The seed, no matter what size, already has the ingredients and parts to grow and nourish the plants. The part of the plant that is inside the seed is very tiny and not nearly as big as it can get yet. With the right environment (water, soil, temperature, sunlight), the plant will come from it's shell (the seed) and begin to grow. As the plant begins to strengthen and grow, it will no longer need the nutrients that the seed provides as it will get it from the soil and grow into the plant it is supposed to.
I hope this explanation helps you. If not, please say so and I'll edit it. Hope this helps.