Plant Life | ICSE Class 6 Biology Notes

This note covers leaf structure, kinds of leaves, vein patterns, leaf functions and modifications, new plants from leaves, flower parts, pollen transfer, fertilisation, fruit structure, seed parts and seed germination.

What are the main parts of a leaf?

A leaf is a lateral, generally flattened structure borne on a stem. A node is the region of the stem where a leaf arises. Leaves differ in shape, size, surface and the depth of cuts in their blades.

How is a leaf attached?

A typical leaf has three main parts: the leaf base, which attaches it to the stem; the petiole, or leaf stalk; and the lamina, or expanded leaf blade. The petiole helps hold the blade towards light.

The leaf base may bear stipules, small leaf-like structures at its sides. The axil is the angle between a leaf and its stem. An axillary bud is a small developing shoot in this angle that can later form a branch.

What can we see on the blade?

The lamina contains veins, channels that support the blade and transport water, minerals and food. Their smaller branches are veinlets. There is usually a prominent middle vein called the midrib. The leaf's edge is its margin, and its tip is its apex.

Long, thin, flexible petioles allow leaf blades to flutter in wind. This cools the leaf and brings fresh air to its surface. Thus, a petiole's position and flexibility help the blade perform its functions.

The drawing labels the lamina, petiole, stipule, leaf base and axillary bud. Two separate leaf drawings show a branching network of veins and a pattern of veins running alongside one another.

See Fig. 5.4 in your NCERT textbook

When observing a leaf, trace its connection to the stem before labelling the blade. This distinguishes the stalk from the leaf base and helps locate the bud. Then examine the veins without confusing them with the outer margin.

How do simple and compound leaves and their vein patterns differ?

A simple leaf has a single blade that is entire, meaning undivided, or has cuts that do not reach the midrib. A compound leaf has a blade divided into separate smaller units called leaflets.

How can a leaflet be recognised?

In a compound leaf, the divisions of the blade reach the midrib. An axillary bud occurs at the base of the whole leaf's petiole, but not at the base of each leaflet. This is useful when distinguishing a compound leaf from a branch carrying simple leaves.

In neem, the leaflets occur along a common axis called the rachis, which represents the midrib. In silk cotton, the leaflets attach at a common point at the tip of the petiole. Both arrangements belong to compound leaves.

What does venation describe?

Venation is the arrangement of veins and veinlets in the lamina. Reticulate venation forms a network. In parallel venation, the veins run parallel to one another within the blade. These terms describe vein patterns, not the number of leaflets.

Describe the blade and venation separately. First decide whether the blade is simple or compound. Then look for a network or parallel veins. A divided appearance and a network of veins answer different questions about the same leaf.

Note: A cut margin alone does not make a leaf compound. The depth of the cuts and the position of the axillary bud are important identifying features.

How do leaves make food and release water?

Photosynthesis is the process in which green plants use light energy to make food from carbon dioxide and water. Chlorophyll, the green pigment in leaves, captures sunlight. A pigment is a colouring substance. Carbon dioxide is a gas present in air.

What materials reach the leaf?

Roots absorb water and minerals from the soil. These travel through the stem and branches to the leaves. Carbon dioxide enters through stomata, tiny pores on the leaf surface. Stomata also allow gases to move between the plant and its surroundings.

1. Water absorbed by the roots travels through the plant to the leaves.
2. Carbon dioxide from the surrounding air enters through the stomata.
3. Chlorophyll captures light energy, which is used to make food from carbon dioxide and water.
4. Carbohydrates, a group of food substances that includes sugars and starch, are formed, and oxygen gas is released.

Starch is a carbohydrate that can be stored in plant parts. Its presence in leaves indicates that photosynthesis has occurred. Green stems and green branches can also carry out photosynthesis. Food-making is therefore not restricted to leaves.

What is transpiration?

Transpiration is the loss of water as water vapour from the plant's aerial, or above-ground, parts, especially through leaf stomata. Water vapour is water in its gaseous form. Not all the water absorbed by a plant is used within it.

Water evaporating from leaves produces a pull that helps move water upwards through the plant. Transpiration also cools the plant. Leaves therefore participate in food preparation, exchange of gases and water loss, rather than performing just one function.

Leaves that appear red, violet or brown also contain chlorophyll. Other pigments can mask the green colour, so colour alone does not show whether a leaf can photosynthesise. A leaf's visible colour and its capacity to make food must not be treated as identical.

