Notes – Seed: Structure, Types & Germination

Life of a Plant Starts with a Seed:

Every plant begins its life as a seed.
A seed is crucial for the development of a new plant.

Seed Formation:

A seed develops from an ovule.
This happens after fertilization (the fusion of male and female gametes).

Dormancy in Seeds:

Under unfavourable conditions (like lack of water or suitable temperature), seeds enter a dormant state.
In dormancy, the seed is inactive and does not germinate.

Germination in Favourable Conditions:

When favourable conditions (like proper moisture, warmth, and air) are available, the seed begins to germinate.
Germination is the process where the seed starts to grow into a new plant.

Food Storage in Seeds:

During the initial stages of germination, the seed uses the food stored within it to grow.
This stored food helps the seedling develop until it can perform photosynthesis.

Definition of Seed:

A seed is the ripened ovule of a plant.
It contains an embryo that germinates into a new plant.

Parts of a Seed:

A mature seed consists of two essential parts:

Seed coat
Embryo

Seed Coat:

A protective outer covering of the seed, developed from the ovule's integuments.
The seed coat has two layers:

Testa: Outer layer
Tegmen: Inner layer

Hilum: A scar on the seed, marking the attachment point to its stalk.
Micropyle: A small opening near the hilum, helps in absorbing water during germination and allows the diffusion of respiratory gases.

Embryo:

The young or miniature plant enclosed within the seed coat.
The embryo consists of five distinct parts:

Cotyledons: Seed leaves; two in dicots, one in monocots.
Plumule: Gives rise to the shoot upon germination.
Epicotyl: The part of the embryonic axis above the cotyledon attachment, holds the plumule.
Hypocotyl: The part of the embryonic axis below the cotyledon attachment.
Radicle: Located at the tip of the hypocotyl, develops into the root during germination.

Dicotyledonous Seeds:

Have two cotyledons.
Can be either exalbuminous (without stored food, e.g., pea, gram, bean) or albuminous (with stored food, e.g., castor, cotton).

Monocotyledonous Seeds:

Have one cotyledon.
Can be either exalbuminous (e.g., Alisma) or albuminous (e.g., maize, wheat, rice).

Germination Process:

Radicle: Develops into the root.
Plumule: Develops into the shoot.

Food Storage in Seeds

Food Storage Locations:

In seeds, food is stored in two main places:

Cotyledons: The first leaves of the seed that store food.
Endosperm: A special tissue that also stores food.

Types of Seeds Based on Food Storage:

Non-Endospermic (Exalbuminous) Seeds:

Example seeds: Gram, Pea, Bean, Mustard.
Food is stored in cotyledons.
Endosperm is absent, indicating that it has been fully used by the developing embryo.

Endospermic (Albuminous) Seeds:

Example seeds: Castor, Maize, Rice.
Food is primarily stored in the endosperm.
These seeds contain a significant amount of stored food for the embryo's development.

Structure of Fruits and Seeds:

Fruit:

It is the enlarged, ripened ovary of a flower.
The outer layer of the ovary forms the fruit wall, which encloses the seed.
The fruit protects the seed and aids in its dispersal.

Seed:

It is the ripened ovule that contains the embryo, which will develop into a future plant.
The seed coat serves as a protective layer for the embryo.

Grain:

It is a fruit where the fruit wall and the seed coat are fused together, creating a protective layer around the seed.

Structure of Dicotyledonous Bean Seed

Shape and Covering:

The bean seed is kidney-shaped.
It is covered by a hard seed coat.

Seed Coat Layers:

The seed coat has two layers:

Testa: The outer layer of the seed coat.
Tegmen: The inner layer of the seed coat.

These layers are fused together to provide protection to the embryo.

Hilum:

Located on the concave side of the seed.
It is a whitish oval scar where the seed was attached to the pod via a structure called the funicle.

