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Our control plants
This experiment was to see the effects of phototropism on plant growth.
On the plants that survived, this effect was very noticeable, with the
plants growing far to the sides in an effort to reach the light. However,
we learned how dependant plants are on light, as two of our pots's soil
levels were too low, and so no light reached the plants. They died astoundingly
To determine in which direction plants will grow given a bottom light
source. Back to the top
If a plant is given a bottom light source, the plant will grow towards
the light due to "phototropism", regardless of the orientation of the pot.Back
to the top
In order for any organism to survive, it must find a source of food.
For plants, this food is water, sunlight, and carbon dioxide. Carbon dioxide
is ever present in the atmosphere, so it is in constant supply. But plants
must put some effort into getting sunlight, and to some extent water. The
simplest way for a plant to get sunlight is to grow towards the sun. In
the video that was shown to our class, the point was made that on most
bushes, there is very little overlap between leaves. This is because each
leaf creates shade behind it, and so the other leaves grow off to the side,
to where it is bright. This effect is called heliotropism, and is part
of a more general group known as tropism. Webster's defines tropism as
any "involuntary orientation of an organism in response to a source of
stimulation", and it is an essential part of life on this earth, both plant
and animal. Tropism comes in two forms: positive and negative. Positive
tropism is when the organism orients itself towards the stimulus, as a
plant to the sun or a fly to warm potato salad. Negative tropism is, naturally,
when the organism moves away from the stimulus, such as plant roots moving
away from the sunlight or the fly moving away from smoke. An organism can
behave positively and negatively to the same stimulus at different times.
For example, a partial shade plant may exhibit positive heliotropism for
a time. It may decide later, however, that it is in too much sun, and begin
to grow away from the sun: negative heliotropism.
There are many types of tropisms. The one mentioned before, heliotropism,
is the growth of an organism towards or away from sunlight, and occurs
in nearly every plant. This is part of the more general phototropism, which
is growth towards or away from any light. Most stems are positively phototropic,
while most roots are said to be negatively phototropic. Negative phototropism
is so common that it has its own special name: skototropism. Another behavior
evident in most plants is gravitropism: where the plant tends to grow towards
or away from the the force of gravity. Most roots are positively gravitropic;
they grow towards the center of the earth regardless of the direction the
seed is planted in, because this helps them get an anchor on the soil.
Conversely, most stems are negatively gravitropic. Another response closely
related to gravitropism is geotropism. Geotropism is the growth of the
plant relative to its position on the earth. This is often indistinguishable
from gravitropism, except when the plant is not horizontal. For instance,
if a plant were growing out of the side of a cliff, and grew upwards, it
would be gravitropic, as it grows away from gravity. If that same plant,
however, grew outwards, it would be said to be negatively geotropic, as
it grew away from the soil. A fifth important response of a plant is hydrotropism,
where the plant moves towards a source of water. This is a critical response
in plants, because plants require water to produce food. Two more less
common tropisms are thigmotropism and centripetal-force tropism. Thigmotropism
is evident mostly in vines such as ivy. It is the growth of a plant towards
another surface. Ivy is positively thigmotropic because it clings to the
sides of buildings and trees. Centripetal-force tropism is when the growth
of a plant is influenced by rotation. This was the main result of your
experiment last year with the plants on the turntables.
Plants use specialized hormones known as auxins to influence the direction
of growth. Auxins have two important properties: they stimulate cell division,
and thus increased growth in plants, and they die in the presence of a
certain stimulus, whether it be the presence of sunlight, water, or gravity.
There are many different types of auxins, as many types as there are forms
of tropisms, because a single type of auxin is only sensitive to one stimulus.
Several different forms of auxins are present in most plants at the same
time. The way auxins work is very simple. Auxins are present in the tips
of shoots. These auxins die in the presence of a certain stimulus. Phototropic
auxins die in the presence of sunlight. Assuming the stimulus doesn't move,
they stay there, and stimulate growth. This growth causes one side of the
shoot to be longer than the other, and as a result, it bends over away
from the auxins and towards the stimulus. Sometimes, several auxins can
counteract each other. Take for instance a shoot of ivy growing along a
deck. Its feelers grip the underside of the deck, and its thigmotropic
auxins die by the contact. Left alone, this would cause the ivy to grow
underneath the deck. However, the phototropic auxins are dying on the opposite
side of the plant because sunlight is present, and wish to move in the
opposite direction. In cases such as these, the stronger stimulus succeeds
in moving the plant.
