As we've already seen, plants change their direction of growth and they
do so in response to different stimuli in their environment (light, soil,
and water etc.)
Why do you think this is important to a plant?
Can a plant get up and walk to a sunny spot if it gets shaded by a tree,
or jump back up if knocked over by the wind?
Many organisms, including humans and plants, use light and gravity to
provide them with orientation and guidance in their environments.
we've already seen in the earlier activities, plants can change their
direction of growth (tropism). Plant roots grow away from the light
(called "negative phototropism") but toward gravity (called
"positive gravitropism"), whereas plant shoots grow toward
light (called "positive phototropism") and away from gravity
(called "negative gravitropism").
Why do you think it is important for these different parts of the plant
to respond in these different ways?
Think about the function of the roots and shoot and the resources
they need to carry these functions out.
How does a plant respond to light?
Which is stronger - a plant's response to gravity or light?
To help you answer these questions we've included two simple experiments
you can do to test how a stem segment (called the "hypocotyl")
of a young seedling responds to light and gravity and to test which is
the stronger response.
You will need the following materials:
35mm black film canisters with lids 35mm black film canisters with lids
An additional lid for each canister
A single hole punch or cork borer
Paper toweling (pieces cut into 1.5cm squares)
Double-sided foam sticky tape (pieces cut into 1.5cm squares)
3-4 day old radish seedlings
A permanent marker pen
(what to do):
First you'll need to grow some seedlings for the
If you've never done this before, it is very easy to do and seedlings
can be grown in a greenhouse or in relatively warm and well-lit areas,
such as near a window.
a layer of paper toweling at the bottom of a container (a seed tray
or flowerpot) and place a layer of paper toweling at the bottom.
about 2 inches of potting soil and moisten the soil.
the contents of a packet of radish seeds on the top of the soil.
a thin layer of soil on the top.
again lightly to make sure that you don't uncover the seeds.
Saran wrap on top of the container until the seedlings emerge (this
may take a day or two).
a pan of water under the container and water if needed from the top.
a "window' in your chamber, punch a hole in the side of the film
canister about 1/3 of the way down from the top.
a square piece of double-sided tape on the top of one of the spare
the top off the tape, and sit the canister on this lid, to act as
a pedestal to stop the canister from rolling about.
half of your canisters so that the window is on the side and half
with the window facing downward, placed over the double-sided tape.
Setting up the Experiment
a piece of double-sided tape (a 1.5cm square) onto the inside of another
film canister lid and peel off the backing, to expose the second sticky
piece of paper toweling (again, a 1.5 cm square) by dipping in water,
and then squeezing to remove the excess water.
this square onto the double-sided tape (which is on the inside of
the film canister lid).
or cut the stem of a 2-3 day old radish seedling, just above the soil.
Try to pick a straight seedling whose 2 seed leaves (called "cotyledons")
have a span of no more than 2 cm across.
the cotyledons of the seedling onto the piece of paper toweling in
place the lid onto the canister so that the stem extends straight
out into the canister.
up half of your canisters so that the "window" is on the
side and half with the "window" facing downward.
the canister and with the marker pen, mark a dot on the lid at the
same side as the "window" and an arrow facing upward.
the plants in this position for a few hours, then carefully open the
chamber and look at the seedlings.
the paper towel if necessary, close the chamber and observe again after
one day, two days, three days etc.
you are waiting to observe the results, you canmake predictions about
what you expect to happen.
Instead of just waiting to see what the plants will do, you can make predictions
about the possible outcomes.
In the chambers with the "window" on the side
Do you think the stem will grow toward or away from the "window"
will the stem move toward gravity?
In the chambers with the covered "window"
Do you think the stem will grow toward or away from gravity, that is
move down or up?
Or do you think the stem will remain straight out toward the other end
of the canister?
Q: Do you think the response to gravity will be stronger than to
You can design additional experiments on your own to test other hypotheses.
For example, will the seedling still respond to light if it is a certain
color? To test this hypothesis, different colors of cellophane can be
placed over the window in the canister.
In phototropism, molecules in the plant (called "receptors")
perceive certain colors, or wavelengths, of light (primarily blue light).
The receptor is activated which leads to a change in the direction of
growth through a series of steps. These steps appear to involve the plant
hormone auxin. Auxin moves from the lighted side of the stem to the darkened
side, where it stimulates cell elongation. It may surprise you to know
that the details of the steps between light per ception and response are
not yet fully understood.
The laboratory exercises are modified versions of "Bottle Biology
and Wisconsin Fast Plants Projects" by Drs. Coe and Paul Williams,
Department of Plant Pathology, University of Wisconsin, Madison.
| Tropisms | Phototropism
| Gravitropism | Calcium