A seed is a biological package designed for dispersal and survival. It contains a tiny embryonic plant, a stored supply of energy and nutrients, and a protective seed coat. This structure allows a seed to remain dormant, waiting for suitable conditions to sprout and grow.
The Seed’s Energy Storehouse
The primary energy for a developing seedling comes from storage tissues within the seed. These energy reserves are typically stored as complex molecules like starches, fats (lipids), and proteins. The specific locations for this stored food vary among different types of seeds.
One major storage location is the endosperm, a nutritive tissue formed inside the seeds of most flowering plants. The endosperm surrounds the embryo and provides nutrition, often being rich in starch, though it can also contain oils and protein. For instance, cereal grains like corn, wheat, barley, and rice store significant amounts of starch in their endosperm. Castor beans and coconuts are also examples of seeds where the endosperm is a primary energy reservoir, storing fats or a combination of nutrients.
In many other plants, particularly dicotyledonous seeds like beans, peas, and peanuts, the energy is stored directly within the cotyledons. Cotyledons are often referred to as “seed leaves” and are a significant part of the embryo. These cotyledons become enlarged and fleshy, serving as the main storage organs. Soybeans, for example, can store up to 40% protein by weight in their cotyledons. Whether the energy is in the endosperm or the cotyledons, these structures act as a food supply for the young plant before it can produce its own food through photosynthesis.
Fueling Early Plant Growth
The stored energy within a seed fuels its early growth, especially before the seedling can perform photosynthesis. The process begins with the seed absorbing water, a process known as imbibition. This water absorption causes the seed to swell and softens the seed coat, allowing the embryo to emerge. Water activates enzymes within the seed that were previously in a dormant state.
These activated enzymes play a role in breaking down the complex stored food reserves. For example, amylase enzymes break down starches into simpler sugars like glucose, while lipases break down fats into fatty acids and glycerol, and proteases break down proteins into amino acids. These simpler molecules are then transported to the growing parts of the embryo. They serve as both fuel for cellular respiration, providing energy (ATP) for growth, and as building blocks for new cells and tissues.
This initial energy supply is essential because the developing seedling cannot begin photosynthesis until it emerges from the soil and develops true leaves. The stored energy powers the growth of the radicle, the embryonic root, which is typically the first part to emerge from the seed. The radicle grows downward to anchor the plant and absorb water and nutrients from the soil. Following the radicle, the plumule, the embryonic shoot, begins to grow upward, eventually developing into the stem and leaves that will reach for sunlight. Without this pre-packaged energy, the seedling would not be able to establish itself and begin independent life.