Micropyle:

A small opening located close to the hilum.
It allows the seed to absorb water during germination.
It also serves as a passage for the diffusion of respiratory gases.

Raphe:

An elongated ridge found on the testa at the other end of the hilum.

Inner Structure:

When the seed coat is peeled off, a white fleshy body is revealed, consisting of:

Two fleshy cotyledons: These store food for the plant.
Short axis: To which the cotyledons are attached.

Axis Components:

The axis consists of:

Radicle: Located outside the cotyledons; it develops into the root system.
Plumule: Develops into the shoot system.

The axis is divided into:

Hypocotyl: The part below the cotyledon attachment.
Epicotyl: The part above the cotyledon attachment.

Structure of Monocotyledonous Maize Grain (Zea mays)

Type of Fruit:

Maize is a one-seeded fruit known as a caryopsis.
The fruit wall (pericarp) is fused with the seed coat.

External Appearance:

The maize grain has a flat, almost triangular, and oblong shape.
It has a large yellowish upper area indicating the position of the endosperm.
The lower side contains a small whitish area which houses the embryo.

Seed Coat:

The seed coat is not distinct as it is fused with the fruit wall (pericarp).
Micropyle and hilum are also not distinguishable.

Internal Structure:

In a longitudinal section, it reveals two distinct regions:

§ Upper region: Endosperm

§ Lower region: Embryo

Endosperm:

The endosperm is surrounded by a one-cell thick layer called the aleurone layer.
It contains aleurone grains, which are proteinaceous in nature.

Embryo Structure:

The embryo consists of:

§ Single Cotyledon called scutellum: it is shield-shaped & flat, closely pressed against the endosperm.

§ Function: Absorbs food from the endosperm and transfers it to the growing parts of the embryo.

§ Plumule: The growing tip of the shoot, accompanied by a few embryonic leaves.

§ It is covered by a sheath called coleoptile.

§ Radicle: Located at the base of the grain and covered with a sheath called coleorrhiza.

§ Hypocotyl: Very short and represented by a short axis between the radicle and plumule.

Seed Germination

Seed Condition at Shedding:

Most seeds are shed from the parent plant in a dry condition.
In this state, physiological activities are very slow, and minimal food is utilized.
Respiration in seeds is also slow during this dry phase.

Dormancy:

Seeds can remain alive and dormant for extended periods when dry.
Some seeds can germinate only a few days after being shed, after which they lose the ability to germinate.
Dormant seeds do not germinate even when provided with all necessary conditions for germination. This condition is known as dormancy.

Germination Process:

Germination is the process where the food reserves in a seed are broken down, allowing the embryo to start growing.
It begins when the seed receives favourable conditions (e.g., water, warmth, and oxygen).

Irreversibility of Germination:

The process of germination is irreversible.
Once germination starts, it cannot stop, and the seed cannot return to the dormant state.

Key Terms:

Dormancy: A state in which seeds remain alive but do not germinate.
Germination: The process of growth from a seed into a new plant.

Conditions Necessary for Seed Germination

Water

Rapid Uptake: Germination begins with the absorption of water through the micropyle and seed surface.
Functions of Water:

Softens the seed coat, allowing it to rupture for plumule (shoot) and radicle (root) emergence.
Accelerates metabolic activities within the seed.
Aids in hydrolysis, converting stored food into soluble products.
Facilitates translocation of soluble nutrients from storage tissues to the developing embryo.

Hastening Germination:

Soaking seeds in water before sowing.
Removing the seed coat.

Availability of Oxygen

Energy Requirement: The embryo needs energy to resume growth during germination.
Source of Energy: Energy is obtained through the oxidation of food stored in cotyledons or endosperm, which requires oxygen.

Favourable Temperature

Physiological Processes: Various physiological processes take place during germination; therefore, a suitable temperature is essential.
Optimum Temperature Range:

Varies by seed type, but most seeds do not germinate below 0°C or above 48°C.
Ideal temperature range for most seeds: 25°C to 35°C.

