Sure, most of us know that green plants provide the oxygen and food that sustain us, but how many of us truly appreciate their penchant for pollution? Imagine radioactive elements being grabbed by cabbage or lead being mopped up by mustard. From humble grasses to potent poplars, scientists have (so far) found more than 400 plants that can actually clean and restore contaminated soil, water, and air!

These powerful plants are called phytoremediators. Invite your students to be language sleuths and try to puzzle out the meaning of the word. (It comes from the Greek word phyto, meaning plant, and the Latin remediare, meaning to remedy.) Phytoremediation is a strategy for cleaning up pollutants — heavy metals, pesticides, oil, and even explosives — by putting plants to work doing what they do naturally. Don't get us wrong; they don't go looking to fight pollution. But the life processes that make plants tick also make them ideal avengers of toxic substances.

This month, we fill you in on what makes plants such great pollution-fighting allies and just how they do it. Use the activity ideas and resources (as is, or adapted to your grade level) for engaging students in investigating these dynamic denizens of planet Earth.

More...

What Makes Plants Tick, 101
Explore This! Plant Sweat

The Pollution Connection
Explore This! Pollution Sleuths

Getting to the Root
Explore This! Investigating Powerful Partnerships
Explore This! Showcasing Pollution-Purging Plants

Imagine That!
Take Action!

What Makes Plants Tick, 101
To understand how plants are able to mop up pollution, it helps to review some of the processes that sustain them. Each plant is, in essence, a solar-driven pump. Recall that during photosynthesis, plant leaves use light energy to convert water and carbon dioxide (CO2) into energy-rich sugars and they release oxygen through tiny pores (stomata). Meanwhile, plant roots reach into the soil and absorb water and mineral nutrients (which they use for growth and repair). Water is quickly pushed up the stem and pulled into the leaves. Warmed by the sun, excess water in the leaves turns to vapor and passes into the atmosphere through stomata in the leaf surface.

Explore This! Plant Sweat
If you haven't already done so with your students, have them secure clear plastic bags over a stem and leaves of several types of plants in sunny locations. They should water the plants well and keep their keen eyes peeled. If students notice evidence of water inside what was once a dry bag, ask them, What's the deal? How can you explain this observation?

The Pollution Connection
Imagine contaminants in the form of gases, solutions, and particles, slipping right into this natural cycling process. Some are bound to find their way in through the stomata in plant leaves. As plant roots do their job — reaching deep into soil or water in search of water and mineral nutrients — other unsavory substances are also "sucked up."

Once they're in the plant, contaminants can be 1.) stored in roots, stems, leaves; 2.) changed into less harmful chemicals within the plant; 3.) changed into harmless gases, which are released into the air.

That's the basic equation. But there's an even more exciting and fruitful pollution-fighting partnership that occurs between plant roots and microorganisms that "eat" soil contaminants. But before delving into that piece in Getting to the Root, consider inviting students to ponder plants and pollution by conducting the following exercises. Adapt them to your context and grade level.

Explore This! Pollution Sleuths
1.) Ask the class, What is pollution? List students' ideas on a chart and challenge the class to arrive at a definition together.

2.) Next, brainstorm a list of possible sources of pollution. (You may want to use these categories: air, soil, and water.) To get students' juices flowing, toss out some examples, like cars and industries. Have your explorers think about signs of pollution they can observe: exhaust from cars, lawnmowers, and so on; waste motor oil; output from smokestacks; storm runoff; particles of ash or dust; and laundry detergents, for instance. You may want to give them a day or two to be detectives before completing a class list. Also ask, Can you think of any pollutants that are invisible? They may want to link to some of the resources listed at the end of this page, or conduct their own research.

3.) Reveal some of what we've shared about plants' roles as pollution fighters. Then ask, What do we already know about plants and their needs that could help us understand how they take in pollutants? You might spark students' thinking by having them imagine they are molecules of nasty soil contaminant, like mercury. Ask, How might a plant lap you up? (You may need to observe a few roots and think about how they do their jobs.) If you were carbon monoxide from car exhaust suspended in air, how might you end up inside a tree?

