Physical Properties of Seeds
Plant seeds are the transitional stage of a plant’s life. Seeds size ranges from very small to very large. For example, certain epiphytic orchids of the tropical rain forest produce the world's smallest seeds weighing only 35 millionths of an ounce. They are dispersed into the air like minute dust particles or single-celled spores, eventually coming to rest in the upper canopy of rain forest trees.to very large and heavy types. On the other hand, the world's largest seed comes from the coco-de-mer palm (Lodoicea maldivica), native to the Seychelles Archipelago in the Indian Ocean. Although it belongs to a different genus from true coconut palms (Cocos), this enormous seed is often called the "double coconut." A single seed may be 12 inches (30 cm) long, nearly three feet (0.9 m) in circumference and weigh 40 pounds (18 kg).
Seeds are composed for three parts: the seed coat, stored food supply, and an embryo. The seed coat is a hard surface designed to protect the seed from pests and diseases as well as regulate and initiate the process of germination. It is made up of two layers of tissue. The outer layer, called the testa, is hard, dry, and generally darker in color than the inner layer.
Plants produce stored food supply as either tissue surrounding the embryo, called the endosperm, or tissue formed by the embryo called the cotyledon. In either case, the tissue store carbohydrates, fats, and proteins needed by the plant until it emerges from the soil and begins to use photosynthesis to produce its own food.
A young plant or embryo develops from the fertilized egg inside the ovule. It consists of four parts, the plumule, hypocotyls, radicle, and cotyledons. The plumule is bud and develops into a shoot that emerges from the seed.
The hypocotyl is located at the bottom of the embryo and causes the cotyledons and plumule to emerge from the seed during germination.
The radicle is located at the base of the hypocotyl and forms the first root of the plant. The tip of the radicle is always the first part of the embryo to emerge from the seed during germination.
The cotyledon stores food and advanced stages of germination give rise to the first true leaves.
Seed Germination
Seed dormancy prevents premature germination, yet encourages germination when all the environmental factors required for growth are present. Research has shown that moisture, warmth, and light are important factors in breaking seed dormancy. During seed dormancy, the seed undergoes physiological changes that help it germinate under the proper conditions.
Seed dormancy may also be caused by conditions created by the seed coat. In some cases the seed coat may be very hard or it may contain a chemical inhibitor that must be removed before germination can start. Some seeds must be treated to break seed dormancy and start germination. Very hard seed coats or seeds that are physiologically dormant must be treated with either scarification or stratification methods or both to initiate germination.
Germination is a complex process that requires a change in the seed’s physiology and biochemical processes. Water is absorbed, specific enzymes are synthesized, cells divide and elongate, stored food is digested and absorbed, and new plant parts develop as the process of germination unfolds. Research has shown that seeds must have water, oxygen, favorable temperature, and in some cases light while others require darkness for germination.