1.0 Seed: The Embryonic Unit
A Seed is a mature, fertilized ovule that contains a dormant embryo and a food reserve. It serves as the bridge between two generations, capable of withstanding unfavorable environmental conditions through a state called Dormancy.
The Anatomy of a Typical Seed
Every seed consists of three fundamental components:
- Seed Coat: Composed of the outer tough Testa and the inner thin Tegmen. It protects the embryo from mechanical injury and pathogens.
- Embryo: The "baby plant," consisting of:
- Radicle: The embryonic root (grows downward).
- Plumule: The embryonic shoot (grows upward).
- Cotyledons: Seed leaves that store food or absorb it from the endosperm.
- Hilum & Micropyle: The Hilum is the scar of attachment to the ovary. The Micropyle is a tiny pore that allows water and oxygen entry.
Albuminous vs. Exalbuminous:
• Albuminous (Endospermic): Seeds where endosperm is retained at maturity (e.g., Castor, Maize, Wheat).
• Exalbuminous (Non-Endospermic): Endosperm is consumed during development; food is stored in cotyledons (e.g., Pea, Gram, Bean).
Dicot vs. Monocot Seed Comparison
| Feature | Dicot Seed (e.g., Bean) | Monocot Seed (e.g., Maize) |
|---|---|---|
| Cotyledons | Two; usually fleshy. | One; thin, shield-shaped (Scutellum). |
| Endosperm | Absent in most. | Large and prominent. |
| Protective Sheaths | Absent. | Present: Coleoptile (plumule) and Coleorhiza (radicle). |
In Maize grains, the endosperm is surrounded by a protein-rich layer called the Aleurone Layer. This layer secretes enzymes like Alpha-Amylase during germination to break down starch into glucose for the growing embryo.
Botanically, a "Maize grain" is not just a seed; it is a one-seeded fruit where the seed coat is fused with the fruit wall (pericarp). This type of fruit is called a Caryopsis.
2.0 Germination: The Awakening of the Embryo
Germination is the process by which the dormant embryo within the seed resumes active growth. It is a physiological transformation where the metabolic rate spikes, converting stored food into energy to establish a new seedling.
The Essential Environmental Trio
Three abiotic factors must be present simultaneously for germination to occur:
- Water (Moisture): Required for Imbibition (swelling of the seed), which ruptures the seed coat and activates enzymes like amylase.
- Oxygen: Essential for Cellular Respiration to provide energy (ATP) for rapid cell division.
- Temperature: An optimum range (usually 25°C to 35°C) is needed for enzymatic activity. Extreme cold inhibits enzymes, while extreme heat denatures them.
The Seedling Axis: The part of the axis above the point of attachment of cotyledons is the Epicotyl (becomes the plumule), while the part below is the Hypocotyl (becomes the radicle).
Comparative Germination Physiology
| Feature | Epigeal Germination | Hypogeal Germination |
|---|---|---|
| Cotyledon Position | Pushed above the soil. | Remain below the soil. |
| Elongating Part | The Hypocotyl elongates rapidly. | The Epicotyl elongates rapidly. |
| Examples | Bean, Castor, Tamarind. | Pea, Maize, Gram. |
Vivipary: A unique type of germination seen in Mangroves (e.g., Rhizophora). The seed germinates while still attached to the parent tree. The radicle becomes heavy and drops into the saline mud, ensuring survival in marshy habitats where normal germination is impossible.
Experiments on germination often use Pyrogallic Acid (to absorb Oxygen) or Anhydrous Calcium Chloride (to absorb Moisture). In the "Three-bean experiment," only the middle bean germinates because it alone receives all three factors: water, air, and warmth.
3.0 Seed Physiology and Experimental Biology
The transition from a dormant seed to a vegetative seedling involves a massive shift in bioenergetics. To understand this, we must look at the biochemical stages of germination and the classical experiments that prove the necessity of external factors.
The Biochemical Cascade
Germination occurs in three distinct physiological phases:
- Imbibition: Physical uptake of water through the micropyle, causing the seed to swell and the testa to rupture.
- Enzymatic Activation: Gibberellins (plant hormones) trigger the production of α-amylase, which converts stored starch into maltose/glucose for the embryo.
- Radicle Emergence: The first visible sign of germination. The radicle emerges through the micropyle, exhibiting positive geotropism.
Classical Experiments in Germination
In the ICSE curriculum, experimental verification is key. Below are the standard setups used to isolate germination factors:
| Experiment | Mechanism of Control | Result / Inference |
|---|---|---|
| To prove Oxygen is needed | Using Alkaline Pyrogallic Acid in a conical flask to absorb all $O_2$. | Seeds fail to germinate; Respiration is halted. |
| To prove Water is needed | Seeds kept on dry cotton wool vs. moist cotton wool. | Dry seeds remain dormant; enzymes remain inactive. |
| Three-Bean Experiment | Three beans on a glass slide placed in a beaker of water. | Only the Middle Bean germinates (gets air + water). |
The G0 State of Seeds: Dormancy is often regulated by Abscisic Acid (ABA). This "stress hormone" prevents germination even if conditions are favorable, acting as an internal clock to ensure the seed doesn't sprout in the middle of a brief winter thaw.
When drawing the Bean Seed vs. Maize Grain, remember: In the Bean, the food is stored in Cotyledons (Exalbuminous), whereas in the Maize, the food is stored in the Endosperm (Albuminous). Do not mislabel the Aleurone layer in a dicot seed!