4.) Consider setting up a simulation to help students visualize how pollutants absorbed by roots move up stems. Put white carnations or freshly cut celery stalks in a jar of water with five drops of red food coloring. Tell students to imagine that the coloring is a pollutant in the soil or a wetland. Ask, What do you predict will happen to the water, coloring, and flowers or stalks in the next 24 hours? Have youngsters explain their answers. After they observe and document their findings, invite the class to discuss new "ahas" about plants and pollution.

A more realistic simulation (though perhaps less visually dramatic) is to try suspending a six-week-old tomato plant in a clear soda bottle half-filled with colored water. Make sure the roots dip into the water and observe changes in the water and plant over the next five days.

Getting to the Root
Harnessing the power of plant roots is one of the cornerstones of phytoremediation. But it's not just the ability of dense or deep root systems to "suck up" lots of contaminants; it's also the company they keep!

Many plants, such as grasses and clovers, have a win-win relationship with microorganisms in the soil surrounding their roots. Here's the deal: A plant releases some of its energy-rich sugars and other compounds from its root tips, which stimulate soil bacteria and fungi to multiply. These organisms, alone or in combination with plant processes, break down "organic" contaminants (those with carbon, such as gasoline) to use for their own growth and reproduction. In turn, they release nutrients the plant needs. But the partnership doesn't always end there.

Certain plants, such as legumes (pea family plants), cultivate partnerships with bacteria (called rhizobia) that can actually heal nutrient-poor soils damaged by overplanting, wind and water action, pesticide contamination, and too much tilling. In this amazing collaboration, bacteria harvest nitrogen directly from the air and make it available in a form that plants can use.

When farmers and gardeners later dig these plants back into the soil, bacteria, fungi, and other decomposers break them down, enriching the soil by releasing nutrients and improving soil structure. The spongy organic matter in the soil helps it hold onto water and nutrients and allows roots to easily probe the soil. What's more, this process ties up carbon in the soil, so less can escape and contribute to global warming. With a resume like that, these remarkable plants are worth exploring!

Explore This! Investigating Powerful Partnerships
So, you've heard that bold bacteria associated with pea family roots can improve soil, but where's the evidence? If you have a school garden, habitat, or small schoolyard plot, you can engage your young scientists in exploring the results of this amazing alliance. The following investigation starts in the fall and picks up again the following spring. (For a simplified version that can be completed indoors in just a few months, see Powerful Partnerships Indoors.)

Materials
Garden plot, seeds of clover and ryegrass (from a farm supply store or nursery) and sunflowers. (If you have older students, you might want to get test strips or a commercial soil test kit from a science supplier for measuring soil nitrates.)

Laying the Groundwork
Invite students to share what they know about grasses and pea or bean plants (legumes), list questions they have, and try to describe each type of plant (growth, leaf type, life cycle, flowers, on so on). The class can visit and update the chart they created in step 1 of Explore This! Pollution Sleuths once they've wrapped up their study. Share with students that many farmers and gardeners use grasses and legumes to keep soils healthy. Challenge your young scientists to set up an investigation to explore the impact of these plants on soil. Here's an approach we recommend:

Exploration
1.) In the early fall, mark out four plots in the garden (at least 1 foot by 3 feet each) and rake out the soil. Label and plant them as follows: A) clover (white, sweet, or red clover), B) rye, C) clover and rye, D) nothing planted (control).

Note: (If you plan to test for nitrates — the most available form of nitrogen — do so now and again at the end of the investigation.)

2.) When plants are about five weeks old (before they've flowered), have students gently dig up one of each type and closely examine the roots. If they carefully brush or rinse soil from the clover roots, they should notice the small bumps or nodules. (The plant roots form these nitrogen factories around the bacteria.) Have students count the nodules, cut them open, and draw or describe them. (Active nodules will look pink. They contain a red pigment called leghemoglobin — similar to the pigment in our blood — that captures nitrogen for the plant. Nodules that are fleshy, grey, or brown are no longer active.)