Why are some leaves modified for special functions?

A leaf modification is a change in leaf structure that helps it perform a special function. The modified structure may look very different from a broad, flat blade. Support, protection, reduced water loss, storage and insect capture are different purposes served by modifications.

How do tendrils, spines and scales help?

A tendril is a slender, coiling structure that helps a plant hold a support. In pea, tendrils are modified leaves. Their coiling supports the climbing plant. Calling the structure a tendril describes its form and role; identifying it as a modified leaf describes its origin.

Spines are sharp, pointed structures. In cactus, leaves are modified into spines. They help protect the plant and reduce water loss. The green stem carries out photosynthesis, so the reduction of broad leaves does not prevent the plant from making food.

Scale leaves are reduced leaves. In an onion bulb, fleshy scale leaves store food, while the outer dry scales protect the inner parts. A bulb is an underground shoot with a short stem surrounded by leaves. The storage leaves should not be mistaken for roots.

Why do insectivorous plants trap insects?

Insectivorous plants trap and digest insects to obtain nutrients, substances needed for growth and other life processes. In a pitcher plant, part of a leaf is modified into a pitcher-like structure. Digestive juices break down trapped insects and their nutrients are absorbed.

The green pitcher plant also photosynthesises. Insect capture supplements its nutrient supply, particularly nitrogen when the soil supplies too little. Nitrogen is an element needed to make proteins, a group of substances used in growth. Catching insects does not replace the plant's use of sunlight.

To explain any modification, give both the changed structure and its function. A pea tendril provides support, a cactus spine protects and reduces water loss, and a fleshy onion scale stores food. The name of the structure alone does not explain its usefulness.

How can a Bryophyllum leaf produce new plants?

Reproduction means producing new individuals of the same kind. Vegetative propagation is reproduction using plant parts such as roots, stems, leaves or buds. It is a form of asexual reproduction, in which new plants are produced without the production of seeds.

Where does the new plant begin?

Bryophyllum, also called the sprout leaf plant, bears buds along its leaf margins. A bud is a developing shoot with a short stem and immature leaves. If a Bryophyllum leaf falls on moist soil, each marginal bud can give rise to a new plant.

1. Observe the margin of a Bryophyllum leaf and locate its buds.
2. Place the leaf on moist soil, reproducing the condition in which a fallen leaf can grow.
3. Watch the marginal buds as they develop into young plants.
4. Record the position of each developing plant and identify the leaf as the part responsible for propagation.

The word can matters: it describes the ability of the buds to develop under suitable conditions. It does not mean that every detached leaf of every kind of plant will form new plants. The example depends on Bryophyllum's marginal buds and moist soil.

This route differs from producing a plant from a seed. The new growth begins on an existing leaf rather than inside a seed. When explaining the observation, include the parent plant, the position of the buds and the condition that supports their growth.

What are the four whorls of a complete flower?

A flower is the reproductive part of a flowering plant. Flowers vary in shape, size and colour. A whorl is a group of floral parts arranged around the centre. A complete flower contains all four whorls: calyx, corolla, androecium and gynoecium.

How are the whorls arranged?

The calyx is the outermost whorl and consists of sepals. Generally, sepals are green and leaf-like; they protect the flower in the bud stage. Inside the calyx is the corolla, the whorl formed by petals.

Petals are usually brightly coloured and help attract insects for pollination, the transfer of pollen from the male pollen-producing part to the female pollen-receiving surface. Their shape and colour vary greatly. Flowers need not all have the same petal colour or shape.

The androecium is the male reproductive whorl, made of stamens. The gynoecium is the female reproductive whorl, made of one or more carpels. Stamens produce pollen, while carpels contain the structures in which seeds can develop after the joining of male and female reproductive cells.

Fertilisation means the fusion, or joining, of male and female reproductive cells. The detailed parts involved are described below. Calyx and corolla protect or assist the flower; androecium and gynoecium directly take part in sexual reproduction, reproduction involving this cell fusion.

A flower drawing labels the calyx, corolla, androecium, gynoecium and pedicel, meaning flower stalk. Beside it, the four whorls are drawn separately, making their different forms easy to compare.

See Fig. 5.10 in your NCERT textbook

How do the male and female parts of a flower work?

