A seed is a fundamental unit of reproduction for many plants. It encapsulates all the necessary components for a new plant to develop, protected and nourished until environmental conditions become favorable for growth.
The Protective Outer Layer
The outermost covering of a seed is the seed coat, or testa. This layer provides robust protection against various environmental threats, including physical damage, harmful pathogens, and excessive drying. Its thickness and texture can vary significantly between different plant species, ranging from delicate, papery coats to hard, woody shells.
Beyond physical defense, the seed coat regulates water absorption, an important process for germination. It prevents premature germination by acting as a barrier until sufficient moisture is available.
The Developing Plant
Within the seed lies the embryo, a miniature, undeveloped plant awaiting the right conditions to grow. This tiny plant possesses several distinct parts, each with a specialized role in its future development. The embryo represents the earliest stage of a plant’s life cycle, formed after fertilization.
The radicle, the embryonic root, is the first part of the embryo to emerge during germination, anchoring the seedling and beginning the absorption of water and nutrients from the soil. Above the radicle is the hypocotyl, which develops into the lower part of the stem. The plumule, at the tip of the epicotyl, consists of embryonic leaves and grows into the shoot system.
Accompanying these structures are the cotyledons, or seed leaves. These structures are either part of the food storage system or facilitate the transfer of nutrients to the growing embryo. The number of cotyledons, typically one or two, helps classify flowering plants into monocotyledons and dicotyledons.
The Stored Food Supply
The stored food supply within a seed provides the essential energy and nutrients for the embryo’s initial growth before it can produce its own food through photosynthesis. This nutritional reserve is critical for the seedling’s survival during its early stages. The form of this stored food can differ depending on the plant species.
In many seeds, particularly those of monocots like corn or wheat, the primary food storage tissue is the endosperm, which surrounds the embryo. This endosperm is rich in carbohydrates, proteins, and lipids, providing a readily available energy source. In other seeds, such as beans, the cotyledons absorb the nutrients from the endosperm during seed development and become the main storage organs.