Light Conditions

Some seeds require light for germination, while others germinate only in darkness.

The Germination Process

Definition: Germination is the process by which a seed develops into a new plant.
Initiation:

Germination begins with the rapid uptake of water by the seed through a small opening called the micropyle.
This water absorption causes the seed to swell, resulting in an increase in weight.

Physical Changes:

The seed coat becomes soft due to the absorption of water.
The first visible sign of germination is the swelling of the seed.

Physiological Changes:

The absorption of water triggers several physiological changes in the seed.
There is an increase in respiratory activity in the germinating seeds.

Enzyme Production:

The embryo inside the seed produces enzymes that convert stored food materials (found in cotyledons or endosperm) into soluble forms.
These soluble forms are essential for the growing embryo's nutrition.

Cell Division:

Once food becomes available, cell division begins in the growing embryo, particularly in two parts:

Radicle (the embryonic root)
Plumule (the embryonic shoot)

Emergence of Structures:

The growth of the embryo tissues causes the seed coat to rupture.
The radicle is the first part to emerge from the seed coat, forming the root system, which grows downwards into the soil.
After the radicle, the plumule emerges and develops into the shoot of the plant.

Result of Experiments on seed Germination: (Refer to your book for the experiments).

Result (Experiment 1) - Pyrogallic acid absorbs oxygen from the air. Without Oxygen, seeds do not germinate in Flask A.
Result (Experiment 2) - Adequate water must be present for germination to start and progress. Excess water prevents germination, probably by excluding oxygen.
Result (Experiment 3) – Suitable temperature is necessary for germination.

Three bean seeds experiment (Refer to your book for the experiment)

The top seed does not germinate because it gets only oxygen and no water.
The bottom seed receives water but very little oxygen.
The middle seed germinates properly as it receives both oxygen and water.

Types of Seed Germination

- Seed germination is the process by which a seed develops into a new plant.

- The behaviour of the cotyledons (the first leaves) during germination distinguishes two main types of seed germination:

1. Epigeal Germination

2. Hypogeal Germination

Epigeal Germination

Definition: A type of germination where the cotyledons are lifted above the ground.
Mechanism:

Occurs due to the rapid elongation of the hypocotyl (the part of the embryonic axis just below the cotyledons).

Examples: Seen in seeds such as:

Kidney bean, Castor, Cotton, Sunflower, Tamarind

Root System:

The primary becomes the main root.

Outcome: The cotyledons are carried above the soil surface during germination.

Hypogeal Germination

Definition: A type of germination where the epicotyl elongates quickly.
Mechanism:

The epicotyl (the part of the embryonic axis above the cotyledons) pushes the plumule (young shoot) up into the air.
The cotyledons remain on or below the ground.

Examples: Observed in seeds like:

Gram, Pea, Mango, Maize

Root System:

The primary root does not become the main root.
Adventitious roots develop close to the primary root, forming a fibrous root system.

Viviparous Germination

Definition: A special type of germination where seeds germinate while still attached to the parent plant.
Example: Common in mangrove plants, such as:

Rhizophora, Sonneratia

EXERCISES

1. Multiple Choice Questions (MCQs)

(i) How is a seed formed?

Answer: (a) It develops from an ovule after fertilisation.

(ii) A mature seed consists of the following parts:

Answer: (c) Seed coat and embryo

(iii) The embryo of a mature seed consists of:

Answer: (d) Cotyledons, plumule, epicotyl, hypocotyl, and radicle.

(iv) Cotyledons are also known as:

Answer: (a) Seed leaves.

(v) The fruit called caryopsis is found in:

Answer: (c) Maize.

(vi) Aleurone layer present in maize seed is rich in:

Answer: (c) proteins.

(vii) Epigeal germination is seen in which of the following seeds?

Answer: (b) Gram.

(viii) Which type of germination takes place in mango?

