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Gravitropism Experiments


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Subject Introduction

Even less well understood than plants' responses to light are their responses to gravity. As already mentioned, roots grow toward gravity (positive gravitropism) and shoots grow away from gravity (negative gravitropism). It appears that plants are able to sense gravity in special cells that contain heavy particles (starch grains) called "amyloplasts". These cells, called "statocytes" are found throughout the stem, but in the root, are located only at the very tip.

  • Q: If a plant is tipped over how does the plant know that it has been tipped over and how is this knowledge changed into growth?
Many scientists think that when the plant is tipped sideways, the amyloplasts sink (because they are very heavy) to the new "bottom" of the statocytes, telling the plant which way is "down". It is thought that the sinking of the starch grains is somehow detected by the plant and eventually leads to a growth response involving the plant hormone "auxin". This model, the "statolith theory", often found in textbooks, is greatly debated by scientists.

Question: Where is Gravity Perceived in the Root?

In this experiment the roots are a really good part of the plant to work with because, as we've said before, they have a very well defined region of cells that sense gravity at the tip.

You will need the following materials:

  • Pea,
  • Broadbean
  • Soya bean or maize seeds

(These are all good to use for this activity because they have lots of reserves and so can be grown on wet paper towels).

  • Paper Towels
  • Water

Procedure (what to do):

  1. Fold a piece of a piece of paper towel in half to get a crease down the middle of the towel.
  2. Place the seeds down the middle of the towel, along the crease.
  3. Fold the paper towel over and sit in a dish of water to keep the towel moist.
  4. Put the paper towel in a tray.
  5. Place the tray as near to vertical as is possible.
  6. Grow the seeds until the root is approximately 3cm long.
  7. Use some roots as your control (that is roots which are handled in the same way but which have not had their root caps taken off)
  8. Flick or cut off the very tip of 3 or 4 roots (if you have used maize seeds the root cap is the pinkish region at the very tip of the root)
  9. Turn the roots horizontally and leave for 3-4h.

  • Q: Did all the roots bend?
  • Q: Which ones did?
  • Q: Which ones did not?
  • Q: Is the root cap necessary for a plant to bend?
  • Q: Is starch necessary for plants perception of gravity ("Graviresponse")?


Question: Is starch necessary for plants perception of gravity ("Graviresponse")?

You will need the following materials:

  • 6-7 day old Seedlings (radish or mint )
  • Dark box or cupboard

Procedure (what to do):

  1. First you need to grow some seedlings for the experiment. If you've never done this before, it's very easy to do.
  2. Put half of your total number of plants in the dark for 24hrs to deprive the plants of light. The plant will use up its starch reserves.
  3. Set the light deprive d plant (starchless) plants and normal plants in the dark and turn them horizontally onto their sides.
  4. It is important to remember to place the plants in the dark for two reasons. It will stop the re-synthesis of starch in the light deprived plant and will also prevent any possible movement in response to light.
  5. Leave the plants in this position for 2 hours.

    • Q: When do the normal plants start to bend?
    • Q: When do the starchless plants start to bend?

  6. Leave the plants in the dark overnight and take another look at them in the morning.

    • Q: Have all the plants bent?
    • Q: What effect do you think starch has on gravitropic bending?


Question:Where does the bending occur?

You will need the following materials:

  • 2 potted plants with long stems (such as mint)
  • Dark box or cupboard Marker Pen

Procedure (what to do):

  1. Mark the stem of your plant at regular intervals (such as one mark every centimeter).
  2. Place it on its side in the dark box or cupboard and wait 2-4h.

    • Q: Where did the plant bend? At the tip of the stem, or further back?

    Now let the plant grow vertically overnight, and repeat this experiment.

    • Q: Where did the plant bend a second time?

If plants bend at the tip of the stem, it's probably due to cell division. Further back, it is due to cells stretching out.


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