Subject: What is Hydrotropism when you have a been seedling?

In school we learned about 3 kinds of tropisms. I think I understand 
Geotropism and Photoism pretty good. But Hydrotropism doesn't really make 
sence to me. I know that Geotropism is when the stem and leaves grow up 
away from gravity and when the plant is tipped on its side, the stem bends 
and continues to grow up away from gravity. I also know that Phototropism 
is where the stem and leaves grow toward the source of light. A plant that 
is placed in the far corner of a box with a hole at the other end where the 
light is grows towards the hole and through it to get to the source of 
light.I know that Hydrotropism is caused by water and happens mostly in 
the roots but that doesn't really make sense to me. Can you explain it to 
me better?   Thank you. 

Re: What is Hydrotropism when you have a been seedling?