A seed represents a fundamental unit of plant reproduction, functioning as a protective vessel for a developing plant embryo. It contains stored food reserves that sustain the young plant during its initial growth. Seeds play a significant role in enabling plants to reproduce and spread across diverse environments.
The Seed Coat
The outermost layer of a seed is known as the seed coat, or testa, which serves as a protective barrier. This layer shields the delicate internal components of the seed, including the embryo and its food supply, from external threats. The seed coat offers defense against mechanical damage and guards against desiccation (loss of moisture).
The texture of the seed coat can vary widely among different plant species, ranging from thin and soft to hard and woody. This outer covering also provides protection from pathogens like bacteria, fungi, and viruses. The seed coat can influence seed dormancy, controlling the passage of water and gases into the seed and regulating the timing of germination.
The Embryo
Within the protective confines of the seed coat lies the embryo, which is essentially a miniature, undeveloped plant. This embryonic plant is the part that will grow into a new, mature plant under suitable environmental conditions. The embryo comprises several distinct components, each with a specialized role in the plant’s early development. These include the radicle, the plumule, and one or more cotyledons.
The radicle is the embryonic root and is the first part to emerge from the seed during germination. Its primary function is to grow downwards into the soil, anchoring the seedling and beginning the absorption of water and nutrients. Following the radicle, the plumule develops, representing the embryonic shoot. This structure grows upwards, forming the plant’s stem, leaves, and other aerial parts.
Cotyledons, often referred to as “seed leaves,” are also part of the embryo. These structures can either store food reserves directly or facilitate the absorption of nutrients from other storage tissues within the seed. In some plants, cotyledons may emerge above ground and become photosynthetic, providing early energy for the seedling.
The Food Supply
A seed’s stored food supply is crucial for sustaining the embryo during its initial growth, before the young plant can produce its own food. This nutritional support is stored in one of two primary forms: endosperm or cotyledons. The endosperm is a nutritive tissue that develops alongside the embryo and serves as a major reservoir of carbohydrates, proteins, and oils. Many cereal grains, such as corn, wheat, rice, and barley, are examples of endospermic seeds, where the endosperm constitutes a significant portion.
In contrast, in some seeds, the endosperm is consumed by the developing embryo during seed maturation. In these cases, the food reserves are stored directly within the cotyledons, which become thick and fleshy. Seeds like beans, peas, and peanuts are considered non-endospermic because their cotyledons serve as the primary storage organs for nutrients. This stored food provides energy for the embryo’s growth until it can establish its root system.