Tropism is a growth
response between a plant and an external stimulus. The
stimulus could be weather, touch, time, gravity or light. A
positive response is indicated by growth toward a stimulus
and a negative response is indicated by growth away from
Gravitropism is a plant's growth response to gravity.
Typically roots grow down into the ground, and stems and
leaves grow up above the ground. If one were to place a
plant on its side, it would begin to bend in an upright
position. This occurs because of the plant hormone also
called auxin that plays a part in gravitropism. When the
plant is laid on its side, the auxin concentration
increases along the lower sides of the roots and stems of
the plant. The auxin stimulates cell elongation on the stem
which makes the stem bend up toward the sky. The auxin
prohibits cell elongation in the roots which causes the
roots to bend down into the soil. Gravitropism allows the
plant to respond to gravity no matter what position the
plant is in.
Biology of Plants,
6th ed. Raven, Peter H., Everert, Ray F.,
Eichhorn, Susan E. Worth Publishing 1999.
Botany: An Introduction to
Plant Biology. Mauseth, James D. Jones and
Bartlett Publishers. Sudbury, Massachusetts 1998.