Geotropism: What Makes Plants Grow up and Roots Grow Down?
Geotropism is the influence of gravity on plant growth or movement. Simply put, this means that roots grow down and stems grow up. Geotropism comes from two words, “geo” which means earth or ground and “tropism” which means a plant movement triggered by a stimulus. In this case, the stimulus is gravity. Upward growth of plant parts, against gravity, is called negative geotropism, and downward growth of roots is called positive geotropism.
What makes geotropism happen?
In plant roots, the very end of the root is called the root cap. It makes the roots turn downward as they grow. The root cap is vital for geotropism since it contains cells with sensors called statoliths. Statoliths are specialized parts of the root cell that settle to the lowest part of the root cap in response to the pull of gravity. This makes the cell expand faster in a downward direction.
A similar mechanism is known to occur in plant stems except that the stem cells are programed to elongate upward, the exact opposite of the cells in the roots.
This upward and downward growth will continue even if the plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth's atmosphere, it will still grow roots down, stem up. The reason for this comes from the nature of a plant, and it's general response to gravity.
Another example of geotropism is the movement of nutrients. minerals and water in a plant. This transport is accomplished by specialized parts of the plant, the xylem (pronounced zylem) and the phloem (pronounced flowem) are the straw like parts of a plant’s stem that move the stuff up and down.
The xylem moves the water and nutrients from the roots to the branches, stems and leaves of the plant. The phloem moves the sugary sap from the leaves to the roots.
An easy way to remember what moves things up or down is to remember what the old native American said - “River flow’em downstream.” Phloem moves things “downstream,” too.
Geotropism is sometimes called gravitropism
How xylem works
The most important cause of xylem sap flow is the evaporation of water from the surfaces cells to the atmosphere. This causes a negative pressure or tension in the xylem that pulls the water from the roots and soil, very similar to the way a drinking straw works.
How phloem works
Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made in the leaves by photosynthesis. These sugars are transported to other parts of the plant, such as the roots, or into storage structures, like tubers or bulbs.
Phloem works like tiny pumps. A high concentration of sugar produced by the leaves of a plant in the cells draws water into the cell. This pushes the sap downward, creating space for more sugar, which draws in more water. The process repeats, moving sap down for storage in the roots of the plant.
As you can see in the diagram of a stem cross section, the phloem tubes are near the outside of the stem. A tree or other plant can be killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots and the plant will die. This is known as girdling. Sometimes animals like beavers chew off the bark of a tree and kill it. Girdling can also be caused by lawn mowers and weed eaters that damage phloem.
Enormous fruits and vegetables like those sometimes seen at fairs and carnivals are produced by controlled girdling. A farmer can place a girdle at the base of a large branch, and remove all but one fruit/vegetable from that branch. All the sugars manufactured by leaves on that branch have no place to go but the one fruit/vegetable which thus expands to many times normal size.
Activity: See xylem in action.
What you’ll need:
- Enough glass containers or plastic cups for each person in your group.
- Food coloring. Try several different colors for some added interest.
- A bunch of celery. The more leaves on the bunch the better the experiment will work.
NOTE: this experiment should be done in an area that will not be damaged if the food coloring gets spilled.
Fill your jar or plastic cup 2/3 with water. Carefully add enough food coloring (at least five drops) to the water. Green food coloring won’t be as easy to see in the green celery as red and blue.
Separate a celery stalk that still has leaves on it from the bunch. Cut off the bottom half inch or so of the stalk and place it in the colored water.
Over the rest of the day check the results. Can you see the color moving up the stalk through the xylem? What happens when it reaches the leaves?
Try this with the celery top down in the colored water. What happens in this experiment?
After allowing the celery to sit in the colored water overnight, remove one and cut through the stalk every inch or so to see the color of the stalk and the xylem. Do this with the upside down stalk and compare the difference.
You can get multiple colored leaves by splitting a stalk longways and putting half in one color and half in another.
Did the colored water move up the stalk through the xylem?
As we saw in the celery experiment, Not all plants have round stems. This can help you identify different plants. If you walk through a wetland, you’ll see lots of different grassy looking plants. Some of these really are grasses, but some are called sedges. Others are rushes. Here’s a little poem that can help you tell which are which.
Sedges have edges,
Rushes are round,
Grasses are hollow,
What have you found?
Even though sedges, rushes and grasses all have xylem and phloem, their stems are different shapes. Sedges have triangle shaped stems, rushes have roundish stems and grasses have stems that are hollow.
If you find a plant with a square stem, it’s most likely a member of the mint family. All mint plants have square stems. You can easily determine if a stem is square or triangle shaped by gently rolling it in your fingers. You’ll feel the corners.
Another interesting thing about mint plants is that they can be used to help keep mice and rats away. Although people usually like the smell of mint, apparently these rodents do not.
Another kind of tropism- Remember that a tropism is plant movement triggered by a stimulus. Let’s take a look at another stimulus.
Phototropism- Photo means light, so phototropism is the movement of a plant as it relates to light. Since plants use sunlight to help make sugar, in order to work best, leaves need to be exposed to as much light as possible to work best. They do this by turning so their leaves face the sun.
Phototropism in action
Activity: Germination and Geotropism
Plastic bag germination project
What you will need: Bean or corn seeds, paper towels, ziplock type plastic sandwich bag, a small piece of cardboard.
What to do: Cut the cardboard to fit inside the plastic bag and slide it in. Tear off three paper towels from the roll, fold them in half and then in half again so you have a square that will fit into the plastic bag. Slide them into the bag so they lay flat. Fill the bag with water. Let the towels soak up as much water as they will and then pour the extra water out. Lay the bag flat and place two bean or corn seeds on top of the paper towels near the center of the bag. You should be able to see the seeds as they rest on the towels. Don’t seal the bag.
Now place the seed bag in a safe place. Find a place that won’t be damaged if it gets wet where you can easily see the bag. A kitchen or bathroom counter would be a good spot. Lean the bag against the wall at an angle with the open side up so you can see the seed without letting it slide down to the bottom of the bag. This will allow you to watch the seed without moving the bag around.
Over the next week or two, keep the paper towels moist, but don’t get the bag so wet the seeds are soaking in water.
Geotropism experiment. Using the seedlings in the plastic bags from the germination activity, look to see what direction the roots are growing. They should be growing down and the stem should be growing up.
Now, turn the bag so the bottom is on the left and the right edge is down. Leave it like that and come back the next day to see what has happened.
Because of geotropism, the root will change direction and turn down and the stem will turn and grow up.
You can keep doing this for several days to see how much you can confuse the growing bean plant. If you are patient, you might get the root and stem to make a complete circle.