Animals also exhibit forms of tropism, though at the animal level these
effects are generally learned behavior and thus more psychological than
physiological. A fly will move towards warm potato salad, and a mosquito
will avoid smoke. By the same token, humans are attracted to the smell
of fresh-baked bread, and a beaten dog will shy away from his master's
Tropism, in all its forms serves two purposes: to encourage growth towards
beneficial stimuli, and to discourage growth to harmful stimuli. In plants,
it does so through the use of auxins, hormones which cause increased growth.
These hormones only grow on one side, and cause the plant to grow in one
direction. In animals, tropism is usually either learned or instinctively
known, making it more a psychological phenomenon than a physiological one.
(Excerpted from Mike Chinigo's report) Back
to the top
Two fish tanks
Stand w/a transparent shelf (to allow light through)
Fluorescent light with timer
Two clear plastic boxes
Journal to record results. Back to the top
Set up the bottom light source. Place the stand with a transparent bottom
wherever you want to. Then, affix the fluorescent light upside down directly
underneath the shelf, so that light shines upwards through the stand.
Cover the inside of the fish tank with construction paper, affixing it
with the tape.
Fill pots with potting soil, making sure to fill them close to the top.
Otherwise, light cannot reach the plants and they will die.
Plant several tomato seeds in each pot, water thoroughly.
Separate the pots into three groups. One pot will be the control. Leave
this pot away from the bottom light source and near a traditional, top
light source. Two pots will be placed into the clear plastic box (make
sure there is no top to it) under one overturned fish tank, above the bottom
light source. The final two pots will be turned on their sides inside the
plastic box, above the light source.
Set timer on the light to eleven hours of sunlight, and thirteen hours
Water plants regularly, and record measurements. Back
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Day 1: We planted our tomato plants, on in a pot facing upright
and one facing side ways.
Day 2: We watered our plants. They still haven't sprouted yet.
Day 3: We constructed our fish tanks by putting construction paper,
cardboard, and thick black tape to keep all of the light out.
Day 7: Our tomato plants sprouted today, we watered them.
Day 14: We have noticed that the plants in the pot facing sideways
have begun to slightly bend towards the light, which is a sign that our
experiment is doing what we expected it to do.
Day 16: Our plants in the right side up pot are not looking to good.
Day 21: It is a sad day because our plants have died, but we do
know the reason for this tragic event. The plants were not tall enough
so they did not over hang the side of the pot, which means they didn't
get any light.
Day 22: We decided to get some plants that were taller than the
other plants so that they could get some light.
Day 27: This is the last day of our project and we have come to
the conclusion that the plants in the sideways pot have done a greater
amount of bending towards the light. This is because they started out closer
to the light so they had a less to bend towards it. With the plants in
the right side up pot the tops of the leaves were facing upward which means
the light takes longer to get them to bend towards it. Back
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Our hypothesis did prove to be correct, the plants did show the growth
response to the bottom light source, as seen on day fourteen. We
did our project on phototropism and the effects it has on plants. We used
a under light source to make the leaves bend towards the light. The first
plants that were standing upright died. We learned that we needed to gather
taller plants so that they could receive light from the top of the pot.
We gather together the taller plants which survived because they received
enough light to live. We also noticed that they began to bend towards the
light. This occurrence is know as phototropism. The plants placed sideways
grew towards the light quickly because they were closer to the light. The
leaves of these plants were already facing more towards the light then
the other plants. This allowed them to receive more light.
The meaning of this is, light, no matter where the plant, will grow towards
the light in order to get as much light, for growth, as possible. An example
of this is when a smaller plant is growing under a porch, the porch restricts
direct light, the plant will then grow into the light so it can thrive
as it was intended to do in nature. Back
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