A seed is the primary mechanism for the dispersal and perpetuation of flowering and cone-bearing plants. Encased within a protective shell, it contains the genetic blueprint and stored energy necessary for a new generation of life to survive unfavorable environmental conditions. The seed allows the dormant plant embryo to wait for the opportune moment of warmth and moisture before initiating growth. This compact reproductive structure has allowed seed-bearing plants to become the dominant form of vegetation across the globe.
The Protective Seed Coat
The outermost layer of the seed is a hardened covering called the testa, which forms a physical barrier against the external environment. This protective shell prevents mechanical damage, guards against pathogens, and significantly reduces water loss, maintaining the seed’s dormant state. The seed coat is not entirely sealed, as two small features mark its surface. The hilum is a visible scar indicating where the seed was attached to the funiculus, the stalk connecting it to the parent plant’s ovary tissue. Adjacent to this scar is the micropyle, a minute pore that initially allowed the pollen tube to enter the ovule and later serves as the primary route for water absorption during germination.
The Embryo: The Plant in Miniature
The embryo is the multicellular, rudimentary plant encased within the seed, representing the future sporophyte generation in a state of suspended animation. Its structure is organized, containing the foundational tissues that will give rise to the roots and shoots of the mature plant. The main axis of the embryo is composed of distinct regions that determine the plant’s initial growth trajectory.
The first part to emerge from a germinating seed is the radicle, which is the embryonic root. Its purpose is to anchor the seedling into the soil and begin the absorption of water and mineral nutrients. The development of the radicle establishes the primary root system that will support the plant throughout its life cycle.
Positioned opposite the radicle is the plumule, the embryonic shoot that will develop into the plant’s above-ground parts, including the stem and leaves. The plumule is protected by rudimentary leaves and contains the shoot apical meristem, the growth center responsible for producing all future shoot tissues. This structure remains inactive until the seed coat is breached and favorable environmental conditions stimulate its upward growth.
Connecting these two extremities are the hypocotyl and the epicotyl, which form the embryonic stem axis. The hypocotyl, meaning “below the cotyledons,” is the segment situated between the point of cotyledon attachment and the radicle. This section often elongates significantly during germination to hoist the cotyledons and plumule through the soil surface.
Conversely, the epicotyl, meaning “above the cotyledons,” is the short stem section located between the cotyledon attachment point and the base of the plumule. This portion of the stem continues to elongate after the seedling has broken ground, producing the first true leaves and contributing to the mature stem structure.
Nutritional Storage and Classification
Plant seeds rely on stored reserves to fuel germination before the seedling can begin photosynthesis. This nutritional material, consisting of carbohydrates, lipids, and proteins, is stored using one of two primary mechanisms, which determines the seed’s classification. The first method involves the persistence of the endosperm, a specialized triploid tissue that forms during fertilization to nourish the developing embryo.
Seeds that retain a substantial, separate endosperm tissue at maturity are referred to as albuminous seeds, a group that includes grains like corn and wheat. In these seeds, the cotyledons, or seed leaves, remain small and function mainly to absorb and transfer nutrients from the endosperm to the growing embryo.
Conversely, in many other plant species, the developing embryo fully consumes the endosperm tissue before the seed matures. These seeds, such as beans and peas, are termed exalbuminous because they lack a persistent endosperm. In this case, food reserves are transferred directly into the cotyledons, causing them to become large, fleshy, and the primary storage organ.
Monocot seeds, characterized by a single cotyledon called the scutellum, typically follow the albuminous pattern, retaining a large endosperm for storage. Dicot seeds, characterized by two cotyledons, are often exalbuminous, with the seed leaves serving as the main repository for nutrients. This difference in food storage and cotyledon number reflects an evolutionary divergence in the plant kingdom.