Plants have a hormone called auxins which stimulate growth. Since plants perform photosynthesis, plants tends to grow in the direction of the sun in a process called phototropism. In this case, the side of the stem that is facing the ground would be simulated by auxin to grow while the side of the stem facing the sky will not grow as much causing the plant to grow upwards. If you are in a forest, you might notice some trees with a curved base because they were also propped on its side at one point
