A seed is a plant embryo enclosed in a seed coat that is usually hard and tough. Also contained within the seed coat is a small nutrient supply. This is just enough to sustain the plant during the earliest stages of growth, until the plant's leaves emerge and begin to provide food for the plant by the process known as photosynthesis.
When a seed is placed in the appropriate environment--one that has the correct amounts of warmth, oxygen, and moisture--two things happen. The moisture softens the seed coat and encounters the plant embryo. This, the oxygen, and the warmth cause the embryo to "wake up" from its dormant state and begin to grow. This stage is called germination.
As the plant tissues absorb moisture they begin to swell, putting pressure on the seed coat. The pressure eventually causes the seed coat to split open, releasing the embryo.
The embryo has two points of growth: One (the radicle) forms the roots, while the other (the plumule) develops to form the stems. In most flowering plants, one or more cotyledons (seed leaves) will emerge from the plumule.
As the radicle grows downward and becomes the root, it seeks moisture. Eventually, tiny hairs will develop on the roots that help the plant obtain nutrients from the soil. This process is helped by a symbiotic relationship in which the plant provides sustenance to soil bacteria, and the bacteria act on soil nutrients to make them available to the plant.
Shortly after the radicle emerges from the seed coat, the stem and cotyledons grow upward, toward the light. During this stage, the plant is still totally dependent on its stored food supply. If it has been planted too deep, it may exhaust its stored food before the cotyledons reach the surface, and die. However, if the cotyledons are able to emerge into the light--the stage known as photomorphogenesis--they immediately prepare for the process known as photosynthesis. As they push toward the light, the tender cotyledons often "wear" the split seed coat as protection.
When the stem and cotyledons emerge into the light, they are usually white or pale yellow. They begin to produce chlorophyll and rapidly become green, a stage known as de-etiolation.
Chlorophyll molecules make it possible for photosynthesis to begin--light is absorbed by the leaf, and that energy is used to turn carbon dioxide from the air into organic compounds, principally sugars, on which the plant feeds.
While all this is happening, the plant's roots are branching and extending wider and deeper into the soil. As more leaves are produced, the rate of photosynthesis increases and the plant begins a surge of growth toward maturity.