A stamen has two main parts: the filament, its stalk, and the anther, the part that produces pollen grains. Pollen grains are small structures in which male reproductive cells develop. A reproductive cell is called a gamete.

What does a carpel contain?

A carpel has a stigma, the surface that receives pollen; a style, the connecting part below it; and an ovary, the enlarged basal part. The stigma is usually at the tip of the style. The style connects it to the ovary.

The ovary contains one or more ovules, structures that contain the female gamete and develop into seeds after fertilisation. The female gamete is the egg. The female part of a flower is also called the pistil, which may consist of one carpel or fused carpels.

Keep the enclosing structure and its contents separate: the ovary encloses ovules, and an ovule contains an egg. An ovary is therefore not the same thing as an ovule. Similarly, a pollen grain carries the male reproductive cells; it is not an anther.

Are all flowers alike?

A bisexual flower has both stamens and a pistil. Mustard, rose and petunia have bisexual flowers. A unisexual flower has either stamens or a pistil. Corn, papaya and cucumber produce unisexual flowers.

Male and female unisexual flowers may occur on the same plant or on different plants. The words bisexual and unisexual describe the reproductive parts present in an individual flower. Complete describes the presence of all four whorls, so it checks a different feature.

How does pollination occur within and between plants?

During pollination, pollen moves from an anther to a stigma. The movement places pollen on a receptive surface; it does not itself join the male and female gametes. This distinction separates pollination from fertilisation.

How do self-pollination and cross-pollination differ?

In self-pollination, pollen reaches the stigma of the same flower or another flower on the same plant. In cross-pollination, pollen reaches a flower on a different plant of the same kind. Identify the plants involved, not just the number of flowers.

Two flowers do not automatically mean cross-pollination. They might belong to the same plant. Equally, pollen carried to a different kind of plant does not fit the definition of successful cross-pollination between plants of the same kind.

Which agents carry pollen?

A pollinating agent carries pollen between flowers. Wind, water and insects are agents. Wind pollination occurs in maize and is quite common in grasses. Light, non-sticky pollen can be carried by wind currents. Wind-pollinated flowers often have well-exposed stamens.

Vallisneria is a water-pollinated plant. Water currents carry its male flowers or pollen at the surface, and some reach female flowers. Water pollination is quite rare in flowering plants. A plant growing in water is not necessarily pollinated by water.

Bees and butterflies are insect pollinators. Mustard provides an example of flowers visited and pollinated by insects. When insects visit flowers, pollen can stick to their bodies and later reach another stigma. The majority of insect-pollinated flowers are large, colourful, fragrant and rich in nectar, a sweet liquid produced by flowers.

The agent and the type of pollination answer different questions. The agent tells us how pollen travels. Self-pollination or cross-pollination tells us whether the receiving flower belongs to the same plant or to a different plant of the same kind.

What happens during fertilisation and fruit formation?

After suitable pollen reaches a stigma, it can develop a pollen tube, a tube through which male gametes travel towards the ovule. Pollination brings pollen to the flower; the pollen tube provides a route towards the female gamete.

What is the sequence?

1. A pollen grain on the stigma develops a pollen tube that grows through the style towards an ovule in the ovary.
2. The male gamete reaches the female gamete, or egg, inside the ovule.
3. The male and female gametes fuse during fertilisation, forming a zygote, the first cell of the new plant.
4. The zygote develops into an embryo, the young developing plant within the seed.
5. The ovule develops into a seed, while the ovary grows into a fruit.

This sequence connects flower structure with its function. The stigma receives pollen, the style lies along the tube's route, and the ovary encloses the ovules. The seed and fruit develop from different structures, although their development is closely connected.

What happens to the four whorls?

Following fertilisation, the ovary of the gynoecium enlarges to form the fruit, and its wall forms the fruit wall. Petals and stamens usually wither and fall. The stigma and style also usually wither. Sepals may fall or remain attached, as in tomato.

A flower therefore does not become a fruit by turning every whorl into fruit flesh. The important changes are the enlargement of the ovary and development of its ovules. Structures that attracted pollinators or produced pollen have already performed their main roles.

Note: Ovary becomes fruit, ovule becomes seed, and zygote becomes embryo. These three relationships describe different levels of development and should not be interchanged.

How do dry and fleshy fruits and their parts differ?

A fruit is a mature or ripened ovary. It generally contains a fruit wall, called the pericarp, and seeds. The pericarp may be dry or fleshy. Some fruits can develop without fertilisation, so fertilisation should not be presented as an exceptionless rule for every fruit.

