Why do trees grow straight up on hills and not angle the way the hill is angled?
Due to plant growth hormone, call auxins. The presence of auxins in plant tissue will stimulate growth. Plants in nature have the ability to balance the presence of auxins in plant parts either in root tissue or shoot tissue. At the same time auxins has negative effect against light. Less auxins is detected on the side of the stems/branches facing the light source and this will slowed down the growth on this side of the stem, while the shadowed side, which contain higher concentration of auxins will grow faster. The uneven growth rate of the stem/branch will l cause the plant to bend and growing towards the sun, and not perpendicular to the surface of the earth where the plant is growing.
Plant cells have a small amount of weight in them. When they are at an angle or even upside down the weight settles and the plant starts to grow upwards. I don’t remember the exact term for it all but to put it in simple terms, plans grow the opposite direction of the gravitational pull.
Gravity. They grow up to get to the sun. But they grow up to balance with gravity, as all plants do.