Seeds are alive, even as they rest in seed packets on store shelves. They are merely in a state of dormancy as they wait for the right conditions in which to germinate. Nature has given them the tools they need to survive until germination is possible. Once those conditions are met, seeds undergo a process in a specific order before becoming full-fledged seedlings.
Seeds contain inside themselves almost everything they need to begin growing. The hard outer coating is called the testa, and its function is to protect the interior of the seed. A tiny hole in the testa that is usually not visible to the naked eye is called the micropyle, which becomes important during germination. Inside the testa are either one or two cotyledons (dependent on species) that store all the food the seed will need to stay alive until germination. Also inside is a tiny baby plant, which has two ends. The radicle (root end) and the plumule (leafy top end) hibernate and live on the stored cotyledon energy until germination or death, if a suitable germination site is not found before that energy runs out.
When a seed finds the proper moisture and temperature conditions, germination begins. Water seeps into the micropyle, signaling the radicle to begin breaking free of the seed in search of nutrients. The radicle punches through the testa, searching for moisture and nutrients in the soil. It is shortly followed by the plumule, which takes the remaining broken testa and pushes it up out of the soil in its quest for light. The radicle grows and sends out new offshoots as it gathers nutrients, developing a strong root system for the seedling. The leaves, in turn, gather energy from the sun (photosynthesis) to power the nutrient uptake of the roots, which powers the overall growth of the young plant--leaves, flowers, fruits, roots and all.
Each species has a specific ideal temperature frame for germination. Since temperatures fluctuate readily in nature, not all seeds of a specific species may germinate at the same time, even if they are planted in the same place. Even minor variances such as wind or sun exposure can affect when seeds will germinate. If seeds do not find a good place to germinate right away, many of them can wait up to a year while living on energy stored in their cotyledons. Some can wait years longer, if stored in cool, dry conditions. Others may die before ever becoming plants if their cotyledon energy runs out.
Planting depth is of great importance to a seed's ability to germinate. Larger seeds have larger cotyledons and can be planted at greater depths than smaller seeds because the baby plant inside will have enough energy to break through to the surface and begin producing its own food supply. Smaller seeds need to be planted very close to the surface, or even directly on the surface. Seeds buried too deeply may still germinate, but the seedlings will die before ever reaching the surface because they will run out of food.
Some seeds also require a certain amount of sunlight to germinate, and so need to be planted on or very close to the surface. Their planting instructions will indicate if this is the case.
Seeds kept for a year or more, even if kept under ideal conditions, may not have a very successful germination rate. To test these seeds, gardeners should dampen a paper towel and place 10 seeds on it, then place the seeded paper towel in a zip-top bag. The bag should be left at room temperature for a week or two, depending on how quickly the plant in question normally germinates (found on the seed packet). The number of seeds that germinate is a good indication of what percentage of the seeds a gardener can expect to be viable.