Download Plant Tropism Lab

Plant tropism lab
4.03 Assess, describe and explain adaptations affecting survival and reproductive success of
plants:
Introduction and Purpose: This lab is designed to elucidate the following vital understandings of
plant adaptations and physiology:
Understand the language of life as it is manifested in plants, i.e. communication within
and without the plant.
Know the true equivalence of animal processes in plants.
Recognize the work being done by and the value of local plants.
This lesson is also designed to help students achieve the following learning standards:
Students will be able to explain or reasonably predict the methods by which plants assess
their environment and the ways in which a plant might respond to various environmental
conditions.
Students will be able to explain, diagrammatically and/or verbally, the role played by
major parts of a plant (roots, stem, leaves, seeds, flowers, etc.) both in interacting with
the environment and in interacting with the other parts of the plant by describing the
transport system that links these parts in both vascular and nonvascular plants.
More specifically, this lab is designed to show that plants adapt to their environment in several
ways based on their ability to discern the direction of gravity and sunlight. Seeds will be planted
in various orientations in relation to the force of gravity and the light source. These plants will be
observed as they adapt to their environmental conditions. This should make the ideas of
phototropism and gravitropism clear while keeping the focus on the practical side of the subject
rather than the more arbitrary and fleeting knowledge that comes with vocabulary focused
lessons. Depending on the pace of the class, this activity could serve as the basis of a full lab
report or as just a visual representation of the information being taught. The repetition of the
same experimental design by a couple groups within each class will also provide many data sets
for analysis. This should be a good example of the inevitable error of single trials and the value
of repetition in scientific research.
Activity Overview: Depending on time, space, and resources, this project can be done by each
student or in small groups, but, for the sake of convenience, the project will be presented as an
individual assignment. The planters in this experiment are divided into blocks of nine individual
cells. Each student should plant three seeds right side up, three seeds upside down, and three
seeds on their sides. A transparent rooting medium will be used so the students can observe the
plants as they react to their environment. Each day the students should observe each of the plants
and note the length and angle of the root and shoot. The initial setup may take most of a class
period, but the rest of the project can be done in a few minutes each day. Since each student will
essentially be repeating the same experiment, the data from all the students can be combined and
analyzed as a class activity or each student can turn in a report.
Once the planting materials have been brought into the classroom, there are several other
experiments that can be done to incorporate other curriculum points: a few of these are discussed
in the variations section below. Like the first setup, these could provide a set of data to really
analyze mathematically or just an interesting visual.
Materials and Procedure:
Materials:
Compartmented seed starting tray (The ones used here are made by Jiffy and have 40 cells)
Potting Soil
SoilMoist—non-toxic, MSDS and purchasing info available at www.soilmoist.com.
Corn Seeds
Toothpicks
Spoon
Eye dropper or small syringe
Fluorescent light bulb covers cut into ~3 inch sections (available at most home improvement
stores)
A cup or bowl in which to soak the SoilMoist
Procedure:
*A note to teachers * Cutting the light bulb covers is somewhat tricky and time consuming, so
this step should be done ahead of time. The best method I have been able to find is to use a utility
knife. Push the blade through the tube and rotate it to cut the lengths. The blades work best when
the tube is pressed firmly against the casing of the knife during cutting. The tubes can be washed
and reused.
Depending on the type of seeds you use the amount of time it takes to see results will
vary. Seeds must be large enough to be easily examined and have an obvious asymmetry. I have
used both corn and sunflower seeds successfully. Both of these seeds have a very clear top and
bottom that makes it easy to orient them properly. The more rigidly vertical growth habit of corn
makes these plants more manageable after they have grown to a few inches tall. Seed packages
will say that it takes 7-10 days for either of these seeds to germinate, but, since we have a
window into the normally subterranean activities, results with be visible within 48 hours. So, if
the plants are started on a Friday, the growth tips should be visible by Monday. Planting the
seeds mid-week, however would mean most of the early visible change would happen over the
weekend. It seems a Monday or Friday would be the best time for this activity. By one week,
almost all the plants should have emerged. Depending on the length of time you plan on
spending on plants, the setup can be done as part of an exploration lesson at the beginning of the
section or, if you want the results early in the section, a few days before the plant section begins.
From one week on, the plants will have fully corrected themselves and can be used to show any
number of other curriculum aspects: venation, gas exchange, apical dominance, etc.
1. Fill the cup about ¾ of the way full with warm water and add ~1/8 tsp. of SoilMoist per
plant. Allow plenty of room for the crystals to expand; each crystal will grow from about
the size of a grain of salt to about the size of a pea. With warm water the crystals should
be fully expanded after twenty minutes, but the plants seem to work best in crystals that
are only about half saturated. Any excess water can be poured off.
2. Fill all of the cells with potting soil.
3. Place a tube in each cell so that the lower ~1/2-1 inch is below the soil level.
4. Using the spoon, fill each tube with SoilMoist.
5. Plant a seed in each tube, and use a toothpick to orient the seeds (1/3 right side up, 1/3
upside down, and 1/3 sideways). The closer the seeds are to the side of the tube the better
view you will get; however contact with the tube will restrict the movement of the root
and the shoot. The seeds should be ~1 inch down. Try to arrange the seeds so that they
can all be viewed without blocking one another.
6. Fill any remaining space on top with the expanded SoilMoist.
7. The dropper can be used to add water if the gel seems to be drying out during the growth
period.
8. Using a toothpick and a piece of paper, make a flag with your name on it to identify your
plants.
Left over SoilMoist can be added to the watering tray of the planter to help keep the plants
moist. To allow for air flow, don’t add more than a layer ~1/4 inch thick.
