Re: Why does a tree grow verticly when growing on an angle.EX:on a mountain.

This is a good question, and you've done well to think of a possible 
answer. Plants don't have sensory organs like animals, but they do sense 
and respond to their environment. A tree growing on a slope grows straight 
up because plants' growing tips (meristems) at the ends of roots and 
shoots respond to both gravity and light. Root tips respond positively to 
gravity growing toward the center of the Earth (and in the case of 
dandelions, the roots nearly reach the Earth's center).  Shoot tips 
respond negatively to gravity by growing upward.  Shoot tips also respond 
positively to light, the sun overhead, and many roots respond negatively 
to light.

Experiments on tropisms are easy to conduct with growing seeds.  If you 
have some corn or beans, examine the seeds.  In corn the embryo forms an 
oval patch low on one side with the root closest to the end.  The bean 
embryo consists of a pair of leaves, cotyledons (most of the bean) and 
they are connected to a short root and a short shoot with small leaves.  
You can soak them in water over night, and then position them in various 
ways and let them germinate.  Even in the dark, shoots will turn upward 
and roots downward.  The turning of a root or shoot is caused by cells on 
the opposite side growing more than those on the near side, which is turn 
is caused by different concentrations of growth substances. This is handy 
because you don't have to carefully position seeds when you plant them to 
get them to grow properly.  

So a tree is straight on the slope because it grows in alignment with 
gravity and light.  You could germinate a seed on a slope in the dark and 
see this happen on a small scale.