Seeds - Structure and Germination

• Albuminous (Endospermic) Seeds: Seeds in which the nutritive endosperm persists as a distinct, prominent food storage tissue in the mature seed. The cotyledons remain thin, membranous, and small. Examples: Maize, Wheat, Rice, Castor.
• Exalbuminous (Non-Endospermic) Seeds: Seeds in which the endosperm is completely digested and absorbed by the growing embryo during seed development. Food is stored in the two large, fleshy cotyledons. Examples: Gram, Pea, Bean, Groundnut.

• A true seed is surrounded only by its seed coats (testa and tegmen).
• In a maize grain, the true seed coat is inseparably fused with the outer fruit wall (Pericarp) to form a single thin protective skin.
• Furthermore, it is formed directly from the ripening of the entire ovary.
• Such an indehiscent, single-seeded fruit with a fused pericarp and seed coat is botanically termed a Caryopsis (Grain).

• Experiment: Three bean seeds are tied at different heights to a glass slide and placed in a beaker containing water such that:
- The top bean is completely in the air (receives air and warmth, but NO water).
- The middle bean rests exactly at the water surface (receives water, air, and warmth).
- The bottom bean is completely submerged in water (receives water and warmth, but NO dissolved oxygen).
• Observations: Only the middle bean germinates healthily. The top bean fails to sprout due to lack of water; the bottom bean rots or fails to sprout due to lack of oxygen.
• Conclusion: Proves that Water, Oxygen, and Warmth are all three simultaneously mandatory for seed germination.

• Epigeal Germination:
- The Hypocotyl (region below cotyledons) elongates rapidly and curves upward into an arch.
- It pulls the cotyledons ABOVE the ground into the sunlight, where they turn green and photosynthetic before withering.
- Examples: French Bean, Castor, Cotton, Sunflower.
• Hypogeal Germination:
- The Epicotyl (region above cotyledons) elongates rapidly.
- The cotyledons remain UNDERGROUND beneath the soil, serving as passive food reservoirs.
- Examples: Gram, Pea, Maize.

• (i) Micropyle: The tiny pore in the seed coat through which water and oxygen are absorbed by imbibition during germination, and through which the radicle emerges first.
• (ii) Coleoptile: A protective conical sheath that encloses and shields the tender growing plumule (shoot tip) as it pushes through abrasive soil.
• (iii) Aleurone Layer: A single-cell thick, protein-rich outer mantle surrounding the starchy endosperm that secretes hydrolytic enzymes (amylase) to digest starch during germination.

• In waterlogged soil, all microscopic air spaces between the soil particles are completely flooded and displaced by stagnant water.
• This cuts off the supply of atmospheric Oxygen to the buried seed.
• Deprived of oxygen, the embryonic cells cannot perform aerobic cellular respiration to generate ATP energy for cell division and growth, causing the embryo to asphyxiate and rot.

• Before germination, food is stored as insoluble, non-diffusible macromolecules (starch, proteins, lipids) that cannot cross cell membranes.
• Upon absorbing water, hydrolytic enzymes are synthesized:
1. Diastase (Amylase): Hydrolyzes insoluble starch into soluble, diffusible Maltose and Glucose.
2. Proteases: Break down complex proteins into soluble Amino Acids for building protoplasm.
3. Lipases: Hydrolyze fats into Fatty Acids and Glycerol for energy.
• These soluble nutrients are transported to the growing radicle and plumule tips.

• Seed Dormancy: A state of suspended metabolic inactivity in which a viable seed fails to germinate even when provided with all favorable environmental conditions (water, oxygen, warmth).
• Methods to Break Dormancy:
1. Mechanical Scarification: Nicking, scratching, or chipping the impermeable hard seed coat with a file or sandpaper.
2. Chemical Treatment: Soaking in dilute acids or treating with plant growth hormones like Gibberellins ($ ext{GA}_3$) or Cytokinins.