Do tree roots always grow downwards?

I remember doing that experiment as a kid!

Are you saying that if the tree was spinning as in the bucket demonstration, the water would stay above the surface and the roots would grow upwards towards the water? I think the bean and blotting paper experiment would demonstrate whether roots grow upwards. If the blotting paper only extended half way down the glass container (jar) and the bean was wedged between glass and paper, close to the bottom edge of the blotting paper, the root should grow upwards to take up the water in the blotting paper ... but would it?

Shelagh,
The bean and blotting paper experiment sounds like a good idea to try this out. I'll see if there are some links on the net that lead the way.

The theoretical principles is what I'm trying to illustrate here.

In this example I gave above of water in a bucket being spun at high speed :



The reason why the water does not spill is because it is pushing against the bottom of the bucket (the direction of the arrow in the diagram where it says 'centrifugal force').

So if the bucket was spun like this:



At position C, when the bucket is upside down for a split second, we see the water to be pushing upward against the bottom of the bucket, hence the water will not spill out. So as long as someone keeps the bucket continuously spinning in this way, the water will not spill out, even when it goes upside down for a split second in each rotation.

Therefore, the same principle should apply to the roots of a tree planted in such a spinning bucket. The same centrifugal forces will be at play to cause its roots to grow towards the bottom of the bucket:




Question is: has any experiment been done yet that demonstrates this in practice?

Plants and gravity:

http://www.rbg.ca/cbcn/en/cbcn4kids/kid_activ.htm

Scroll down the page to find "Plants and gravity".

That little kiddies experiment is not quite what I had in mind.

The way to answer this would be to put a plant into an astronaut training centrifuge and have it spinning 24/7 for a whole month non-stop. Then to see if the roots of the plant have grown outwards as I predicted they would in my interstellar ark back in 2004:





Or there is this kiddies experiment, where one could place a plant on the inner rim of a tyre and roll it continously for a few hundred miles:

https://www.youtube.com/watch?v=dg7s6Ohrp0c

On the link I provided it says:
"Can you think of an example of positive geotropism? What about the roots of the plant? They are growing towards the force of gravity. How does it work? Plants have hormones that respond to the pull of gravity and tell the plant which way to build cells and grow. So now you know that you can't plant a seed upside down."

I remember doing the bean experiment when I was a child. The upside down beans had shoots that started to grow in the wrong direction and then started to grow in a curve and eventually grew in the right direction. When I asked about the blotting paper only half-filling the jar, I asked if the root would grow upwards to find the water. The answer is that it wouldn't. The root assumes that water is below ground and will grow downwards, expecting to find water. It is the force of gravity that determines the direction of growth not the presence or absence of water.