Interactions of Plants and Gravity - Geotropism Lab
Gravitropism (also known as geotropism) is a movement by a plant in response to gravity. Roots show positive
gravitropism (moving with the force of gravity) while stems and leaves show negative gravitropism (growing against the
force of gravity). Although this phenomenon commonly occurs in nature, it can be easily demonstrated with a potted
plant in a classroom. When laid onto its side, the growing parts of the stem begin to display negative gravitropism,
growing upwards. Herbaceous (non-woody) stems are capable of a small degree of actual bending, but most of the
redirected movement occurs as a consequence of root or stem growth in a new direction.
Gravitropism in Roots
Root growth occurs by division of stem cells in the root meristem located in the tip of the root, and the subsequent
expansion of cells in a region just proximal to the tip known as the elongation zone. Differential growth during tropisms
mainly involves changes in cell expansion verses changes in cell division. Gravity is sensed in the root tip and this
information must then be relayed to the elongation zone so as to maintain growth direction and mount effective growth
responses to changes in orientation and continue to grow its roots in the same direction as gravity.
Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant
hormone auxin. Auxin exists in nearly every organ and tissue of a plant. In roots, an increase in auxin concentration
generally inhibits cell expansion. Therefore the redistribution of auxin toward the lower flank of a root, that has been
reoriented in the gravity field, can initiate differential growth resulting in root curvature.
Gravitropism in Shoots
A similar mechanism is known to occur in plant stems, except that the shoot cells have a different response with respect
to auxin. In shoots, increasing the local concentration of auxin promotes cell expansion; this is the opposite of root
cells.
The differential sensitivity to auxin helps explain observatios that stems and roots respond in the opposite way to
gravity. In both roots and stems auxin accumulates towards gravity on the lower side. In roots, this results in the
inhibition of cell expansion on the lower side and the curvature of the roots towards gravity (positive gravitropism). In
stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion and results in the
shoot curving up (negative gravitropism).
Materials:
1 shoe box
2 square flower pots containing radish seedlings
Procedure:
In this experiment you will measure the impact of gravitropism on radish seedlings stems for a 24 hour period.
Procedure Day 1:
Step 1: Turn one flowerpot containing radish seedlings on its side and leave it on the lab table for 24 hours.
Step 2: Turn another flowerpot containing radish seedlings on its side inside a shoebox for 24 hours.
Create a Hypothesis:
In the space below, write a hypothesis regarding this lab procedure. Be sure to write your hypothesis as an “If
(independent variable) then (dependent variable)” statement. Be specific!!
Name the independent variable(s) of this experiment:
Name the dependant variable(s) of this experiment:
Draw a sketch of your experimental set up. Label the parts of your experimental including the control.
Analysis Questions:
1. Define gravitropism.
2. Define positive gravitropism.
3. Explain how roots use auxin in order to exhibit positive gravitropism.
4. Explain how stems use auxin in order to exhibit negative gravitropism.
5. Explain this statement: “Differential growth during tropisms mainly involves changes in cell expansion
verses changes in cell division.”
6. Explain why it is necessary for roots to exhibit positive gravitropism and for stems to exhibit negative
gravitropism.
7. Using your intuition and the diagram below, explain what a clinostat is and why it is used as a control in
a gravitropism experiment.