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Plant Gravity

October 11, 2007

A listener asks: Regardless of how you plant a seed, the roots always grow downward and the shoots upward, so how does the plant orient itself?


The gravity of plants. I’m Bob Hirshon and this is Science Update.

Regardless of which way you plant a seed, its roots always grow downward and its stem upward. This inspired listener B-J Stone of White Plains, New York to ask how the seedling manages to orient itself. We turned to plant physiologist Cary Mitchell of Purdue University. He says that gravity is the environmental signal plants use to determine up and down.

CARY MITCHELL (Purdue University):
If you were to launch a seed into space, in the absence of gravity, you would get an aimless germination and it would just wander.

But in the presence of gravity here on earth, dense starchy bodies inside the plant settle to the bottom of each cell. The plant senses up and down based on the location of these dense bodies, triggering its roots to grow toward water and mineral nutrients and its shoots to grow toward the sun. I’m Bob Hirshon, for AAAS, the science society.