A seed is a self-contained unit produced by flowering plants and conifers, fundamental to their reproductive cycle. It encapsulates an embryonic plant and stored food reserves, protected within an outer covering. This structure allows plants to disperse offspring and ensures the survival of the next generation until conditions are suitable for growth.
The Seed’s Outer Protection
The outermost layer of a seed is the seed coat, also known as the testa, which acts as a protective barrier. This tough covering shields the delicate embryo from physical damage and guards against desiccation by preventing excessive water loss. The seed coat also provides defense against various pathogens like bacteria, fungi, and viruses, ensuring the embryo remains viable.
The hilum is a scar marking where the seed attached to the parent plant. Near the hilum, a small pore called the micropyle allows water to enter the seed, which is essential for initiating germination. During seed ripening, the hilum can also regulate the seed’s moisture content. Both the hilum and micropyle can serve as entry points for pathogens if compromised.
The Embryo: A Plant in Miniature
Within the protective seed coat lies the embryo, a miniature, undeveloped plant. It consists of several primary parts, all organized along a central embryonic axis. The embryonic axis itself is the main stem of the embryo, connecting its various structures and eventually developing into the mature plant’s stem.
One end of the embryonic axis terminates in the radicle, the embryonic root. The radicle is the first part of the embryo to emerge during germination, growing downwards to anchor the seedling and absorb water and nutrients from the soil. At the opposite end is the plumule, the embryonic shoot. The plumule develops into the plant’s future stem and leaves, growing upwards towards sunlight and eventually becoming responsible for photosynthesis.
Connected to the embryonic axis are the cotyledons, often called “seed leaves.” Their primary role in many species involves nutrient management. The number of cotyledons present distinguishes major plant groups: monocots have one, while dicots have two.
Nourishment for Early Growth
Before a seedling can produce its own food through photosynthesis, it relies on stored energy and nutrients within the seed. This nourishment is typically provided in one of two forms: the endosperm or the cotyledons themselves. The endosperm is a specialized nutritive tissue that surrounds the embryo in many seeds, providing carbohydrates, proteins, and oils. It supports embryonic growth by supplying nutrients and can act as a mechanical barrier during seed development and germination.
In monocot seeds (e.g., corn), the endosperm is the primary food storage tissue. The single cotyledon (scutellum) absorbs these nutrients from the endosperm and transfers them to the growing embryo. Conversely, in dicot seeds (e.g., beans), the endosperm is largely consumed during seed development, and cotyledons become thick and fleshy, serving as the main storage organs for food reserves. These stored resources fuel the initial growth of the radicle and plumule, enabling the young plant to establish itself before it can sustain itself photosynthetically.