into soluble sugar for the embryo.
• B. Air (Oxygen):
  • Function: Required for Cellular Respiration. The energy released is used for rapid cell division and growth.
• C. Suitable Temperature:
  • Function: Enzymes are temperature-sensitive. The optimum temperature (usually 25°C to 35°C) is where enzymes are most active.

Experimental Setup: Conditions for Germination

3. Chemical Changes During Germination

Initial Form	Converted Form	Enzyme Involved
Starch (Insoluble)	Maltose/Glucose (Soluble)	Amylase / Diastase
Proteins	Amino Acids	Proteases
Fats	Fatty Acids & Glycerol	Lipases

Critical Note: Light

Light is generally not essential for the early stages of germination. However, it becomes vital once the first leaves (plumule) emerge for photosynthesis.

❌ Don't Confuse:

Very high temperatures (above 45°C) will destroy the enzymes and kill the seed. Low temperatures (near 0°C) only inactivate them (seed stays dormant but alive).

⚡ Quick Revision : Epigeal vs. Hypogeal Germination

1. Epigeal Germination

In this type, the Hypocotyl (region below cotyledons) elongates rapidly and arches upwards.

• ✔ The cotyledons are pushed above the soil surface.
• ✔ The cotyledons become green and act as the first leaves (photosynthetic) before eventually falling off.
• ✔ Examples: Bean, Castor, Tamarind, Cotton, Gourd.

2. Hypogeal Germination

In this type, the Epicotyl (region above cotyledons) elongates rapidly.

• ✔ The cotyledons remain below the soil surface.
• ✔ The plumule is the part that grows out of the soil to form the shoot.
• ✔ Examples: Pea, Maize, Gram, Mango, Rice.

3. Comparison Summary

Feature	Epigeal	Hypogeal
Elongating Part	Hypocotyl	Epicotyl
Cotyledon Position	Above the ground	Below the ground
Cotyledon Function	Becomes photosynthetic	Remains purely for storage

Exam Key

Pay attention to the Bean seed (Epigeal) vs. Pea seed (Hypogeal). Even though both are dicots, their germination types are different.

❌ Don't Confuse:

Don't assume all Monocots are Hypogeal. While Maize is Hypogeal, some Monocots like Onion actually show Epigeal germination!

⚡ Quick Revision : Vivipary & Seed Dormancy

1. Viviparous Germination (Vivipary)

This is a unique type of germination where the seed germinates while still attached to the parent plant. It is an adaptation for plants growing in marshy or salty areas (mangroves).

• ✔ The Process: The embryo grows out of the fruit as a long, heavy seedling. When it falls, it pierces the soft mud and fixes itself instantly.
• ✔ Why? Marshy soil is high in salt and low in oxygen, making it impossible for a normal seed to survive and germinate on the ground.
• ✔ Example: Rhizophora, Sonneratia (Mangrove plants).

2. Seed Dormancy

Even if all environmental conditions (water, air, warmth) are met, some seeds still fail to germinate. This state of "suspended animation" is called Dormancy.

Causes of Dormancy:

• Impermeable Seed Coat: Water or oxygen cannot enter.
• Hard Seed Coat: The coat is too tough for the embryo to break through.
• Immature Embryo: The embryo needs more time to develop.
• Presence of Inhibitors: Chemical substances like abscisic acid prevent growth.

Biological Significance

Dormancy is a survival strategy. It ensures that seeds only germinate when the conditions are favorable for the entire life cycle of the plant, not just the first few days.

3. High-Yield Summary: Seed vs. Fruit

Structure	Seed	Fruit
Origin	Mature Ovule	Ripened Ovary
Protective Layer	Seed Coat (Testa/Tegmen)	Pericarp
Function	Protects/Nourishes Embryo	Protects/Disperses Seeds