Seeds: Structure and Germination | ICSE Class 9 Biology Notes

This note covers the ICSE Class 9 Biology topic Seeds: Structure and Germination: what a seed is and why it matters, the structure of a dicot seed (the bean) and a monocot grain (maize), the differences between monocot and dicot seeds, how seeds germinate, the difference between epigeal and hypogeal germination, and the conditions for germination with the experiments that prove them.

What is a seed, and why is it important?

After fertilisation, the ovules of a flower develop into seeds, and at the same time the ovary develops into a fruit.

A seed is the final product of sexual reproduction in flowering plants and is often described as a fertilised ovule.

The wall of the ovary becomes the wall of the fruit, called the pericarp, and the seeds are formed inside it. A fruit is thus the ovary that has developed and matured after fertilisation; it protects the seeds inside it and, in many plants, helps to disperse them.

A seed is made up of a seed coat and an embryo. The embryo consists of an embryonal axis and one cotyledon (as in wheat and maize) or two cotyledons (as in gram and pea).

The integuments of the ovule harden into the tough, protective seed coat, and the micropyle remains as a small pore in it.

Significance of seeds

• Dependable reproduction: pollination and fertilisation do not depend on water, so seed formation is more dependable.
• Dispersal: seeds can be carried to new habitats, helping the species colonise other areas.
• Food for the seedling: stored food nourishes the young seedling until it can photosynthesise on its own.
• Protection: the hard seed coat protects the young embryo.
• Variation: as products of sexual reproduction, seeds carry new genetic combinations.
• Basis of agriculture: dry, dormant seeds can be stored as food through the year and sown to raise the next crop.

As a seed matures, its water content falls to about 10 to 15 per cent by mass and the embryo's metabolic activity slows down.

The embryo may then enter a state of inactivity called dormancy, or germinate if conditions are favourable.

Some seeds stay alive for a very long time: a lupine seed from the Arctic tundra germinated after an estimated 10,000 years, and a 2,000-year-old date palm seed has also been grown.

What is the structure of a dicot seed such as the bean?

The bean is a typical dicotyledonous seed. It is kidney-shaped, and when soaked in water overnight it swells and its parts are easy to see.

External features

• Seed coat: the outermost covering, made of two layers: the outer, thick testa and the inner, thin tegmen. It protects the embryo.
• Hilum: a scar on the seed coat, on the concave side, where the developing seed was attached to the fruit by a stalk.
• Micropyle: a small pore just above the hilum. When the seed is soaked and pressed gently, a drop of water comes out through it. It lets water and oxygen enter the seed during germination.

Internal features

When the seed coat is removed, the embryo is seen. It consists of an embryonal axis and two large cotyledons, which are fleshy and full of reserve food. At the two ends of the embryonal axis are the plumule and the radicle.

• The plumule is the future shoot. It lies between the two cotyledons and ends in the stem tip, with a few tiny folded leaves.
• The radicle is the future root. It lies outside the cotyledons, with its tip pointing towards the micropyle, and ends in the root tip, covered by a root cap.
• The part of the axis above the cotyledons is the epicotyl, and the part below them is the hypocotyl.

In the bean, gram and pea, the endosperm is used up as the seed develops, and the food is stored in the cotyledons. Such seeds are non-endospermic (ex-albuminous). In some dicot seeds, such as castor, the endosperm remains as a food-storing tissue, and they are endospermic (albuminous).

On the left, the outside of a bean-shaped seed, with the seed coat, the hilum and the micropyle labelled.

On the right, the seed opened out to show the two cotyledons joined to a small central axis, with the plumule at one end, between the cotyledons, and the radicle at the other.

See Fig. 5.14 in your NCERT textbook

What is the structure of a maize grain?

A maize grain looks like a seed, but it is actually a one-seeded fruit. Its thin seed coat is membranous and fused with the fruit wall, so the two cannot be separated. The grain is monocotyledonous and endospermic.

• Seed coat and fruit wall: the fused outer covering of the grain.
• Endosperm: a bulky tissue that stores food, mainly starch, and occupies most of the grain.
• Aleurone layer: a proteinous layer at the outer edge of the endosperm, which separates the endosperm from the embryo.
• Embryo: small, and situated in a groove at one end of the endosperm.
• Scutellum: the single, large, shield-shaped cotyledon, which lies against the endosperm.
• Plumule and radicle: on a short embryonal axis. The plumule is enclosed in a sheath called the coleoptile, and the radicle, with its root cap, in a sheath called the coleorhiza.

If a maize grain is cut lengthwise and a drop of iodine solution is added, the endosperm turns blue-black, showing that it stores starch.

On the left, a small outline of the grain with the endosperm and the embryo marked. On the right, a large longitudinal section.