Answer: (a) Hypogeal germination.

(ix) Which of the conditions are essential for seed germination?

Answer: (d) All of these (Water, Oxygen, Optimum temperature).

(x) The first visible indication of germination is:

Answer: (a) The swelling of the seed.

(xi) Assertion (A): The bean seed is kidney-shaped and remains covered by a hard seed coat. Reason (R): Bean seed is dicotyledonous.

Answer: (a) Both A and R are true.

(xii) A bean seed is placed in a moist paper towel and kept in a warm, dark place. A few days later, the seed germinates, and a small plant begins to grow. Which of the following is/are involved in the process described above?

Answer: (e) All the above

2. Definitions

(i) Embryo: The embryo is the young plant that develops from the fertilized ovule and consists of the radicle, cotyledons, and plumule.

(ii) Exalbuminous seed: An exalbuminous seed is a seed that does not contain endosperm. The cotyledons store the food and are fleshy.

(iii) Germination: Germination is the process by which a seed develops into a new plant when the conditions are favourable.

(iv) Seed dormancy: Seed dormancy is a period during which seeds remain alive, but do not germinate, even under favourable conditions.

3. Functions of the Following Parts

(i) Coleoptile: The coleoptile is the protective sheath covering the emerging shoot of a seedling and helps it grow upwards through the soil.

(ii) Coleorrhiza: The coleorrhiza is the protective sheath surrounding the radicle in monocot seeds, aiding in its emergence.

(iii) Aleurone layer: The aleurone layer stores proteins and enzymes and plays a crucial role in the early stages of germination by providing nutrients.

(iv) Micropyle: The micropyle is a small opening in the seed coat through which water and oxygen can enter, facilitating the germination process.

(v) Endosperm: The endosperm provides nutritional support to the developing embryo, supplying it with the necessary energy for growth.

(vi) Plumule: The plumule is the part of the embryo that develops into the shoot and leaves of the new plant.

4, Differentiate between the following:

(i) Epicotyl and hypocotyl

Question	Epicotyl	Hypocotyl
Definition	The part of the seedling above the cotyledons.	The part of the seedling below the cotyledons.
Location	Located above the cotyledons.	Located below the cotyledons & above the radicle.
Function	Develops into the stem & leaves of the plant.	Supports the plant & connects the radicle to the epicotyl.
Appearance	Typically elongated & greenish in color.	Generally short & cylindrical.

(i) Radicle and plumule

Question	Radicle	Plumule
Definition	The embryonic root of the seed.	The embryonic shoot of the seed.
Location	Located at the lower end of the seed.	Located at the upper end of the seed.
Function	Develops into the primary root of the plant.	Develops into the stem and leaves of the plant.
Appearance	Generally straight and may be fleshy.	Generally short and may have leaf-like structures.

(iii) Bean Seed and Maize seed.

Feature	Kidney Bean Seed	Maize Grain
1. Seed Coat	- Thin and brownish in colour.	- Very thin and fused with the pericarp.
2. Hilum & Micropyle	- Both are visible.	- Both are not visible.
3. Cotyledons	- Contains two thick fleshy cotyledons.	- Contains a single cotyledon.
4. Endosperm	- Absent in the seed.	- Present as a part of the seed.

5 Answer the following:

(i). Describe the structure of a seed.

A seed typically consists of three main parts:

Seed Coat: The outer protective layer that shields the seed from damage and prevents water loss.
Cotyledons: These are the seed leaves. In dicot seeds, there are usually two cotyledons, while monocot seeds have one. Cotyledons provide nourishment to the developing plant during germination.
Embryo: The part of the seed that develops into the new plant. It contains the embryonic root (radicle), which will grow into the root, and the embryonic shoot (plumule), which will develop into the stem and leaves.

(ii). What conditions are essential for seed germination? Describe their importance.

