NCSU-NSCORT Outreach - Tropism Experiments

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Subject Introduction

Plants live in a very slow world, in which time is measured in months and seasons, not hours and seconds. Did you know that plants can move?

They usually do so by slowly growing in one direction or another, towards light, the soil, water or away from other plants. These growth movements are called "Tropisms" and are all based on the same principles of "asymmetric growth" (a scientific term which means that plants grow unevenly on one side of the stem as compared to the other).

Scientists know what causes plants to move (factors such as light, temperature, chemicals & gravity) and roughly how this is achieved, but the details of the steps between perception and response are not fully understood.

We are studying gravitropism. It’s important for shoots to grow upwards, and roots to grow downwards, and that in itself is an interesting question.

There are also economic implications: Wind damage to crops ("lodging") causes millions of dollars of loss each year. Is there a way to get the plants to right themselves faster? Does the process involve a substantial amount of energy that would otherwise be put into seeds or fruit? The floral and logging industries pay more for straight flowers and trees. Is there a way to inhibit or reduce the effects of gravity to get straighter plants?

Question: How do tropisms, that is plant growth movements, work?

To help you understand the answer to this question, you can build a model that shows how plants move.

You will need the following materials:

Your hands

Procedure (what to do):

Clasp your hands together in front of you, keeping your elbows together and your wrists nice and relaxed.
Keeping your right hand still, push upwards with your left.
Remember to keep your wrists relaxed

Q: What happened?

Did your hands tilt to the right?

Now do the opposite, keep your left hand still and push up with your right.

Did your hands tilt to the left?

Observations:

You have just done what plants do every time they want to grow somewhere!
They stop growing on the side of the stem where they want to turn and keep on growing on the other side. This causes them to tilt in the direction they want to grow, just like your hands!

Building a Model of a Plant to see how plants grow.

You will need the following materials:

A slinky
Strong Cardboard
Scissors
"Duck Tape" (or other strong adhesive tape)
A Balloon
An elastic band
A straw (or a length of rubber tubing)
Some wire twists (used for closing plastic bags or tying up flowers)

Procedure (what to do):

Attach the balloon to the straw (or rubber tubing) using the elastic band.
Try to make the balloon more supple by blowing into it.
Cut the cardboard into two squares or circles - cut them large enough to cover the ends of the slinky.
Punch a hole into the center of one of the pieces of card. The hole needs to be big enough to squeeze the balloon through.
Tape the pieces of card to each end of the slinky
Stuff the balloon into the slinky and place the slinky vertically on one end.

Now you are ready to make your plant grow!
Inflate the balloon by blowing into the straw.

Q: If you keep blowing what happens?

The whole slinky will extend more or less upwards, just like a growing plant.

Now that you’ve got your "plant" to grow, try to make it bend!

8. Using one of your wire twists, hook the wire round several of the slinky’s hoops and inflate the balloon again by blowing into it.

Q: What do you notice?

Just as in a real plant, the slinky will bend to the side that does not grow (where the wire is hooked round the hoops of the slinky)

Observations:

The great thing about this model is that it also represents the main force used by plants to move, that is internal pressure acting on a semi-rigid box.

In the case of the balloon, air pressure is the force that drives the rings of the slinky apart. Real plants use water pressure, or in scientific terms, "hydrostatic pressure", to get their cells to elongate.