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.
Seeds wait to germinate until three needs are met: water, correct temperature (warmth), and a good location (such as in soil). During its early stages of growth, the seedling relies upon the food supplies stored with it in the seed until it is large enough for its own leaves to begin making food through photosynthesis. The seedling's roots push down into the soil to anchor the new plant and to absorb water and minerals from the soil. And its stem with new leaves pushes up toward the light:The germination stage ends when a shoot emerges from the soil. But the plant is not done growing. It's just started. Plants need water, warmth, nutrients from the soil, and light to continue to grow.