The essential conditions for seed germination include:

Water: It is necessary for the seed to swell and soften the seed coat. Water activates enzymes that begin the metabolic processes necessary for growth.
Oxygen: Seeds require oxygen for respiration. This process provides energy needed for growth and development.
Temperature: Most seeds require a specific temperature range (usually warm) to germinate effectively. Optimal temperatures enhance enzyme activity and metabolic processes.
Light (for some seeds): Certain seeds need light to germinate, while others do not. Light can trigger specific germination processes in some species.

(iii). Differentiate between hypogeal and epigeal germination.

Aspect	Epigeal Germination	Hypogeal Germination
Cotyledon Position	Cotyledons are carried up and out of the soil.	Cotyledons remain below ground.
Growth Mechanism	Caused by the growth of the hypocotyl	Caused by the growth of the epicotyl
Plumule Position	The plumule tip lies between the cotyledons.	The plumule is carried up in the air above the cotyledons.
Example	Kidney bean, castor, cotton, sunflower, tamarind, etc.	Gram, pea, mango, maize, etc.

(iv). Give two examples each of the following types of seeds:

(i) Dicotyledonous albuminous seed: Castor bean, Sunflower seed.

(ii) Dicotyledonous exalbuminous seed: Pea seed, Bean seed.

(iii) Monocotyledonous albuminous seed: Maize grain, Rice grain.

(v). How would you demonstrate that oxygen and water are necessary for seeds to germinate?

We can conduct a simple experiment:

1. For Water:

- Let us take two identical seeds (e.g., beans) and place one in moist soil and the other in dry soil.

- Observation over a few days - The seed in moist soil will germinate, while the one in dry soil will not.

2. For Oxygen:

- Let us take two containers with moist soil and place a seed in each.

- Let us seal one container with a lid (preventing oxygen entry) and leave the other open.

- Observation over a week - The seed in the open container will germinate, while the sealed one will not.

(vi). Why does a farmer plough his field and make the soil loose before sowing seeds?

A farmer ploughs the field to:

· Improve Soil Aeration: Loose soil allows better air circulation, which is essential for root growth and helps in oxygen availability for germinating seeds.

· Enhance Water Absorption: Loosened soil absorbs water more effectively, ensuring that seeds have adequate moisture for germination.

· Reduce Soil Compaction: Ploughing breaks up hard soil, preventing the roots from having difficulty penetrating the ground.

6. Draw labelled diagrams showing the structure of the following seeds:

(i) Kidney Bean - Do it Yourself

(ii) Maize Grain - Do it Yourself

7. Outline the stages in the germination of the following seeds with the help of labelled diagrams:

(i) Kidney Bean Seed - Do it Yourself

(ii) Maize Grain - Do it Yourself

8. Observe the figure given below and answer the following:

(i) Label the parts marked 1-5.

1 – Cotyledon

2 – Plumule

3 – Epicotyl

4 – Hypocotyl

5 – Radicle

(ii) Identify whether the given seed is a monocot seed or a dicot one. Give one reason in support of your answer.

The given seed is a dicot, as it contains two cotyledons.

9. Observe the given figure and answer the following:

(i) Identify the given seed as monocot or dicot.

Monocot

(ii) Give one reason in support of your answer.

It contains only one cotyledon

(iii) Label the parts marked 1-7.

1 – Pericarp and seed coat fused

2 – Endosperm

3 – Coleoptile

4 – Plumule

5 – Cotyledon (Scutellum)

6 – Radicle

7 – Coleorrhiza

10. Observe the figure given below and answer the following:

(i) What does the given figure show?

Epigeal Germination

(ii) Label the parts marked 1-5.

1 – Radicle

2 – Cotyledon

3 – Hypocotyl

4 – Cotyledon

5 – Foliage Leaf

(iii) Name one plant in which this takes place.

Kidney bean

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High-Quality Study Notes on Seed Structure, Types, and Germination for ICSE Class 8