Which fruits are dry or fleshy?

In a dry fruit, the fruit wall becomes dry at maturity. Pea and bean pods are examples. A pod is the fruit enclosing their seeds. In a fleshy fruit, the wall or part of it remains fleshy; mango and tomato are examples.

Classification depends on the mature fruit wall. A young green pea pod should not be used to conclude that mature pea fruits belong to the fleshy group. The peas inside the pod are seeds, while the enclosing pod is the fruit.

What are the layers of a fleshy fruit?

The epicarp is the outer layer of the pericarp, the mesocarp is its middle layer, and the endocarp is its inner layer. A mango shows these layers clearly: thin outer skin, fleshy edible middle region and a stony, hard inner layer around the seed.

The seed lies inside the endocarp; it is not itself a layer of the pericarp. Coconut also has these fruit-wall layers, but its mesocarp is fibrous. This comparison shows why the middle layer should not be defined as always soft and juicy.

When describing a cut mango, work from outside to inside. Name the skin, pulp, hard layer and seed in that order. Connect each everyday description to the correct term, while keeping the fruit wall distinct from the seed it encloses.

What does a seed contain, and how do monocot and dicot seeds differ?

A seed develops from an ovule after fertilisation. It contains an embryo enclosed in a protective seed coat. The embryo includes a young root, a young shoot and one or two seed leaves called cotyledons.

Which parts grow into the new plant?

The radicle is the embryonic root, and the plumule is the embryonic shoot. Embryonic means belonging to the embryo. These parts must not be confused with the cotyledons, which are seed leaves. Cotyledons are often fleshy and contain stored food.

A dicotyledonous seed, or dicot seed, has two cotyledons. Gram and pea are examples. A monocotyledonous seed, or monocot seed, has one cotyledon. Wheat and maize are examples. The terms refer to cotyledon number, not the number of seeds in a fruit.

Where is food stored?

In pea and gram, the cotyledons contain food reserves. In maize, a food-storing tissue called the endosperm provides a reserve. A tissue is a group of cells working together; cells are the small units from which living bodies are built.

Do not define every seed as having thick storage cotyledons. Seeds differ in the location of their food reserves. The single cotyledon of a monocot is still a cotyledon even when a separate endosperm stores much of the food.

One view shows the seed coat, hilum and micropyle. An opened view labels a cotyledon, plumule and radicle. The hilum is the attachment scar; the micropyle is a small pore in the seed coat.

See Fig. 5.14 in your NCERT textbook

Leaves of dicotyledonous plants generally have reticulate venation, while parallel venation is characteristic of most monocotyledons. Preserve generally and most in this comparison. Cotyledon number defines the seed group; vein patterns provide a useful associated feature.

What conditions allow a seed to germinate?

Germination is the beginning of growth of a seed into a sprout. A seedling is the resulting young plant. A seed needs suitable moisture, air and warmth for germination. Warmth here means a suitable temperature, not strong heating.

Why are moisture and air important?

Water softens the seed coat and allows the embryo to carry out the processes necessary for growth. Seeds in soil use air in the spaces between soil particles. Too much water can fill these spaces and reduce the air available to the seeds.

Keeping seeds moist is different from keeping them submerged. The word moist means containing some water without being flooded. In a germination activity, slightly moist soil supplies water while leaving air available. Both conditions matter when explaining whether bean seeds germinate.

Is sunlight essential at this stage?

Light is not essential for bean seed germination. In general, most seeds do not require light for germination. After germination, sunlight is required for further growth of the seedling. The conditions for starting growth should therefore be distinguished from those for continued growth.

1. Place seeds on wet blotting paper in a shallow dish, or sow them in soil kept slightly moist.
2. Keep the seeds where they receive air and suitable warmth.
3. Observe the appearance and growth of the radicle and plumule as the embryo develops.
4. Record the changes and continue observing the young seedling as its root and shoot grow.

To investigate water and light, compare dry soil, soil flooded with excess water, slightly moist soil in darkness and slightly moist soil in light. Keep the seed kind and other conditions the same. Record observations rather than assuming that every seed will germinate at an identical time.

Do not infer that germination has failed merely because a seed is in darkness. Check moisture and air as well. Likewise, sunlight alone cannot replace missing water. Each explanation should identify the actual condition experienced by the seeds.

