Theme: Nurturing Young
Scientists
Luminous Lessons: Investigating
Light
"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.
Out of the Darkness
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.
Let There Be Light
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:
- Will different types of light (e.g., fluorescents, sunlight) promote the same types of phototropic responses?
- How long will it take plants to bend back toward light after being rotated?
- Do some types of plants move more quickly or markedly in response to light than others?
- Can plants actually "seek" light through a hole when grown in an enclosed box?
- Can we find evidence of phototropism outdoors?
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.
Is All Light Created Equal?
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.
How Much is Enough?
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.
Indoors...
-
How does the height of lights in the GrowLab affect plant growth?
What is the ideal light height?
-
How do different numbers of hours of light/darkness affect
plant growth? Do plants seem to need darkness?
- Do different areas of the GrowLab/classroom seem to provide
different amounts of light?
- How will different degrees of shading affect plant growth?
- How can we increase the amount of light reaching plants
in a GrowLab?
Outdoors...
-
What differences can we notice in the same type of plant
growing in shady, sunny or partially shaded places?
- How do tree leaves that receive a lot of sun compare with
leaves from the same tree that are typically shaded? (Often,
leaves in the shade are larger and darker than those on the
same plant that receive more sunlight. It's believed that
a larger surface area and clustering of green chlorophyll
pigment helps leaves in low light more efficiently intercept
the limited light rays.)
- When do most woodland flowers bloom? How might that relate
to our understanding about light?
Author: Eve Pranis
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