The Amateur Scientist; February 1996; Scientific American Magazine; by Carlson; 2 Page(s)

Something remarkable happens when you tip a plant. Special hormones, called auxins, begin to collect in the underside of its roots and stem. Auxins stimulate stem cells to grow and divide. The bottom of the stem then outgrows the top, causing the stem to bend skyward. Auxins in the root cells act differently: they retard growth. The auxin-poor cells near the top of the root then outgrow the auxinrich cells near the bottom, and the root bends downward. In this way, a tipped plant makes internal adjustments to realign itself with the pull of gravity.

Botanists call a plant's response to gravity geotropism. In the early 1800s experimenters explored geotropism by growing plants on a rotating wheel, thereby exposing them to both the earth's gravity and centrifugal forces. The plants grew against the vector of the resultant force--that is, against the direction of the combined forces.