Seeds are crucial for plant reproduction and survival. A seed is a self-contained unit, equipped to develop into a new plant under suitable conditions. It safeguards an embryonic plant, providing nourishment and protection until it can grow independently. This package allows plants to disperse their offspring and endure unfavorable environmental periods.
The Embryo
The embryo is the miniature, undeveloped plant nestled within the seed. It is the result of fertilization and will grow into a mature plant when conditions are right. This tiny plant contains the beginnings of roots, stems, and leaves.
The embryo consists of several distinct parts, each with a specialized role. The radicle is the embryonic root, the first part to emerge from the seed during germination, anchoring the seedling and absorbing water and nutrients from the soil. The plumule is the embryonic shoot, which develops into the plant’s stem and true leaves, growing upwards towards sunlight for photosynthesis. The embryo also includes one or two cotyledons, often called “seed leaves.” These cotyledons can function as part of the food supply, either by absorbing nutrients from an external storage tissue or by storing food themselves.
The Seed Coat
The seed coat is the protective outer layer of the seed, formed from the ovule’s outer layers after fertilization. Its primary function is to shield the delicate embryo and its food reserves from external threats. This includes protection against physical damage, mechanical stress, dehydration, extreme temperatures, and excessive water.
The seed coat also acts as a barrier against pathogens like bacteria, fungi, and insects. Variations in its thickness and texture are common across different plant species. A thick, hard seed coat can contribute to seed dormancy, delaying germination until conditions are optimal for survival.
The Food Supply
A seed’s food supply provides energy and nutrients for the embryo’s initial growth before it can produce its own food through photosynthesis. This stored energy primarily exists in two forms: endosperm and cotyledons.
Endosperm is a specialized nutritive tissue that surrounds the embryo in many flowering plants. It is rich in carbohydrates, proteins, and oils, acting as a direct food source for the developing embryo. For example, in corn and wheat, endosperm is a major component. In other seeds, like many legumes (beans and peas), the endosperm is largely absorbed by the developing embryo before the seed matures. Here, the cotyledons become thick and fleshy, serving as the primary storage organs for food.