Subject: what direction do plant roots grow in and why do they grow this way?

I have done an experiment and found that the roots grow downwards.
I did this by placing four beans inbetween wet newspaper and plastic. 
The beans were in different positions two side ways one upwards and 
one downwards.
I would like to know what causes the roots to grow in this direction?
Is it the effects of positive geotropism? Can you explain this to me?
Thank you.

Re: what direction do plant roots grow in and why do they grow this way?