3.) As soon as they can work the ground in the spring, have students use shovels to chop and turn each crop into the soil, leaving the stakes or other markers so they can keep an eye on each plot. To warm the soil and speed up the decomposition process, consider putting a black weed mat over the plots. (The class should also perform these same steps on the control plot.)

4.) After about three or four weeks, invite students to plant the same number of sunflowers, spaced 6 inches apart, in each plot. (They will germinate when the soil temperature is at least 45 degrees.) Ask, What do you predict will happen to the sunflowers? Have them explain their responses.

5.) Give students time to routinely measure and record the sunflower seedlings' growth, describe their leaf color, and keep alert for other differences. They should also examine the soil in each plot, noticing the texture and color and looking for signs of life.

Making Connections
As small groups of students compare their data, have them grapple with these questions before making a presentation to the class: How do the plots compare? Do you notice any patterns? What do you think might have caused the differences? What conclusions can you draw based on your data? How could you further test your assumptions? What new questions do you have?

If students don't touch on these topics, consider asking, What do you think the plant gets from partnering with the bacteria? What do you think the bacteria get out of the deal? Why do you think a farmer or gardener would want to grow these kinds of crops? Be sure to have the class update the chart with new insights and questions the activity inspired.

Plants Stem Global Warming Carbon dioxide (CO2) is another "pollutant" green plants take in and store. Although CO2 is an important component of our atmosphere, many scientists conclude that high levels of it are responsible for the phenomena of global warming. Why the buildup? It's believed to result, in large part, from growing populations of humans burning more fossil fuels. But when we help green plants thrive and watch our energy use, the planet wins. Did you know that an acre of trees can absorb the amount of CO2 generated from someone driving 26,000 miles?!

What to Expect
Students are likely to find that the sunflowers growing in strips A (clover) and C (clover and rye) grow faster and look healthier (darker green, for instance) than those grown in plain soil. The ones grown in ryegrass alone should fall somewhere in between. They may also notice that the plots with rye have fewer weeds than the others do and that the plots that hosted plants have more evidence of living things.

What's the scoop? The legume/bacteria partnership makes nitrogen from the air available to plants (in the form of nitrates). When the plants are turned into the soil, fungi and other decomposers break them down, releasing nitrogen and other nutrients that sunflowers need to grow.

When rye and clover are grown together, they make an even meaner team. The rye, which keeps down weeds, thrives with the nitrogen released by the legume, and it adds carbon to the soil. Decomposers help incorporate carbon and nitrogen into a rich stable soil complex, and that's a good deal! The carbon is less likely to escape into the air where it can contribute to global warming; nitrogen will remain available to plants for a long time. (Excess nitrates from chemical fertilizers can more readily run off and pollute water bodies. They spur rapid growth of algae, which ties up oxygen, killing other aquatic plants and fish.)

Powerful Partnerships Indoors You can modify the activity for indoors by filling four 6-inch pots with a mixture of half garden soil and half sand. Plant the pots according to step 1 of "Exploration." Once plants have grown for 60 days or longer, invite students to "mow down" the plants in each pot and mix them into the soil. Keep the soil moist and warm and allow 30 days for decomposition. Next, plant a sunflower seed in each pot and compare growth and other factors as described in step 5.

Explore This! Showcasing Pollution-Purging Plants
Don't just learn about these powerful plants — shout about it! Challenge students to section off an area of the schoolyard or garden, or set up a series of containers filled with soil mix, and grow a display of pollutant-battling crops. Featured fighters might include sunflowers, ryegrass, barley, bush beans, alfalfa, rye, soybeans, cabbage, and mustard. (Scientists are also researching whether tomatoes and pumpkins have the right stuff.) Have the class create signs explaining how plants can cancel out contaminants. They can glean information from the Internet or the following chart, which reveals the main ways in which scientists use green plants to remove, degrade, or contain pollutants. (Before sharing the chart, consider having students puzzle out how each process works based on its name!)