The outer boundary is labelled "seed coat and fruit-wall", with the aleurone layer just inside it. The large endosperm fills most of the grain.

On one side lies the embryo: the scutellum next to the endosperm, and the axis with the plumule inside the coleoptile at the top and the radicle inside the coleorhiza at the bottom.

See Fig. 5.15 in your NCERT textbook

(a) A dicot embryo with two cotyledons on either side of the axis, the plumule at the top between them, and the hypocotyl running down to the radicle and root cap.

(b) A longitudinal section of a grass embryo, with the scutellum on one side, the coleoptile around the shoot apex at the top, a small epiblast, and the radicle and root cap inside the coleorhiza at the bottom.

See Fig. 1.14 in your NCERT textbook

How do monocot and dicot seeds differ?

The bean and the maize grain are the standard examples of a dicot seed and a monocot seed. They differ in the number of cotyledons, in where food is stored and in the sheaths around the embryo.

Note: Not every dicot seed is non-endospermic. Castor is a dicot with an endospermic seed, and orchids are monocots with non-endospermic seeds. The bean and maize are chosen because they show the typical pattern.

A useful way to remember the difference is to ask where the food is. In the bean it is inside the embryo itself, in the cotyledons. In maize it is outside the embryo, in the endosperm, and the scutellum absorbs it for the embryo during germination.

What happens when a seed germinates?

Germination is the process by which the embryo of a seed resumes growth and develops into a seedling. It happens when a seed that is not dormant gets favourable conditions: adequate moisture, oxygen and a suitable temperature. The general sequence is:

1. Absorption of water: the dry seed absorbs water, through the micropyle and the rest of the seed coat, and swells. The seed coat softens and bursts.
2. Activation: with water, the embryo's metabolism restarts. Enzymes convert the stored insoluble food, such as starch, into soluble forms that can reach the growing parts.
3. Respiration: the embryo respires rapidly, using oxygen to release the energy needed for growth.
4. Emergence of the radicle: the radicle comes out first and grows downwards to form the primary root, which anchors the seedling and absorbs water.
5. Emergence of the shoot: the plumule then grows upwards to form the shoot, which develops green leaves and starts photosynthesis.

Until its leaves can make food, the seedling depends on the food stored in the cotyledons or the endosperm. Germination can be divided into two types, depending on whether the cotyledons are carried above the soil or stay below it.

How do epigeal and hypogeal germination differ?

In epigeal germination, the cotyledons are pushed above the soil. This happens because the hypocotyl grows rapidly. Examples are the bean, castor and sunflower.

In hypogeal germination, the cotyledons remain below the soil. This happens because the epicotyl grows rapidly while the hypocotyl stays short. Examples are the pea, gram and maize.

Note: Link the type to the part that grows. "Epi" means above and "hypo" means below: when the hypocotyl lengthens, it lifts the cotyledons up, giving epigeal germination.

How do the bean seed and the maize grain germinate?

Germination of the bean seed (epigeal)

1. The seed absorbs water through the micropyle, swells, and the seed coat bursts.
2. The radicle emerges near the micropyle and grows down into the soil, forming the primary root with branches.
3. The hypocotyl grows rapidly and forms an arch or hook that pushes up through the soil.
4. The arch straightens, pulling the cotyledons and the plumule above the soil.
5. The cotyledons turn green and supply food until the first leaves open; then they shrivel and fall off.

Germination of the maize grain (hypogeal)

1. The grain absorbs water and swells. The scutellum absorbs the food made soluble in the endosperm and passes it to the growing embryo.
2. The coleorhiza emerges first; the radicle then breaks through it and forms the primary root. More roots soon grow from the base of the stem.
3. The coleoptile grows straight up through the soil, protecting the young plumule inside it.
4. The first green leaf then emerges from the tip of the coleoptile.
5. The grain, with the endosperm and the scutellum, stays below the soil throughout.

The maize seedling does not form an arch, because the pointed coleoptile protects the plumule as it pushes up.

Comparing the two shows how structure and germination are linked. In the bean, the food is in the cotyledons, which are lifted into the light and can even photosynthesise for a short time.

In maize, the food is in the endosperm, which stays in the soil, and the scutellum draws on it for the growing seedling.

What conditions do seeds need to germinate, and how can this be shown?

A seed needs three external conditions to germinate:

• Water: it softens the seed coat, makes the seed swell, activates the enzymes and dissolves the stored food so it can move to the growing parts.
• Oxygen: the germinating embryo respires rapidly and needs oxygen to release energy for growth.
• Suitable temperature: enzymes work best within a moderate temperature range; very low or very high temperatures prevent germination.

In addition, the embryo must be alive (viable), any period of dormancy must be over, and there must be enough stored food. Most seeds do not need light to germinate, which is why they can germinate under the soil.

