Have you ever wondered why roots grow down, and stems grow
upno matter how you plant a seed? The term geotropism
(sometimes called gravitropism) describes how
plants respond to gravity. Roots are termed positively geotropic; that is, they
grow toward the direction of the pull of gravity. Shoots are negatively
geotropic because they grow away from that force. If you plant a seed on its
side, the shoot and root will emerge horizontally, but will quickly change their direction
of growth. Within hours of germination, the shoot will bend to grow upward and the root
will bend to grow downward.
corn seed planted vertically, sideways, and upside down
The mechanism for this change in the direction of growth
is similar to that for phototropism. Under the influence of gravity, auxin migrates to the
lower side of the stem. Here, the hormone stimulates growth, causing the lower side to
grow more quickly than the upper, bending the stem upwards.
Remarkably, the opposite happens in roots. As in the stem,
the auxin collects in the lower side of a horizontal root. However, root cells behave
differently than stem cells. Root cells elongate in response to the lower levels of
auxin in the upper surface, causing the roots to bend downward.