Do you Know Why do stems grow up and roots grow down because Plants can sense which direction is up and which direction is down. This is because of a phenomenon called geotropism that helps a plant to respond to gravity. Geotropism is controlled by certain plant hormones.
When a seed starts to germinate, no matter which may it is positioned the roots will always go down and the stem will always go up. The roots need to grow downwards into the soil to obtain the water and the nutrients present in the soil. In same way, the stem must grow upwards towards the light, because the leaves must get sunlight. Sunlight is essential for photosynthesis to take place and the plant can manufacture its food and grow only through photosynthesis.