Why do trees grow straight?

Why do the trunks of trees located on the sides of hills and slopes,still grow straight upward, instead of perpendicular to the slope of the land?

I was told that the answer is NOT because they grow toward the sun.

Is it because they grow atop their roots, and the roots are straight because they are reaching toward the water source?

Geotropism is the response to an organsism toward or away from the direction of gravity. Plants can tell up from down. The trunk and grows up in response to gravity.

The direction of growth of plants in general is dictated by gravity, not by the sun. (How would seeds underground otherwise know which way to grow to get the leaves above ground ASAP?)

IIRC, there’re growth hormones that are both less and greater density than plant cell contents in general.

So, a root that is growing sideways out of a seed (yes, the structures growing out of a seed are initially in a specific direction) will accumulate the less dense hormone on the side of its cells that is facing up. Those cells will grow a little lopsided, slowly pusing down the direction of the growth at the root tip, which is directed by a different process. (Imagine an opposite effect in the stem cells using the heavier hormone.)

One experiment I did in HS biology was to fill a paper towel with soil and stuff it into a Mason jar. Then I pushed beans down the sides of the jar (between the glass and the paper towel). The beans were in various orientations. When they sprouted, the roots came out in one specific direction relative to the beans orientation and the stem in the opposite direction. Then, after both had cleared the bean by about 1/2 inch, they started growing up (stems) or down (roots) as usual.

IIRC, experiments on the Space Shuttles have show that seeds lose this direction sense in zero gravity (aka free fall). They grow any which way, and would probably be influenced if the ship had a extended acceleration in a particular direction, simulating gravity.

AWB wrote: *
One experiment I did in HS biology was to fill a paper towel with soil and stuff it into a Mason jar. Then I pushed beans down the sides of the jar (between the glass and the paper towel). The beans were in various orientations. When they sprouted, the roots came out in one specific direction relative to the beans orientation and the stem in the opposite direction. Then, after both had cleared the bean by about 1/2 inch, they started growing up (stems) or down (roots) as usual.*

You can see this in established plants, as well. A technique I use to create a stronger root system in my tomato plants is to strip off the lower leaves of the seedlings and plant them at a shallow angle right up to the leaves. The stem that is underground will sprout new roots and the stem that is above ground will straighten up in just a few days. The new roots that grow out of the buried stem don’t sprout randomly around the stem. The grow only on the lower side and grow down into the soil.

Also, while planting your bulbs the right way up will certainly increase your chances of success, bulbs planted upside down can still grow. And they are generally 6-8 inches underground when they send out their stems and roots, so light has nothing to do with the direction of growth.

Of course, it is also true that when light is insufficient, plants will lean toward the light source. I grew an amaryllis this year and kept in in the middle of the dining room table. Each day it would lean toward the window and I would give it a half turn every so often so it would straighten out. And when I grow seedlings, I keep the light source close to them so they grow bushy instead of spindly and rotate them so they grow evenly. So light does have an effect on direction of growth, but it’s not why the stems go up and the roots go down in the first place.

Actually most plants - including trees - will grow towards the sun. For example, take a small potted plant and stick it halfway in an area the only receives sunlight on one side, such as in the path of a sunbeam in a window. Give it about a year and you’ll notice the plant has gotta pretty good ‘lean’ to it.

A side note–

Trees grow straighter–fewer bends, forking, etc.–when they’re in a forest and have to compete with other trees for sunlight. They need to devote most of their energies to getting up to the canopy where the sunlight is. Trees out in an open field are free to grow any which way.