A seed is to a plant what an embryo is to an animal: an organism in its earliest stages of development. As lifeless as seeds may look before germination, the potential they hold is clear. Simply pick up an acorn and gaze at the oak tree from which it has fallen, and you can see the potential for yourself.

A seed consists of three main components: the embryo, the food source surrounding the embryo that will nourish it as it develops, and the seed coat that protects the embryo and food source during dormancy. Most seeds require a period of dormancy before they will germinate. Dormancy provides a protected state in which a seed can "wait" for favorable growing conditions. And some seeds are able to wait a very long time. The oldest known viable seeds were from an East Indian lotus. They were 466 years old when they germinated.

Germination usually starts when the embryo is exposed to water. However, the ease with which this happens depends to a large extent on the toughness of the seed coat. In many seeds, water seeps easily through this protective shell. But in some cases, the coat is watertight and requires some other action, such as fire, sand or soil abrasion, or the digestive processes of an animal, to weaken it. When water enters the seed coat, it swells the embryo inside, bursting the seed coat and setting primary growth into motion.

The embryonic root, or radicle, is the first structure to emerge from the seed during germination. It penetrates the soil very rapidly, forming a slender, usually unbranched taproot, which, in some plants, may penetrate several feet into the soil during the first few weeks of growth. This rapid stage of development, known as primary growth, is made possible by dramatic rates of division and elongation of cells in the radicle. New cells originate at the meristem, a structure near the tip of the growing root. The root cap at the very tip of the radicle protects the meristem as the root moves through the soil. Cells that are worn off of the root cap are replaced by cells from the meristem. The main function of the meristem, however, is to produce the cells that will make up the critical tissue systems of the root.

These cells, called sieve-tube members (the conducting cells of the phloem) and vessel members (the conducting cells of the xylem), undergo dramatic changes shortly after they are produced in the meristem. Most important of these changes is the elongation of the cells themselves. A single cell may grow more than 10 times its original length in just a few hours. Not only does this elongation contribute to the overall length of the root, allowing it to reach water deep underground, but also helps to create the system of tubules that will conduct water and nutrients throughout the plant during the course of its life.