How do seed cells know the difference between top and bottom?

[Dr E. Buxbaum]

ComDep at lottery.powernet.co.uk  (Matthew) wrote:
>If you plant a germinating seed in soil, how does the seed know where
>is the top and where is the bottom? 
>
>In other words, the "leaf part" grows upwards, and the "root part"
>downwards. What sensory device does the seed appear to have to know
>where to grow.

This is called geotropism (growth orientation in a gravity field). If you look at plant roots 
under the microscope, you will find crystalls in the cells near the root tips. It is assumed, 
that they are gravity sensors, however, to the best of my knowledge the mechanism how this 
works has not been elucidated.

If you want to experiment with this, you can use a "geostat", a turntable (like the one used 
for vinyl records) on which the plants are mounted. The plants will then orient themselfs in 
the centrifugal field as they would do in the gravity field of the earth: Roots growing 
outward, stems growing inward, in other words the plants will grow horizontally. The speed of 
the turntable will determine the strength of the centrifugal force.