Classroom Activities

Gravity and Plants: Experiments for the Classroom

Gravity is definitely very important for plant roots and shoots.  It isn't too hard to see what plant shoots do.  They grow up and out of the soil, but roots are harder to study.  They grow down into the soil making it impossible to watch them and see how they grow.  Right?  Well, maybe not.  Here's an idea for an experiment to study plant roots.  I've always wanted to try it but never had the time.  Maybe you would like to try it and let me know how it works?

What we want to do is make a clear planting box with clear "soil" so we can actually look in on the roots whenever we like and see what they are doing.

1.  The Planting Box: the perfect planting box for this experiment is an ant farm.  That's right.  It's tall and thin with a clear face so you can look right in.  If they aren't too expensive, go out and buy one.  You could also make one out of some clear plastic if you want.  Take two flat pieces of clear plastic about 12 inches wide and 6-12 inches tall.  Glue some plastic spacers around three sides of one piece of plastic, then glue the other piece of plastic on top to create a very thin, tall box.  Use silicon sealant for glue, it's clear and rubbery and makes a nice waterproof seal.  Next, glue some triangular pieces on the edges of the box so it can stand up. 

Figure 1: Planting Box for Root Studies

2.  The clear "soil":  if we put regular black soil in the planting box, we could only see the roots if they came close to the windows.  That's not good enough.  Instead, we can make clear soil, which isn't really soil at all, but the plant should grow very nicely in it.  First, go to the grocery store and buy some gelatin or pectin (it's the same thing).  You can find it in the baking section.  Essentially, it's Jell-O without the food coloring and sugar.  Next, buy some plant food from the garden section of the store.  While you're there, pick up some lettuce and radish seeds, as well as any other seeds you want to try growing for your experiments (lettuce and radish are really good for this kind of experiment).

When you get home, follow the directions on the plant food container to make 1 gallon of nutrient solution.  Usually this means you put about 1 teaspoon of plant food into a gallon of water.  Mix it up completely.  This full strength plant growth solution is much too strong for seeds.  You need to dilute it.  Take 4 cups of the full strength solution and pour it into a second 1-gallon container, then fill the container up the rest of the way with water.  That's a 1/8 strength solution and it should be perfect for germinating seeds.  Label the two containers, "full strength plant food" and "1/8 strength plant food."

Next, figure out how much water your plant growth box holds.  Fill it up and then pour the water out into a cup measure to see how many cups of water it holds.

Now, follow the directions on the gelatin package to make as many cups of gelatin as the plant growth box can hold.  But, instead of using water to make the gelatin, use the 1/8 strength plant food.  This should require you to boil the plant food in a pot on the stove and then pour in a certain amount of gelatin.  Stir constantly and boil until the gelatin is dissolved.

Let the plant/plant food mix cool for a while on the stove before you move on to the next step.

Finally, and very carefully, pour the plant/plant food mix into your planting box.  Use a funnel or cup measure or something so you don't spill, and be very careful not to burn yourself.  Let the box set over night.  The plant food/gelatin mix should set and voila!  You've got clear Jell-O plant food!

3.  Planting and growing seeds:  now for the fun part.  Plant your lettuce or radish seeds about 1/4 or 1/2 inch below the surface of the Jell-O/plant food in our planting box.  Make a nice row of seeds across the top about 2 inches apart.  Set the box on a well-lit windowsill.  Lettuce and radish grow very quickly.  The lettuce should germinate in 1-2 days.  The radish in 4-5 days.  The roots will grow down into the gelatin and you can watch them and measure their progress every day.

4.  What to look for: Watch the roots growing down.  Do they grow straight or do they curve?  You could measure the root growth every day.  Make a mark on the glass at the tip of the root at the same time each day.  Later, you can measure the distance between marks to find out how fast the roots grow.  How long does it take for lateral roots to emerge?  Lateral roots will sprout out from the root after 4-5 days and then turn downward as they grow.  You could count the number of lateral roots and measure their length.  Compare lettuce and radish.  Compare other kinds of plants you might want to grow.  Which ones grow fastest?  Straightest?  Have the most lateral roots?  If you have a regular camera or video camera with a close-up lens, you could take pictures of the roots each day to show in your report.

