Each seed is a self-contained unit capable of developing into a mature plant under suitable conditions. This fundamental role in plant reproduction is important for the maintenance of ecosystems and has implications for human life, especially as a primary food source.
The Seed Coat
The seed coat, also known as the testa, serves as the outermost protective layer. Its primary function is to safeguard the delicate internal components, the embryo and its food supply, from physical damage and environmental stressors like desiccation.
The thickness and texture of the seed coat can vary significantly among different plant species. Some seeds, like beans, have relatively thin and soft coats, while others, such as coconut or locust seeds, possess thick and hard coverings. This variation influences the seed’s resistance to damage and its ability to regulate processes like dormancy. The seed coat also acts as a barrier against pathogens, including bacteria, viruses, and fungi, preventing them from infecting the embryo.
Beyond physical protection, the seed coat plays a significant role in controlling seed dormancy, a state where germination is temporarily suspended. It can regulate the passage of water and gases into the seed, thereby preventing premature germination. In some species, the seed coat may even contain chemical inhibitors that must be leached out or broken down by specific environmental cues, like fire or cold temperatures, before germination can occur.
The Embryo
Within the protective confines of the seed coat lies the embryo, which is essentially a miniature, undeveloped plant. This embryonic plant contains the rudimentary structures that will eventually grow into the adult plant upon germination. It is the living core of the seed.
The embryo consists of several distinct parts, each with a specific role in future development. The radicle is the embryonic root, growing downward to anchor the plant and absorb water and nutrients. The plumule is the embryonic shoot, which will develop into the stem, leaves, and other aerial parts of the plant.
Also part of the embryo are the cotyledons, often referred to as seed leaves. These structures are integral to the embryo’s form and can vary in number; monocot plants have one cotyledon, while dicots have two. While cotyledons can later become photosynthetic, their initial role within the embryo is primarily structural.
The Food Supply
The seed contains a food supply that provides nourishment to the embryo during its dormant phase and the initial stages of germination. This stored energy is needed until the seedling can produce its own food through photosynthesis. The food supply is composed of starches, oils, and proteins.
There are two primary forms in which this nourishment is stored: endosperm and cotyledons. The endosperm is a separate nutritive tissue that surrounds the embryo in many flowering plants. It is particularly prominent in cereals like wheat and corn, where it constitutes a significant portion of the mature seed and provides the bulk of the stored food.
In other plants, particularly many dicots like beans, the endosperm is largely absorbed by the developing embryo before the seed matures. In such cases, the cotyledons themselves serve as the primary storage organs for food reserves. These cotyledons directly provide the necessary energy until the young plant can begin photosynthesizing and sustain itself.