Experiment: to show that water, oxygen and a suitable temperature are needed

1. Take four flasks, A to D, each with a few healthy seeds, such as bean or gram.
2. Flask A: dry seeds only, kept in a warm room. No water.
3. Flask B: seeds on moist cotton wool, kept in a warm room. Water, air and warmth.
4. Flask C: seeds completely covered with boiled and cooled water, with a thin layer of oil on top, in a warm room. Boiling drives out dissolved air, and the oil keeps fresh air out. No oxygen.
5. Flask D: seeds on moist cotton wool, kept in a refrigerator. Low temperature.
6. After three or four days, only the seeds in flask B have germinated.
7. Conclusion: seeds need water, oxygen and a suitable temperature for germination. Each of flasks A, C and D lacks one condition, and flask B, which has all three, acts as the control.

A simpler version uses three seeds tied at different heights on a strip in a beaker of water: the top seed has air but no water, the middle seed at the water surface has both, and the bottom seed under the water has no air. Only the middle seed germinates.

What are dormancy and viability, and why do germinating seeds respire so fast?

A mature seed does not always germinate as soon as it is formed. Its water content falls, its metabolism slows, and the embryo may enter dormancy, a resting state in which it stays alive but does not grow.

Dormancy and dryness are crucial for agriculture: they allow seeds to be stored as food through the year and kept to raise the next season's crop.

If seeds germinated as soon as they formed, they could not be stored at all.

Viability is the ability of a seed to stay alive and able to germinate. The period varies greatly between species:

• In a few species, seeds lose viability within a few months.
• Seeds of a large number of species live for several years.
• Some seeds remain alive for hundreds of years. A seed of the lupine Lupinus arcticus, dug out of the Arctic tundra, germinated and flowered after an estimated 10,000 years of dormancy, and a 2,000-year-old date palm (Phoenix dactylifera) seed found near the Dead Sea has also been grown.

So, besides water, oxygen and warmth, a seed will germinate only if its embryo is viable and its dormancy is over.

Germination and respiration

Once a seed absorbs water, the embryo becomes very active. It breaks down its stored food and respires rapidly to release the energy needed for cell division and growth.

This is why germinating seeds use a lot of oxygen, give out carbon dioxide and release heat.

Soaked, germinating seeds are therefore used in experiments on respiration: in a closed flask they turn lime water milky, and in a vacuum flask they raise the temperature.

It also explains why seeds sown in waterlogged or tightly packed soil often fail to come up.

The spaces between soil particles fill with water or disappear, the embryo cannot get enough oxygen, and it cannot respire fast enough to grow.

Farmers plough and loosen the soil before sowing partly for this reason, so that air can reach the seeds.

Glossary

• Seed — The fertilised ovule of a flowering plant, made of a seed coat and an embryo, formed inside the fruit.
• Testa and tegmen — The outer thick layer and the inner thin layer of the seed coat.
• Hilum — The scar on the seed coat where the seed was attached to the fruit.
• Micropyle — A small pore near the hilum through which water and oxygen enter during germination.
• Cotyledon — A seed leaf of the embryo; in the bean it is fleshy and stores food.
• Plumule — The part of the embryo that grows into the shoot.
• Radicle — The part of the embryo that grows into the root.
• Endosperm — The food-storing tissue of a seed, bulky in maize and absent in the mature bean seed.
• Scutellum — The single, shield-shaped cotyledon of the maize embryo, which absorbs food from the endosperm.
• Coleoptile — The protective sheath around the plumule of a monocot embryo.
• Coleorhiza — The protective sheath around the radicle of a monocot embryo.
• Epigeal germination — Germination in which the hypocotyl elongates and brings the cotyledons above the soil.
• Hypogeal germination — Germination in which the epicotyl elongates and the cotyledons remain below the soil.
• Dormancy — A state of inactivity of the embryo in a mature, dry seed.

Common errors and misconceptions

• Misconception: A maize grain is a seed. Correct: It is a one-seeded fruit, since its seed coat is fused with the fruit wall.
• Misconception: In epigeal germination the epicotyl grows fast. Correct: The hypocotyl grows fast in epigeal germination; the epicotyl grows fast in hypogeal germination.
• Misconception: The plumule comes out of the seed first. Correct: The radicle emerges first, so the seedling can anchor itself and absorb water.
• Misconception: The bean stores its food in the endosperm. Correct: In the bean the endosperm is used up, and food is stored in the two cotyledons.
• Misconception: Seeds need light to germinate. Correct: Most seeds need water, oxygen and a suitable temperature, not light, which is why they germinate under the soil.
• Misconception: The micropyle is where the seed was attached. Correct: The hilum is the attachment scar; the micropyle is the small pore just above it.

