A seed is a remarkable biological package, serving as the fundamental unit of reproduction for flowering plants. It encases a dormant embryonic plant, a stored food supply, and is protected by an outer covering. This compact structure allows for plant dispersal and survival, enabling new life to emerge when conditions are favorable.
The Seed’s Protective Shell
The outermost layer of a seed is the seed coat, or testa, providing protection for the delicate structures within. This tough covering guards against physical damage, such as abrasions or impacts, and prevents desiccation by regulating water loss, important for viability. It also acts as a barrier, deterring pathogens like fungi and bacteria. Seed coat characteristics, including thickness, texture, and color, vary widely among plant species, reflecting adaptations to diverse environmental conditions.
The Miniature Plant Within
Within the protective seed coat lies the embryo, a miniature, undeveloped plant ready for growth. This tiny plant consists of several distinct parts, each with a specific role in seedling development.
Embryo Components
The radicle is the embryonic root, typically the first part to emerge during germination, anchoring the plant and absorbing water and nutrients from the soil. Above the radicle is the hypocotyl, which develops into the lower stem. The plumule, at the opposite end, is the embryonic shoot that gives rise to the plant’s stem and true leaves. Surrounding these structures are the cotyledons, which are embryonic leaves. Depending on the plant species, cotyledons can serve as initial photosynthetic organs upon emergence or remain underground, functioning as storage organs for early nutritional needs.
The Seed’s Food Supply
For successful germination and initial growth, the embryo relies on a stored food supply within the seed, as it cannot immediately produce its own food through photosynthesis. This energy reserve is typically stored as endosperm or in the cotyledons. The endosperm is a specialized nutrient-rich tissue, packed with starches, oils, and proteins, that surrounds or lies adjacent to the embryo. In seeds like corn or wheat, the endosperm remains the predominant food source, providing continuous nourishment as the embryo develops. Conversely, in many other seeds, such as beans or peanuts, the cotyledons absorb nutrients from the endosperm during seed development. These enlarged, fleshy cotyledons then become the primary storage organs, supplying energy and building blocks for the radicle and plumule to grow and establish the young plant.
Different Kinds of Seeds: Monocots vs. Dicots
Seeds are classified into two major categories based on their internal structural organization and cotyledon number: monocotyledonous (monocot) and dicotyledonous (dicot) seeds. Monocot seeds, like corn, rice, and wheat, possess a single cotyledon. In these seeds, the large endosperm typically serves as the primary food reservoir, with the cotyledon transferring nutrients to the developing embryo. Dicot seeds, including beans, peanuts, and sunflowers, have two cotyledons. For many dicots, the endosperm is largely absorbed during seed development, and the two cotyledons become enlarged and fleshy, holding most stored food reserves. This difference in cotyledon number and food storage location influences the seedling’s initial growth patterns and emergence from the soil.