"My third graders wanted to experiment to find out whether plants need light," reports Jane Carver from Boston, MA. "So we placed one bean plant on the windowsill, and left another in the closet. The students were amazed a week later to find that the one in the closet actually seemed to be growing better than the one in the light!" But when they observed more closely, reports Jane, some students noticed that although the plants in the dark were taller, they had thinner stems, smaller leaves, and didn't, in fact, look all that healthy. "We decided to keep growing them to find out what would happen over time," she reports. "This also got us wondering about how we might explore other aspects of the plant/light relationship."
Light - so omnipresent and indispensable to our lives - is often taken for granted. Yet the relationship between plants and light is essential to the very existence of life on Earth, since light energy fuels the most important chemical process in the world -- photosynthesis -- upon which other life forms depend. The old "bean plants in and out of the closet" is a familiar classroom routine to underscore plants' need for light. But whether you're growing plants outdoors, on classroom windowsills, or in GrowLabs or other light units, there are a whole host of light-related classroom observations and active investigations with which to engage students.
A plant's growth, responses, and its ability to make food is affected by light (and its absence). The amount and type of light plants receive is also significant. Read on for some details that we hope will inspire investigations and illuminate learning in your classroom.
So why do some students discover (to their surprise) that plants can actually grow taller in a dark closet than in a GrowLab or on a windowsill? In the absence of light, cells in plant stems grow rapidly and leaf growth is supressed. If you take a closer look or let a plant continue to grow in the dark, you may notice thin and weak stems. One may also discover that without light, leaves tend to be small and pale, as they lack the green pigment, chlorophyll, and thus the ability to make food or photosynthesize (but more on that later).
To further explore how darkness affects stem growth, consider asking students to observe and describe how a bean or peanut seed emerges from the soil. What plant part do students predict will emerge first? What part actually appears first? With beans and many other types of dicots, the curved stem actually breaks through the soil first. Why don't plants simply shoot straight up out of the ground? Like the plants in the closet, the stems cells grow quickly in the dark underground, forcing the stem to curve and push up first. This is advantageous for the plant, since the tender shoot and delicate bud with young leaves are protected from having to push through abrasive soil. But when the plant emerges from the soil, light triggers a change, causing the stem hook to straighten, its growth to slow, and the leaves to expand in preparation for making food.
Monocots like corn and other grasses have a sheath protecting their growing tip until it hits the light and the leaves emerge. Students may want to investigate how a monocot (like corn) compares with a dicot (beans) when emerging from darkness into light.
Invite your students to explore outdoors or at home to find evidence of plants that have been deprived of light (e.g., potatoes in the drawer will have long white sprouts; grass under a rock will appear white or yellow). Have them describe what clues indicated that the plants were growing in dark conditions.
Once plants have emerged from the darkness, light affects them in many ways, some of which scientists don't fully understand. Phototropism and photosynthesis are two of the important factors regulated by light.
Phototropism. Most of us have seen plants on a windowsill appearing to lean toward the light. This response, called phototropism, results because plants have a growth hormone near the stem tips and the young leaves that is highly reactive to light. This hormone tends to migrate to the "dark" side of the stem, causing the stem cells to elongate, much like those of plants in the closet. This forces the stem typically to bend toward the light (and sometimes, in turn, causes leaves also to orient to the light) - a helpful response for a living thing that needs light to make food!
When scientists were first exploring phototropism, some hypothesized that something in the tip of the stem responded to light, then tried testing their hypothesis. They took corn seedlings that were leaning toward a window and turned them around 180 degrees. They then cut 1/2" from the tip of half the seedlings, left the others alone, then observed for a few days. Ask your students what they predict might have happened, then consider trying a similar experiment yourselves.
As students observe phototropism in the classroom, encourage them to ask questions that they might test themselves. Some that have been explored by other growing classrooms include:
Photosynthesis: Light Fuels Foodmaking. A discussion of plants and light wouldn't be complete without at least mentioning photosynthesis. Light provides the energy for this complex and vital process whereby CO2 and H2O combine chemically in plant leaves and other green plant parts, in the presence of green chlorophyll pigment, to form carbohydrates. Without light, the process of photosynthesis cannot occur, nor can the production of food that plants and other living things need to survive.
Leaves are the primary sites for photosynthesis and most are well designed for the task. They have large surface areas to intercept light rays, contain pores or stomata that can open and close to let CO2 in and limit water loss out, and are well supplied with veins through which water and the food products of photosynthesis are exchanged with the rest of the plant. Some leaves even adjust their position to intercept light. We've discovered that bean leaves can shift dramatically between high and low (or no) light conditions! Your classroom scientists may enjoy exploring leaf designs and movements and inferring how they might help a plant to photosynthesize.
