Seeds Structure And Germination Chapter Notes | Biology Class 9 ICSE PDF Download

What is a Seed?

• Fruit: A fruit is the ripened ovary of a flower, with the ovarian wall forming the fruit wall that encloses the seed. Fruits protect seeds and aid in their dispersal. Examples include mangoes and pea pods.
• Seed: A seed is a mature ovule containing an embryo, which can develop into a new plant. The seed coat shields the embryo from mechanical damage. Examples include bean seeds and peas.
• Grain: Found in plants like maize and wheat, a grain is a type of fruit where the fruit wall and seed coat are fused into a single protective layer.

More About Seeds

• A seed is a fertilized, mature ovule.
• It contains a tiny living plant called the embryo, formed from the fusion of sperm and egg nuclei.
• The embryo remains dormant until conditions are favorable for germination.
• Seeds store food material to nourish the embryo during germination.
• The embryo can endure adverse conditions, such as extreme temperatures or drought. Some seeds may remain dormant for over 100 years.

Classification and Structure of Seeds

• Seeds are classified into two main types: monocotyledonous (one cotyledon, e.g., maize, grasses) and dicotyledonous (two cotyledons, e.g., pea, bean).
• Seeds vary in size, from tiny (e.g., poppy, orchid) to large (e.g., coconut, double coconut).
• Based on endosperm presence, seeds are categorized as:
  • Albuminous (endospermic): Cotyledons are thin, and endosperm persists (e.g., dicot: poppy, custard apple; monocot: cereals, millets, palm).
  • Exalbuminous (non-endospermic): Cotyledons store food and become thick and fleshy (e.g., dicot: gram, pea, mango; monocot: orchids, Vallisneria).

The Bean Seed

Various types of beans, such as broad beans, lima beans, and French beans, share a similar seed structure. Typically, bean seeds are kidney-shaped, featuring a convex and a concave side.

• Seed Coat: The seed coat comprises two layers: the testa, a hard, brownish outer layer that safeguards the seed’s inner parts from physical damage, bacteria, fungi, and insects, and the tegmen, a thin inner layer that provides additional protection.
• Hilum: A noticeable white, oval scar on the concave side of the seed, marking the point where the ovule (now the seed) was attached to the ovary wall via the placenta.
• Micropyle: A small pore near the hilum, indicating where the pollen tube entered the ovule. The micropyle has two key roles: (1) it allows water absorption during soaking, aiding embryo germination, and (2) it facilitates the exchange of respiratory gases for the developing embryo.
• Cotyledons: Beneath the seed coat lie two thick cotyledons, which store food for the embryo and offer protection.
• Embryo: By gently separating the cotyledons, the tiny embryo can be observed attached to one of them. The embryo consists of two main parts: the radicle, which develops into the root, and the plumule, which forms the shoot. The plumule includes a short stem, a pair of small leaves, and a growing point between them.

Maize Grain

• Maize grain is a one-seeded fruit with fused fruit wall and seed coat, forming a protective layer, hence called a grain.
• Features a light-colored oval area on one side, indicating the embryo's location.
• Contains a large endosperm rich in starch, separated from the embryo by a thin epithelial layer.
• The outermost endosperm layer, called the aleurone layer, is rich in protein.
• Embryo includes:
  • Scutellum: A single cotyledon that absorbs food from the endosperm.
  • Radicle: Located at the pointed end, enclosed in a protective sheath called coleorhiza.
  • Plumule: Located at the broader end, enclosed in a protective sheath called coleoptile.

Major Differences Between Bean Seed and Maize Grain

• Bean seed has two cotyledons; maize grain has one cotyledon (scutellum).
• Bean seed lacks endosperm; maize grain has a large endosperm.
• Bean seed has a large embryo; maize grain has a small embryo.
• Bean seed has folded plumule leaves; maize grain has rolled plumule leaves.
• Bean seed has a large plumule; maize grain has a very small plumule.
• Bean seed has visible hilum and micropyle; maize grain has no visible hilum or micropyle.
• Bean seed is contained in a pod; maize grain has fused seed and fruit walls, forming a single grain.