Glossary

• Lamina — The expanded blade of a leaf, containing its veins and veinlets.
• Venation — The arrangement of veins and their smaller branches within a leaf blade.
• Compound leaf — A leaf whose blade is divided into separate units called leaflets.
• Photosynthesis — The process in which green plants use light energy to make food from carbon dioxide and water.
• Transpiration — Loss of water as water vapour from aerial plant parts, especially through leaf stomata.
• Vegetative propagation — Production of new plants from parts such as roots, stems, leaves or buds.
• Calyx — The outermost floral whorl, consisting of sepals that protect the flower bud.
• Corolla — The floral whorl made of petals, which are usually brightly coloured.
• Pollination — Transfer of pollen grains from an anther to the stigma of a flower.
• Fertilisation — Fusion of a male gamete with a female gamete to form a zygote.
• Pericarp — The fruit wall, which develops from the wall of the ovary.
• Cotyledon — A seed leaf forming part of the embryo within a seed.
• Radicle — The embryonic root that grows into the root of the developing plant.
• Plumule — The embryonic shoot that develops into the shoot of the young plant.
• Germination — The beginning of a seed's growth into a sprout under suitable conditions.

Common errors and misconceptions

• Misconception: Every leaf with cuts is compound. Correct: A simple leaf can have cuts that do not reach the midrib. A compound blade is divided into leaflets.
• Misconception: A leaflet is identified by its own axillary bud. Correct: Leaflets lack axillary buds. The bud occurs at the base of the whole compound leaf.
• Misconception: Insectivorous plants cannot photosynthesise. Correct: The green pitcher plant photosynthesises and also obtains nutrients by digesting insects. These processes serve different nutritional needs.
• Misconception: Pollen moving between two flowers must be cross-pollination. Correct: If both flowers belong to the same plant, this is self-pollination.
• Misconception: Pollination and fertilisation are identical. Correct: Pollination transfers pollen to a stigma; fertilisation joins male and female gametes to form a zygote.
• Misconception: The ovary becomes a seed and the ovule becomes a fruit. Correct: The ovary develops into the fruit; the ovule develops into a seed.
• Misconception: Every seed needs sunlight to germinate. Correct: In general, most seeds do not require light for germination, although sunlight is needed for further seedling growth.
• Misconception: Flooding seeds supplies better germination conditions. Correct: Excess water can reduce available air. Seeds need appropriate moisture as well as air and suitable warmth.

Exam-style questions with model answers

Q1. Define reticulate venation and parallel venation, referring to the arrangement of veins in each case. [2 marks]

1. Reticulate venation is an arrangement in which the veins and veinlets form a network within the leaf blade.
2. Parallel venation is an arrangement in which the veins run parallel to one another within the leaf blade.

Q2. State the special function of each of these leaf modifications: pea tendrils, cactus spines and fleshy onion scale leaves. [3 marks]

1. Pea tendrils coil around a support and help hold up the climbing plant. They are modified leaves rather than ordinary broad blades.
2. Cactus spines help protect the plant and reduce water loss. Its green stem carries out photosynthesis despite the leaves being reduced to spines.
3. Fleshy onion scale leaves store food within the bulb. They should be distinguished from the outer dry scales, which protect the inner parts.

Q3. Name the four whorls of a complete flower, proceeding from outside towards the centre. State the members and main function of each. [4 marks]

1. The calyx is the outermost whorl. Its members are sepals, which protect the flower in the bud stage.
2. The corolla consists of petals. These are usually brightly coloured and help attract insects that can carry pollen.
3. The androecium consists of stamens, the male reproductive parts. Their anthers produce pollen grains in which male gametes develop.
4. The gynoecium consists of one or more carpels. It receives pollen at the stigma and contains ovules in the ovary.

Q4. In one transfer, pollen moves between two flowers on the same mustard plant. In another, pollen moves between flowers on two different mustard plants. Identify each type of pollination and give the reason. [2 marks]

1. The first transfer is self-pollination because both flowers belong to the same plant, even though two flowers are involved.
2. The second transfer is cross-pollination because the flowers belong to different plants of the same kind, namely mustard.

