1.0 Seed: The Miniature Plant
A seed is a mature integumented megasporangium (ovule) containing an embryo and stored food. It remains in a state of dormancy until favorable conditions are met.
Seed: It is a ripened ovule which contains a dormant embryo and serves as a means of dispersal and reproduction for the plant.
Basic Structure of a Seed
A typical seed has three main parts:
- 🛡️ Seed Coat: Consists of an outer hard Testa and an inner thin Tegmen. It protects the embryo.
- 🌱 Embryo: The "baby plant" consisting of the Radicle (future root) and Plumule (future shoot).
- 🍴 Cotyledons: Seed leaves that store food. There may be one (Monocot) or two (Dicot).
Classification of Seeds
In "Label the Diagram" questions, look for the Hilum (the scar where the seed was attached to the fruit) and the Micropyle (the tiny pore). The Micropyle is vital as it allows water and oxygen to enter during germination.
2.0 Detailed Study: Bean vs. Maize
Understanding these two models helps us grasp the fundamental differences in how plants store energy and protect their embryos.
1. The Bean Seed (Dicot)
The bean seed is Exalbuminous (non-endospermic). All nutrients are stored in the two large cotyledons.
- Seed Coat: Consists of the Testa (outer brownish) and Tegmen (inner white).
- Embryo Axis: Attached to the cotyledons. The part above the attachment is the Epicotyl, and the part below is the Hypocotyl.
- Micropyle: Clearly visible; it absorbs water during germination.
2. The Maize Grain (Monocot)
A Maize grain is an Albuminous (endospermic) seed. It is technically a fruit called a Caryopsis because the fruit wall (pericarp) and the seed coat are fused.
- Aleurone Layer: A protein-rich outer layer of the endosperm.
- Scutellum: The single, large, shield-shaped cotyledon.
- Protective Sheaths:
- Coleoptile: Protects the Plumule.
- Coleorhiza: Protects the Radicle.
Differences: Bean Seed vs. Maize Grain
| Feature | Bean Seed | Maize Grain |
|---|---|---|
| Number of Cotyledons | Two | One (Scutellum) |
| Endosperm | Absent | Large and bulky |
| Protective Sheaths | Absent | Present (Coleoptile/Coleorhiza) |
In L.S. (Longitudinal Section) diagrams of Maize, always look for the Aleurone layer. It is a one-cell thick protein layer that separates the endosperm from the embryo. It is a common label in ICSE papers.
The word "Caryopsis" specifically refers to a dry one-seeded fruit in which the ovary wall is united with the seed coat. This is why we call corn a "grain" rather than just a seed.
3.0 Germination: From Seed to Seedling
Germination is the process by which the dormant embryo wakes up and begins to grow into a young plant (seedling) under favorable conditions.
Essential Conditions for Germination
A seed requires three specific external factors to germinate. If even one is missing, the seed remains dormant.
| Factor | Role in Germination |
|---|---|
| Water (Moisture) | Softens the seed coat and activates enzymes to digest stored food. |
| Oxygen (Air) | Required for Cellular Respiration to provide energy for growth. |
| Temperature | Optimum temperature (25°C - 35°C) is needed for enzyme activity. |
Types of Germination
Based on whether the cotyledons come above the ground or stay below, germination is of two types:
The Hypocotyl elongates rapidly and pushes the cotyledons above the soil.
Examples: Bean, Castor, Tamarind.
The Epicotyl elongates rapidly, and the cotyledons remain below the soil.
Examples: Pea, Maize, Gram.
Viviparous Germination: Seeds germinate while still attached to the parent plant. Common in mangrove plants like Rhizophora due to salty/marshy soil conditions.
Prepare the "Three-Bean Experiment" thoroughly. It proves that Water, Oxygen, and Temperature are all necessary for germination. ICSE frequently asks questions about which bean germinates (only the middle one!) and why.
4.0 Seed Dormancy & Experiments
1. Seed Dormancy
Sometimes, even when favorable conditions are provided, a seed fails to germinate. This state of "rest" is called Dormancy.
- Hard Seed Coat: Water or oxygen cannot penetrate (e.g., certain legumes).
- Immature Embryo: The embryo needs more time to develop.
- Chemical Inhibitors: Presence of hormones like Abscisic Acid that prevent growth.
2. The Three-Bean Experiment
This classic experiment is used to demonstrate that water, oxygen, and temperature are all required simultaneously for germination.
- 📍 Top Bean: Receives oxygen and light but no water. Result: No germination.
- 📍 Middle Bean: Receives water, oxygen, and warmth. Result: Germinates.
- 📍 Bottom Bean: Submerged in water; receives water but no oxygen (dissolved oxygen is insufficient). Result: No germination.
- Can you identify the Hilum and Micropyle on a bean seed?
- Do you know why Maize is a grain and not just a seed?
- Can you distinguish between Epigeal and Hypogeal germination?
- Do you understand the role of enzymes in digesting stored food during germination?
In experiments to prove oxygen is necessary for germination, alkaline pyrogallic acid is often used. It is used because it absorbs oxygen from the air inside the flask. This is a very frequent "Give Reason" or "Name the Chemical" question.