Germination

• Germination is the process where a dormant embryo becomes active and develops into a seedling.
• In a dry seed, the embryo is dormant, showing minimal chemical activity, consuming little oxygen, and releasing small amounts of carbon dioxide and heat.
• Fresh seeds undergo a dormancy period for physiological maturation before germinating, even under favorable conditions.

Conditions Necessary for Germination

Water:

• Absorbed mainly through the micropyle, causing the seed to swell and rupture the seed coat.
• Enables enzymes to convert stored food into a diffusible form for the embryo.

Suitable Temperature:

• Optimal range is 25°C to 35°C; very low temperatures inhibit growth, and very high temperatures damage delicate tissues.
• Tropical plant seeds require higher temperatures than temperate ones.

Oxygen:

• Essential for respiration to provide energy for cell division and growth.
• Seeds sown too deep in soil may fail to germinate due to insufficient oxygen and weak embryonic force to break through soil.

Some Experiments on Germination

Experiment to Prove Water is Necessary:

• Place green gram seeds on wet cotton wool in beaker A and dry cotton wool in beaker B.
• Keep both in a room; seeds in beaker A germinate, proving water's necessity.

Experiment to Prove Suitable Temperature is Necessary:

• Place green gram seeds on wet cotton wool in beakers A (room temperature) and B (refrigerator).
• Seeds in beaker A germinate, while those in beaker B show no or delayed germination, proving temperature's importance.

Experiment to Prove Oxygen is Necessary:

• Place soaked gram seeds on wet cotton wool in two conical flasks.
• Flask A has a test tube with water; flask B has a test tube with alkaline pyrogallic acid (absorbs oxygen).
• Seeds in flask A germinate, while those in flask B do not, proving oxygen's necessity.

Three-Bean Seeds Experiment:

• Tie three bean seeds on a glass slide: top seed above water, middle seed at water level, bottom seed in water.
• Keep in a warm place; middle seed germinates (gets water and oxygen), top seed does not (no water), bottom seed shows minimal germination (limited oxygen).
• Proves water is essential; oxygen's role is partially demonstrated.

Types of Germination

• Epicotyl: Region of the axis between cotyledons and plumule.
• Hypocotyl: Region of the axis below the cotyledons.

Hypogeal Germination:

• Cotyledons remain underground.
• Epicotyl elongates (e.g., pea, gram).

Epigeal Germination:

• Cotyledons are pushed above ground.
• Hypocotyl elongates (e.g., bean, castor).

Method to Observe Germination Stages:

• Place seeds between filter paper and glass in a sand-filled vessel.
• Add water; seeds absorb it and germinate, allowing observation of stages.

Germination in Some Common Seeds

Pea Seed (Hypogeal):

• Seed swells, testa softens, and bursts.
• Radicle grows downward to form roots.
• Plumule grows upward, initially arched to protect the shoot, forming the shoot system.
• Cotyledons supply food, remain underground, and later wither.

Bean Seed (Epigeal):

• Seed swells, radicle grows downward to form roots.
• Hypocotyl forms an arch above soil, straightens, and pushes cotyledons above ground.
• Cotyledons become the first green leaves and fall off after foliage leaves grow.

Maize Grain (Hypogeal):

• Grain swells, radicle pierces coleorhiza and fruit wall, grows downward, but dies off.
• New roots develop from the base.
• Plumule pierces coleoptile, grows upward.
• Scutellum absorbs food from endosperm; hypocotyl does not elongate.

Viviparous Germination:

• Seen in mangrove plants like Rhizophora and Sonneratia.
• Seed germinates inside the fruit while attached to the parent plant.
• Seedling drops into soil, develops roots, and fixes itself.

The Seedling

• A seedling is a young plant developing from a seed, not yet independent of stored food.
• Roots absorb water and minerals; leaves manufacture food, enabling growth into a mature plant that produces flowers and seeds.

Points to Remember

• Dicot seeds have two cotyledons, an embryo (radicle and plumule), and a seed coat; cotyledons store food.
• Monocot seeds (e.g., maize) have one cotyledon (scutellum), a large endosperm, and a small embryo with rolled plumule leaves.
• Germination requires water, suitable temperature (25°C–35°C), and oxygen.
• Germination types: hypogeal (cotyledons underground, epicotyl elongates) and epigeal (cotyledons above ground, hypocotyl elongates).