Gravitropism: the Role of Roots
Students’ Sheet
Why roots?
Roots are the hidden half of a plant, but they are responsible for providing the plant with all the
water and minerals they need to grow. Understanding roots better can help scientists breed crops
that use fertiliser more efficiently, and which can cope with either drought or water-logging. This
research could be particularly important for small farmers in the developing world, especially as
climate change affects their growing conditions.
Introduction
Plants sense and respond to gravity - gravitropism. In roots, this is positive gravitropism and
they grow towards gravity deeper into the soil. This helps anchor the plant, as well as allowing the
roots to obtain water and mineral ions. Water is absorbed by osmosis through the root hair cells
of roots to then enter the xylem to be transported throughout the plant. In the soil, plant roots will
naturally meet obstructions such as stones and hard lumps of mud, which the roots can sense
and change the course of their growth.
In this experiment, you will grow mustard seedlings on agar to visualise how numerous the root
hair cells are and observe them in 3D. You will then use a graticule to measure their length.
You could then extend your investigation to observe the effect of an obstruction in the path of the
roots, visualise the xylem in roots using food dye, and observe gravitropism in the roots.
Instructions
Step 1: You will be provided with an agar plate to grow your seeds on, but you need to prepare
sterile mustard seeds first.
Wear safety glasses throughout the preparation of sterilising the seeds and wash your hands at
the end of the planting of the seeds. Wipe up any spilt bleach.
1. Place the mustard seeds in a beaker with 6% bleach for 8 minutes.
2. Remove the mustard seeds using sterile forceps and transfer to the container of distilled
sterile water. After 20 seconds, using sterile forceps transfer the seeds to a fresh
container of distilled sterile water and then one more rinse after this. The 3 rinses in water
are to ensure that all bleach residue is removed.
3. Using sterile forceps transfer one mustard seed at a time to the top third of the agar plate,
gently press the seed slightly into the agar, it should not be buried, but you don’t want the
seed to slide down the agar when the agar plate is held vertically. 3 or 4 seeds can be
placed on each agar plate (figure 1).
4. Wrap each plate in clingfilm, to prevent desiccation and then place the agar plate in the
vertical position in a cut plastic bottle. Leave seeds to germinate, a window sill or shelf is
ideal.
5. Agar plates should be observed daily to monitor growth. Growth of roots typically occurs
within 24-48 hours.
Science & Plants for Schools: www.saps.org.uk
Gravitropism: the role of roots - p. 1
Revised 2013
Figure 1: how to plant the seeds
Figure 2: growth of mustard seeds at 48 hours
Step 2: examination of root hair cells and measuring their length using a graticule
1. Remove the clingfilm and lid from the petri dish. Place the petri dish on the stage of a
microscope and focus on low power until you can visualise the root hair cells on the root.
You could then focus using the medium power lens You may be able to observe trapped
water vapour amongst the root hair cells.
2. Identify the zone of elongation at the tip of the root, you should not see any root hair cells
here, but can see individual cells which are dividing by mitosis causing the root to grow in
length.
3. Move further up the root into the zone of differentiation, this is where root hair cells start
to become visible.
4. Gently place a graticule on top of the root where you have root hair cells, ensuring the
measuring scale lies parallel to the root hair cells. Using the scale measure the length of
some root hair cells.
Extension task 1: to observe the response of roots to an obstruction in their path
Roots will encounter obstructions in the soil, such as stones, buried objects and brick walls. Using
your agar plates and your seedlings you can observe the response of roots to an obstruction.
1. Remove the clingfilm and lid from the petri dish of grown seedlings, their roots must not
have reached the end of the petri dish.
2. Using sterile forceps place a sterile obstruction in the immediate path, but not touching
the roots, such as lego brick, ball bearing or glass bead.
3. Re-wrap the petri dish in clingfilm and place in a vertical position in the holder. Leave and
observe at 12-24 hours later.
Extension task 2: to observe the xylem in the roots using food dye
Water is transported from the roots to the leaves via the xylem. Water enters the xylem having
been absorbed by osmosis through the root hair cells.
1. Place approximately 200cm 3 distilled water into a 250cm 3 beaker, add 2-3 drops of food
dye and mix to ensure an even colour.
2. Set up 2 cocktail sticks parallel to each other over the beaker of dyed water, resting on
the edge of the beaker.
3. With sterile forceps remove a seedling that has roots and root hair cells from the agar.
4. Suspend the seedling between the 2 cocktail sticks ensuring the root hair cells are in the
coloured water. Leave the seedling for 24 hours.
5. Remove the seedling using forceps and place onto a microscope slide. Focus on low or
medium power on the root and look for the presence of the xylem running up the root
which will be the same colour as the coloured water.
Science & Plants for Schools: www.saps.org.uk
Gravitropism: the role of roots - p. 2
Revised 2013
Setup of the suspended mustard
seedling between the 2 cocktail
sticks with the root in the coloured
water
Extension task 3: to observe gravitropism in mustard seedling roots
Roots show a positive geotropic response; this is easy to observe with the roots of your seedlings
on the agar plate.
1. Using an agar plate with mustard seedlings with roots, that have not reached the bottom
of the petri dish, rotate the agar plate 90o. Place the agar plate back into the vertical
position in its holder and leave. Observe the roots after 2 hours or longer.
2. Rotate the agar plate a further 90o. Place agar plate back into the vertical position in its
holder and leave. Observe the roots after 2 hours or longer.
Questions
Does the length of the root hair cell vary along the root?
Is there a ratio between the width of the root and the maximum length of the root hair cells?
How do cells become specialised, such as root hair cells?
What was the response of the roots to the obstruction?
What cells does the water have to pass through by osmosis to reach the xylem from the root hair
cells?
How does the plant ensure the water potential of the cells that the water passes through is always
lower than the previous cell to enable the water to enter the xylem?
What is the water that enters through the root hair cells used for in the plant?
Why is it an advantage for roots show gravitropism?
How does a root sense a change in direction of gravity, to cause a change in the direction of
growth of the roots?
Science & Plants for Schools: www.saps.org.uk
Gravitropism: the role of roots - p. 3
Revised 2013