Have you ever looked at a seed? It's amazing how that little, hard thing can be the source of a garden, flower bed or houseplant, even a huge tree. All seeds are covered by a seed coat. That coat is an amazing thing by itself.
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A seed coat protects the embryo of a new plant. It is a membrane that grows and coats the embryo after germination in the mother plant. Nestled inside the coat is the dormant embryo, the cotyledon or seed leaf, the plumule or first true leaves of a seedling and the radicle--the latter which is the beginning root system. Housed within the seed coat are the nutrients if the embryo will need them before it is able to gather nutrition from the soil it is rooted in.
The seed coat, or testa, can be impenetrable, especially in long dormancy seeds. Dormancy is ensured because the seeds can not be easily broken into or out of. These seeds must be scratched (scarified), notched or split open to force them to allow for sprouting and growth of a new plant. Other seeds sprout more easily by being planted directly into moist soil or by being soaked in water for a few hours to soften the testa. The contents of the seed coat are protected until it is time for them to produce new life.
Seeds can be viable only for a few days up to hundreds or thousands of years. In general, seeds are viable for a good 15 years if stored properly in containers that are in a cool, dark place. The seed coat will dry out if in a hot area. If the seed coat is in contact with moisture for a prolonged period, it will be breached and the embryo will deteriorate or be destroyed by bacteria. In some cases, the seed coat can be strong enough to withstand the juices in an animal's digestive tract long enough to be passed through its system and carried to another location. It will then fall to the soil, and a new plant or tree will grow in this new place.
Seeds come in all sizes and shapes. Seed coats also have a variety of textures and appearances. Some have appendages. An aril can be found on a nutmeg seed, an elaiosome on corydalis, and hairs will be found on cotton seeds. You may notice scars called hilum on seeds. This is the area in which the seed was attached to the ovule of the mother plant. When seeds are infected by bacteria or disease, this area may start showing discoloration or mottling.
Animals and humans are able to eat or bite through seed coats. We eat sunflower seeds and peanuts, for example. A seed coat on the peanut is a brown, papery cover, yet the coat on a coconut is so strong we have to break it open, usually after piercing it with a tool. The smallest seeds, such as mustard and petunia, can weigh less than 1 g, while the double coconut palm tree seed weighs up to 60 lbs. each. The seed coat is an incredible thing, as it protects the embryo of the things that we need to grow to survive.