5.  Throw in a curve: one last experiment.  Let the lettuce or radish roots grow for a few days until they are 3-5 centimeters long.  Turn the whole box in its side.  Now all the plants are horizontal.  In a few hours you should see the tips of the roots starting to curve downward.  By the next day, all the roots should have curved and will be growing down again.  Flip the box back up and the roots should curve again.

How many plants do I need to measure? Understanding Results With Statistics

Plants exhibit quite a bit of individual variation in their responses to gravity.  The more plants you measure, the stronger your results. 

Dry weight is a more accurate measure of biomass than fresh weight because it does not measure water content.

In theory, plants use energy and nutrients to curve.  Plants that curve a lot should then grow shorter and use more energy and nutrients than plants that grow straight.  Could

this affect crop yield over the long term?  It might, but I don't know of any clinostat studies where plants were grown for long durations to see how yield was affected. 

Gravity signaling and plant growth response.

As for your questions about different species of plants to test: nearly ALL kinds of plants have developed a very good internal mechanism for sensing gravity so shoots "know" to grow up and roots "know" to grow down.  But, shoots also have a very good internal mechanism for sensing light and growing toward the light (called phototropism).  For almost all plants, there is a constant competition between these two factors; light and gravity that tell the plant shoot which way to grow.  Even more interesting, plants are sensitive to red light and to blue light in different ways. 

Perhaps you could do an experiment to understand how light and gravity work together to tell a plant which way to grow.  I've done this before using a fluorescent lamp (fluorescent bulbs are good for growing plants) inside an overturned cardboard box (keeps other light sources out).  Try growing the plants under constant light from above and then from the side.  How do the plants grow?  Next (I've not tried this before but I think it would be really interesting) you could do the same thing except with a red filter (red tinted plastic would do) and then a blue filter over the lamp.  Any difference in plant growth?  I would measure the total length of the shoot and the direction of plant growth (or maybe the angle of bend in the shoot). 

If you have time you might try this with several species of plants.  Lettuce and radish are good, they germinate fast and in less than two weeks you'll see good results.  Look on seed packages at the store to see which types germinate fastest (you don't want to wait three weeks just to see the seedlings pop up above the soil!).

Plant under hypergravity conditions.

Plants are very well adapted to Earth's constant 1 g pull, but how much gravity can a plant withstand?  Scientists have used the centrifuge as a tool for studying plant growth under hypergravity (greater than 1 g) conditions.  Some scientists have asked, "How much gravity is too much gravity for plants?"  Research has shown that plants can grow in hypergravity environments, but as the gravity level increases, plant growth is inhibited.  At 5-10 times normal gravity, plant growth stops.  In general, most seeds won't even germinate in environments of more than 10 g. It's not too hard to build a centrifuge yourself and study plant growth under hypergravity conditions.   

One good experiment to do with a centrifuge is to test seed germination and growth at different gravity levels between 1 and 10 g.  Start with two different gravity levels.  Grow plants at 1 g in stationary cups next to the centrifuge and grow a second group of plants on the centrifuge in hypergravity.  Water the plants for a week, then harvest them and measure the length of the seedlings.  Is there a difference in length between the plants grown on the centrifuge (experimental) and the stationary (control) group?  If so, this could be due to hypergravity.  Many other factors contribute to the growth of plants, but gravity can be a very important one.

In addition to the direct affects of hypergravity, plants grown on a centrifuge must cope with changes in water and air circulation.  These environmental factors are lumped together and generally referred to as the indirect affects of hypergravity.  In hypergravity, water falls to the bottom of a soil container faster than in normal gravity.  So, at high gravity levels water drains through the soil quickly, drying it out.  Seeds may not stay wet and plant roots may not get enough water.  Also, the increased air circulation on a centrifuge can dry out the plants and topsoil very quickly.  Make sure the soil on the centrifuge stays moist for the duration of any experiment.  At high gravity levels, wind can become a serious factor.  It might be necessary to cover the plants with plastic cups or some other windbreak.