Exam-style questions with model answers

Q1. Name the parts of the embryo that grow into (a) the root, (b) the shoot. [1 mark]

1. (a) The radicle grows into the root, and (b) the plumule grows into the shoot of the seedling.

Q2. What is the function of (a) the micropyle, (b) the scutellum? [2 marks]

1. (a) The micropyle is a small pore in the seed coat through which water and oxygen enter the seed during germination.
2. (b) The scutellum is the single cotyledon of the maize embryo; it absorbs the food from the endosperm and passes it to the growing embryo.

Q3. Give three differences between a bean seed and a maize grain. [3 marks]

1. A bean seed has two cotyledons, while a maize grain has one, the scutellum.
2. In the bean, food is stored in the fleshy cotyledons and there is no endosperm; in maize, food is stored in a bulky endosperm surrounded by an aleurone layer.
3. The bean's plumule and radicle have no sheaths, while in maize the plumule is enclosed in the coleoptile and the radicle in the coleorhiza.

Q4. Distinguish between epigeal and hypogeal germination, with one example of each. [3 marks]

1. In epigeal germination the cotyledons are brought above the soil, while in hypogeal germination they remain below it.
2. Epigeal germination is caused by rapid growth of the hypocotyl; hypogeal germination by rapid growth of the epicotyl.
3. An example of epigeal germination is the bean (or castor), and of hypogeal germination the pea (or maize).

Q5. Describe the germination of a bean seed. [4 marks]

1. The seed absorbs water through the micropyle, swells, and the seed coat bursts. Enzymes make the stored food in the cotyledons soluble.
2. The radicle emerges first and grows down into the soil to form the primary root.
3. The hypocotyl then grows rapidly and forms an arch that pushes up through the soil; as it straightens, it pulls the cotyledons and plumule above the ground.
4. The cotyledons turn green and supply food until the first leaves open, and then shrivel and fall off. This is epigeal germination.

Q6. Describe an experiment to show that water, oxygen and a suitable temperature are necessary for seed germination. [5 marks]

1. Take four flasks, A to D, each with a few healthy bean seeds. In A put dry seeds in a warm room (no water). In B put seeds on moist cotton wool in a warm room (water, air and warmth).
2. In C cover the seeds completely with boiled and cooled water and pour a layer of oil on top, in a warm room; boiling removes dissolved air and the oil keeps fresh air out (no oxygen).
3. In D put seeds on moist cotton wool in a refrigerator (low temperature).
4. After three or four days, only the seeds in flask B germinate; those in A, C and D do not.
5. Conclusion: seeds need water, oxygen and a suitable temperature to germinate. Each of A, C and D lacks one condition, and B, which has all three, is the control.

Key takeaways

• A seed is a fertilised ovule, made of a seed coat and an embryo with an embryonal axis and one or two cotyledons.
• The bean seed has a testa and tegmen, a hilum and micropyle, two fleshy food-storing cotyledons, a plumule and a radicle.
• The maize grain is a one-seeded fruit with a fused seed coat and fruit wall, a bulky endosperm, an aleurone layer and a scutellum.
• In maize the plumule is protected by the coleoptile and the radicle by the coleorhiza; the bean has no such sheaths.
• During germination the seed absorbs water, food is made soluble, the radicle emerges first, and the plumule then forms the shoot.
• In epigeal germination the hypocotyl lengthens and lifts the cotyledons above the soil, as in the bean, castor and sunflower.
• In hypogeal germination the epicotyl lengthens and the cotyledons stay below the soil, as in the pea, gram and maize.
• Seeds need water, oxygen and a suitable temperature to germinate, which the four-flask experiment demonstrates.

Test yourself

What are the two layers of the seed coat called?

The outer, thick layer is the testa and the inner, thin layer is the tegmen.

Give one example each of an endospermic and a non-endospermic seed.

Castor and maize are endospermic; the bean, gram and pea are non-endospermic, storing food in their cotyledons.

What is the aleurone layer?

A proteinous layer at the outer edge of the endosperm in a maize grain, separating the endosperm from the embryo.

Why does iodine turn the endosperm of maize blue-black?

Because the endosperm stores starch, and iodine turns starch blue-black.

Why is boiled water used in the experiment on oxygen?

Boiling drives out the air dissolved in water, so the seeds under it get no oxygen.

What is the moisture content of a mature, dry seed?

A mature seed becomes relatively dry, with about 10 to 15 per cent moisture by mass.

Why does the bean hypocotyl form an arch?

The arch pushes through the soil first, so the delicate plumule is pulled up behind it without damage.

Name two examples of hypogeal germination.

The pea and maize show hypogeal germination; gram does too, as its cotyledons remain below the soil.