Seeds are amazing things. Perfect little portable starters for plants that can be stored until required and activated into life at will by just providing a few basic requirements. Of course, they can't be stored indefinitely and germination success rate does deminish with time, and this varies from species to species. I have always read that parsnip seeds should be bought fresh every year and I have found from experience that sweetcorn and French bean germination rates go down significantly with storage time. However, some seeds, such as carrots, lettuce and brassicas seem to be fine years after the expiration date on the packet.
Seeds are made up of three main parts - the outer protective coating, a store of food that will be needed to fuel germination, and the germ or embryo that will grow when the conditions are right.
The first requirement for germination is water and a seed will spend several days absorbing water until it has enough to activate the enzymes that power germination. However, a seed also needs oxygen so it is important to not just submerge it in water. As such, providing enough water but not too much is critical for the first few days if germination is going to be successful. The final requirement is warmth and this is why seeds don't tend to germinate during winter. Germination rates are better if done inside or in a greenhouse during spring. Again, however, it is important to not over do it and if they are left in a closed greenhouse on a sunny day they can go above 40°C and this can cause the enzymes to be denatured (destroyed).
The first stages of germination happen under the ground and it can be difficult to be patient during this time and to not start to doubt that it will ever happen. This is particularly the case this year when I find myself standing in the greenhouse several times a day, wondering if anything has grown since I last looked! Sometimes just a little indication of disturbance to the soil surface is enough to get me excited.
After the initial stage of absorbing water, the root is the first thing to emerge and hormones control its growth towards gravity (gravitropism). Next the shoot emerges and hormones cause it to grow towards the light (phototropism). This is quite amazing really and very useful so that you don't have to worry about planting a seed the wrong way up!
It is only when the shoot breaks through the surface of the soil do we finally know for sure that germination has been successful. The leaf seeds (cotyledons) unfold and a seedling is born. Cotyledons are weird things because they look like leaves but very often they look nothing like the leaves that the plant will eventually go on to grow. As such, it can be very hard at this stage to know if what has emerged is the thing you planted or some rouge weed seedling. It is only after the true leaves grow that it becomes clearer. However, with experience, it is possible to recognise seedlings at the coyledon stage too and to quickly spot an imposter.
By the time the true leaves start to grow, the supplies in the seed are exhausted and it now necessary for the little solar panels that are leaves unfurl and start photosynthesing to make food. Ah photosynthesis - that awesome process that is probably the only thing that is more amazing than germination!
Left to its own devices, nature is quite capable of taking care of itself and germination happens everywhere during spring and summer without human intervention. However, a little understanding of it can help you to know how to store seeds to keep them viable, the conditions you need to provide to allow seeds to germinate, and how to recognise seedlings in their early stages. But don't worry, understanding the science behind it never takes away from the wonder of it or the thrill of seeing seeds you have sown starting to grow.