Seeds for plants, trees and bushes all grow via the same means. The ingenious structure of seeds protects them from the outside world until conditions are right for them to emerge. Some seeds, such as those from strawberries, even have an extra protective coating because they are intended to be ingested by birds, animals or people before finding their way into soil.
Seeds of all shapes, sizes and species have the same internal structure. The cotyledon is like a food storage pouch inside the seed. The seeds of some species even have two of them. The radicle (root portion) and the plumule (leaf-sprouting above-ground portion) exist in very tiny form within the seed, feeding off the cotyledon until conditions are right for germination. All of this is protected by the hard outer shell, called the testa. It is mostly impermeable, except for a microscopic hole called the micropyle that sucks in water when it becomes available.
When conditions of moisture and temperature are correct for the seed, the germination process begins. Water seeps in through the micropyle, encouraging the radicle and plumule to begin growing. The radicle grows downward, poking through the testa and into the soil. It begins uptake of nutrients as the plant pushes the plumule up so that it can break ground and begin reaching for the sun. The plumule photosynthesizes the energy needed for the root system to grow larger and take up more nutrients. Both the root system and the above-ground foliage system grow as a result.
A seed's energy storage system allows it to stay dormant for a very long period of time. Not all seeds will find a suitable growing environment before the cotyledon is exhausted, but this system allows a relatively long period of time before that happens. If gardeners keep seeds in a cool, dry place, they may stay viable for a long time. They will certainly keep for a year, and may in some instances keep even longer than that. Both the plant species that spawned the seeds and gardeners benefit.
Temperature needs for germination of seeds vary among species. Outdoor temperatures and conditions are variable, so not all seeds from a given planting may germinate at the same time. Water levels are very important for germination. Too little water will simply cause seeds not to germinate at all. Too much water can cause them to drown and rot before the seeds break the surface. Seed planting depth can roughly be calculated according to seed size. Larger seeds have larger cotyledons, and the plumules can spend a longer amount of time poking up to the surface. Seed packets have recommended planting depths printed on them. Some seeds also require sunlight to germinate, which will be indicated on the seed packet as well.
Although most seeds pass their periods of viability after a few years, instances of seeds remaining viable for significantly longer periods of time have happened. Lotus seeds thought to be thousands of years old have germinated under scientists' watchful care. The protective mechanisms of seeds may lead to exciting scientific discoveries of species either thought dead or never before known in the future.