Students can observe one aspect of what happens when leaves do not get light by simply covering a leaf with an opaque material like aluminum foil and observing changes over time. Activities on pages 74-86 of GrowLab: Activities for Growing Minds can help students delve more deeply into the relationship between leaves and light, and into the puzzle of photosynthesis.
Creating "biosphere bottles" (e.g., pond water in a sealed jar) and observing what happens over time can be an exciting way to explore what happens in a closed system where the only input is light.
In nature, plants are exposed to the "white" light of the sun, which is actually a mixture of red, orange, yellow, green, blue, and violet wavelengths. Students can see evidence of this in a rainbow, where light is separated into its component colors. To investigate this concept further, invite students to explore the way light is refracted through prisms.
The different-colored light rays, which in combination form white light, have different effects on plant growth. Blue light and red light are particularly important to plants. Light in the blue end of the spectrum is critical to photosynthesis and also induces phototropism. Blue light tends to promote compact growth and dark green leaves, but few flowers. Too much blue light can stunt plants. Light in the red (and far-red, partially visible) end of the spectrum promotes flowering and rapid stem growth, but too much can produce scrawny plants. Plants don't use much green light, but reflect it, which is why they appear green to us.
Although sunlight is ideal for plants because it contains a balance of light colors, many growing classrooms and other gardeners successfully use fluorescent lights to grow plants. No artificial light exactly reproduces the sun's rays. Standard cool white fluorescent tubes emit more light in the blue end of the spectrum while warm white tubes emit more in the red end. Some people feel that a combination of cool and warm white supply an ideal combination of colors for growing plants. Different types of wide- and full-spectrum "growlights" attempt to combine the best of both.
To explore more about how light rays of different colors affect plant growth, your students might try growing plants under different-colored opaque filters. If you grow a plant under red cellophane or in a clear plastic bottle colored red with a marker, for instance, you can see how a plant reacts to only red light, since the red filter screens out all visible light waves of the spectrum except red. You might find the most dramatic results when comparing plants grown under red, blue, or clear filters with one another and with those grown under a green filter, since plants do not use green light, but reflect it back. Another way to explore the effects of different visible light waves on plants is to compare plant growth under different types of lights.
So we know light's important, but how much is enough? The intensity of light is measured in either lumens or footcandles. Lumens refers to the amount of light emitted at the source and remains fixed. Footcandles is the amount that hits a given area. You can measure footcandles with a light meter or roughly with a camera as described on page 46 in GrowLab: A Complete Guide to Gardening in the Classroom.
Not all plants have the same light requirements. Generally, the 1,000 to 1,500 footcandles of light from a GrowLab is adequate for most vegetables, flowers, houseplants, and herbs that you would grow in a classroom garden. (For comparison, outdoors on a sunny day might be as much as 10,000 footcandles.) Typically, the more light you can get to your indoor plants, the more they can photosynthesize and healthier they'll be. Plants grown for their leaves (e.g., herbs and lettuce) can get by with less intense light than plants from which you want to produce fruit such as tomatoes and peppers.
In a classroom garden, the intensity of light the plants receive will dramatically decrease as the distance of the lights to the plants increases. The light is also most concentrated in the center of the GrowLab, and less so at the edges. Some classes have experimented with ways of increasing light intensity, for instance, by reflecting some light back to the plants with aluminum foil or mirrors. Because the amount or intensity of light your fluorescent tubes emit decreases dramatically over the life of the tubes, consider changing tubes every year.
The duration of light (i.e., hours of light per day) also affects plant growth. Plants growing outdoors are exposed to as many hours of light as there are hours of available sunlight. The duration of light outdoors varies according to time of year and latitude. Because indoor lights have less intensity than sunlight, most indoors growers compensate by leaving them on for 12 to 16 hours per day.
Paul Osmer's seventh graders in Vernon, NJ, wondered, If some light is good, would full-time light be better? To find out, they subjected some plants to 24 hours of light and others to only 12 hours. The students discovered that bean plants grown under 24 hours of light were smaller and seemed less healthy than those with 12 hours of light, reports Paul. Most plants seem to require a dark period, some scientists believe, to turn the products of photosynthesis into usable forms of energy.
Flowering is particularly sensitive to another important factor - photoperiodism, or the plant's response to the proportion of light and dark in a 24-hour period. Some plants require specific daylengths (actually, periods of darkness) to bloom, while others are neutral to daylength. Greenhouse growers take advantage of this habit by controlling hours of light to induce flowering, as for chrysanthemums, for example. Outdoors, seasonal daylength signals changes that affect flowering.
Most classroom garden plants are neutral to daylength. Radishes and lettuce, however, are long-day plants, flowering when days are long and nights are short. This is why midsummer lettuce easily goes to seed. We're curious whether lettuce, a long-day plant, can be coaxed to flower in the GrowLab. Please share your results with us if you try it! Consider some of the other questions your students might investigate regarding the amount of lights plants receive.