How Plants Grapple with Pollutants
Process Fighters	What Happens	Sample Pollution
Phytoaccumulation (extraction)	Plant roots take in water, nutrients, and pollutants (e.g., lead and other heavy metals) from the soil. The mixture is drawn up to stems and leaves. Water evaporates from the leaves, leaving the pollutant in the plant. Mature plants are harvested and disposed of.	Sunflowers, Indian mustard, rape seed, barley, dandelions, poplar trees
Phytodegradation	Organic contaminants (those containing carbon, like oil) and water are absorbed by roots. Plants break them down to nontoxic molecules, which can be released as nontoxic vapor.	Legumes: clover, alfalfa, cowpeas Grasses: rye, bermuda, sorghum, fescue Other: poplar trees
Rhizofiltration	Plant roots act like filters, absorbing polluted water and releasing clean water.	Sunflowers, many types of wetland plants
Microorganism stimulation	Plants exude sugars and other substances from roots that microorganisms (fungi and bacteria) use for growth. Microbes break down organic pollutants so they're harmless.	Legumes: bush beans, alfalfa Grasses with fibrous roots: rye, fescue Other: Mulberry, apples
Phytostabilization	Plant roots prevent contaminants from migrating to groundwater and water bodies.	Poplar trees, grasses and other plants with dense root systems.

Imagine That!

•	Scientists discovered that arsenic, a very toxic substance found in wood preservatives and other compounds, is soaked up in massive quantities by a plant called the brake fern. In fact, each plant can clean, on average, nearly 50 times its weight in soil!
•	Sunflowers have been used to clean a pond teeming with radioactive elements near the site of the Chernobyl nuclear reactor accident in Russia. Scientists float the plants on rafts so the thirsty roots can dip in and soak up the toxic soup!
•	Legumes (pea family plants) are superheroes! They can reduce soil erosion and soil and water pollution, recycle nutrients, encourage earthworms (which burrow and loosen soil), diversify the microscopic life in the soil, and fight pollution.
•	When it comes to popular plants for cleaning pollution, poplar trees are hands-down winners. Fast-growing trees with deep roots, each one can suck up 25 gallons of water every day along with toxic chemicals, such as trichloroethylene, which is used in dry cleaning.
•	Sunflowers and mustard plants were put to work cleaning up decades' worth of lead from a DaimlerChrysler car factory. Within a year, they'd mopped up a quarter of the toxin and saved the company one million dollars in cleanup costs!
•	In the US, there are more than 40,000 sites needing hazardous waste treatment. Scientists continue to research plants that can step up to the plate!
•	An estimated 100,000 premature deaths each year are believed to be related to air pollutants alone.

Take Action!
Plants can't do it alone! Better to prevent pollution when you can, than have to mop it up later! Your young environmental stewards surely have ideas about how they can take action to put a lid on pollution. Let's hear them! Here are a few ideas to get them started:

•	Use biodegradable (phosphate-free) soaps.
•	Use natural pest control in the garden instead of insecticide controls (for instance, pick or wash off pests; plant lots of flowers and herbs to attract beneficial insects).
•	Minimize driving! Walk, bicycle, take the bus, or share a ride.
•	Pick up after pets! Pet waste can get into storm water runoff and pollute streams and lakes.
•	Purchase household and garden products that are the "least toxic" to the environment (water-based paints, for example).
•	Reduce, reuse, and recycle materials whenever possible.
•	Plant trees, native plants, organic gardens, and habitats!

These Web sites might further inspire action:

•	Pollution Prevention Starts at Home
•	We All Cause Air Pollution
•	Stop Pointless Personal Pollution!
•	The Great Green Web Game