Re: Why do roots grow down no matter what position the seed is in?

Bravo!  BOTH hypotheses are correct!  Roots grow towards 
areas of higher moisture.  Roots also grow down because of gravity.
This is called geotropism.  It is caused by abscissic acid, a natural
substance produced by plants that inhibits growth.  The compound settles
to the bottom half of the root.  The bottom half of a root that is growing
sideways will then grow slower than the top half.  It therefore bends 
downward.

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Admin note:

David Hershey adds the following:

Roots do not grow toward areas of higher moisture. The hypothesis of
hydrotropism is obsolete.

References

Hershey, D.R. 1992. Is hydrotropism all wet? Science Activities
29(2):20-24.