Q5. Starting with suitable pollen on a stigma, explain five stages connecting pollen-tube growth, movement of the male gamete, fertilisation, embryo development, and fruit and seed formation. [5 marks]

1. A suitable pollen grain on the stigma develops a pollen tube. The tube grows through the style towards an ovule inside the ovary.
2. The male gamete travels through the pollen tube to reach the female gamete, or egg, located in the ovule.
3. The male and female gametes fuse in fertilisation. This produces a zygote, the first cell from which the new plant develops.
4. The zygote develops into an embryo, the young developing plant. The embryo is contained within the seed that forms from the ovule.
5. The ovule develops into a seed and the ovary into a fruit. These are distinct changes: the seed contains the embryo, while the fruit encloses the seeds.

Q6. Describe the epicarp, mesocarp and endocarp of a mango. For each layer, state its position or appearance and its role. [3 marks]

1. The epicarp is the thin outer skin of the mango. It forms an outer protective covering around the inner fruit layers.
2. The mesocarp is the middle, fleshy, edible region. It stores food and forms the pulp between the skin and the hard inner layer.
3. The endocarp is the stony, hard inner layer. It surrounds and protects the seed, which is distinct from the fruit wall itself.

Q7. Identical healthy bean seeds are kept at suitable warmth in four conditions: dry soil in light; flooded soil with very little air in light; slightly moist, aerated soil in darkness; and slightly moist, aerated soil in light. Explain how each condition affects germination. [4 marks]

1. Dry soil does not provide the water needed to soften the seed coat and support the embryo's growth, so germination is prevented.
2. Flooded soil provides water but very little air. This limits germination because bean seeds need air as well as water.
3. Slightly moist, aerated soil in darkness supplies both water and air. Darkness does not prevent bean seed germination because light is not essential at this stage.
4. Slightly moist, aerated soil in light also provides the required water and air. With the stated suitable warmth, these conditions support germination.

Q8. Explain six features of a pea seed: the seed coat, embryo, cotyledon number, cotyledon function, radicle and plumule. [6 marks]

1. The seed coat forms the outer covering of the pea seed. It protects the embryo enclosed within the seed.
2. The embryo is the young developing plant inside the seed. It includes the embryonic root, embryonic shoot and cotyledons.
3. A pea seed has two cotyledons, which are its seed leaves. It is therefore called a dicotyledonous seed, or dicot seed.
4. The cotyledons contain stored food. These reserves provide nourishment for the young plant during the early stages of its development.
5. The radicle is the embryonic root. During germination it grows to form the root of the developing young plant.
6. The plumule is the embryonic shoot. It develops into the shoot, so its role differs from that of the radicle.

Key takeaways

• A typical leaf has a leaf base, petiole and lamina; its veins support the blade and transport materials.
• Simple and compound describe blade division, while reticulate and parallel describe the arrangement of veins within a leaf.
• Leaves make food through photosynthesis, exchange gases through stomata and lose water vapour through transpiration.
• Modified leaves can provide support, protection, reduced water loss, food storage, insect capture or vegetative propagation.
• A complete flower has calyx, corolla, androecium and gynoecium, with protective, attractive or reproductive functions.
• Pollination transfers pollen to a stigma; fertilisation joins male and female gametes to form a zygote.
• The ovary develops into the fruit, the ovule becomes a seed, and the zygote develops into the embryo.
• Seeds need suitable moisture, air and warmth for germination; in general, most seeds do not require light at this stage.

Test yourself

Where is the axillary bud associated with a compound leaf?

It lies in the axil of the whole leaf's petiole. Individual leaflets do not have axillary buds.

How can a cactus make food despite having spines instead of broad leaves?

The cactus has a green stem that carries out photosynthesis, while its modified leaves help reduce water loss.

Which part of a Bryophyllum leaf can give rise to a new plant?

Buds along the leaf margin can develop into new plants when the leaf falls on moist soil.

What are the two main parts of a stamen?

They are the filament, which forms the stalk, and the anther, which produces pollen grains.

Why is a transfer between two flowers on one plant self-pollination?

Self-pollination includes transfer to another flower on the same plant. The identity of the plant matters, not just the number of flowers.

Is the hard layer surrounding a mango seed the seed coat?

No. The stony layer is the endocarp, part of the fruit wall. The seed lies inside it.

How many cotyledons occur in maize and pea seeds?

Maize has one cotyledon and is a monocot. Pea has two cotyledons and is a dicot.

Why can excess water hinder germination?

Excess water fills spaces that would otherwise contain air, reducing the air available to the germinating seed.