Clear Tubes with
SoilMoist and seeds
Planter cells filled with
potting soil
Expected Outcomes:
Shoot
Root
2-3 days
3-5 days
Below are the student questions with annotations and predicted responses based on a trial run of
the experiment with 46 ninth graders. The students are intended to answer the questions in the
space above the line after they have planted the plants but before they have begun to grow. They
will answer these questions again during/after the experiment using their, hopefully, more
comprehensive understanding of plant physiology. This forces students to both confront their
preconceptions and reconcile them with their new knowledge
How will the seed’s orientation affect the plant’s growth?
Most students agreed that the plants would correct for their orientation. The main logic
was that plants are not always planted straight in nature so they must have some way of
compensating. Some students (about 10-20%) said the plants would not grow straight if they
were planted upside down.
_________________________________________________________________________
The experiment should show clearly that the plants will correct themselves. It should also
show that doing so requires effort on the plants part. The students should see that once the seed’s
energy is used up, the plants will be almost identical in spite of the way they were planted.
If the shoot and root orient themselves correctly, will they do so before emerging from the
seed or after?
The responses were all over for this question. It is really designed to get the students
thinking about what a seed is and how a plant works.
_________________________________________________________________________
The real answer depends on the type of seed. Sunflower seeds seem to break open at the
same place regardless of orientation. As soon as the tip comes out of the shell, the plant starts to
react to gravity. With corn, the root and shoot emerge independent of one another. In this case
the tips do seem to begin establishing a growth vector prior to breaking out of the pericarp, but
they still do most of their angle adjustments after they have emerged.
Will they need to emerge from the ground (i.e. be exposed to light) before they can correct
themselves?
Both this and the question before it are included in response to many students saying that
the seed would only correct itself if it was exposed to light. This probably comes as a result of
students being taught that plants grow toward light. Very few people will be aware of the vital
role played by gravity before the plant ever sees light.
_________________________________________________________________________
Since the gel is clear the idea that the plants are simply responding to light is difficult to
dispel. In the variation section there is a suggestion to grow a set of plants without light to see if
they too correct themselves. Seeing that plants grow straight even in the absence of light makes it
clear to the students that they are seeing gravitropism before they see phototropism.
What external factors/stimuli (e.g. light, radiation, noises, odors) can the plant sense in
order to control its growth pattern?
This is designed to start the students thinking about sensing the world in ways other than
the five senses they are taught about. Students will generally write something about the plants
responding to light and maybe to water. Very few people will write about responses to gravity,
chemicals, touch, etc.
_________________________________________________________________________
After the experiment all students should have something about gravity in their response.
How does your body assess which way is up?
This is a good way to tie in human anatomy. Many students will assume the eyes and the
brain are the vital sensors of position. Some will know that the inner ear is the key organ, but
very few will understand how it is done. It is still not entirely clear how plants assess their
position, but it seems that there are heavy particles in cells that sink to the bottom. The higher
concentrations of these particles tell the cell which side is down. This is very similar to the
otoliths in the mammalian inner ear.
_________________________________________________________________________
After the experiment students should be able to speculate or hypothesize about how an
organism might decide which way is up whether it is a plant or an animal.
What is happening in the seed before the plant begins to photosynthesize? How will this
affect the seed’s shape, size, or orientation?
Many students did not seem to understand that the seed is the energy source for the plant.
Photosynthesis does not need to begin for some time. The cells will be visibly expanding as they
take on water. Most students should be able to predict this from the shriveled state of the corn
seeds.
_________________________________________________________________________
After the lessons on plants students should be able to name a few processes that are going
on in the seeds and how they affect the plant. They should be able to link germination with cell
division, diffusion, cellular respiration, etc.
With no nervous system or specialized sensory organs like our eyes, how do plant cells
communicate? Does each cell operate independently or are there cells that control the other
cells?
This gets at the idea of a hormone as something that is produced in one part of an
organism that acts in another part. No student included a response in the trial run that mentioned
anything more specific than “cell signaling.” Most students had no idea what to put here.
_________________________________________________________________________
After the lesson it should be clear that plants use hormones to control their growth and
development. It is inherent in the definition of a hormone that the chemicals be made in one part
of the plant in order to control the behavior of another part.
Student Handout and Questions:
The purpose of this experiment is to watch plants as they respond to their environment. After
seeing how they respond we will discuss what happens within the plants to allow them to interact
with their environment. This should help you understand the work being done by the plants
around you and the degree to which their internal processes mirror those of your body.
Materials:
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Compartmented seed starting tray (The ones used here are made by Jiffy and have 40
cells)
Potting Soil
SoilMoist—non-toxic, MSDS and purchasing info available at www.soilmoist.com.
Corn Seeds
Toothpicks
Spoon
Eye dropper or small syringe
Fluorescent light bulb covers cut into ~3 inch sections (available at most home
improvement stores)
A cup or bowl in which to soak the SoilMoist
Procedure:
1. Fill the cup about ¾ of the way full with warm water and add ~1/8 tsp. of SoilMoist per
plant. Allow plenty of room for the crystals to expand; each crystal will grow from about
the size of a grain of salt to about the size of a pea. With warm water the crystals should
be fully expanded after twenty minutes, but the plants seem to work best in crystals that
are only about half saturated. Any excess water can be poured off.
2. Fill all of the cells with potting soil.
3. Place a tube in each cell so that the lower ~1/2-1 inch is below the soil level.
4. Using the spoon, fill each tube with SoilMoist.
5. Plant a seed in each tube, and use a toothpick to orient the seeds (1/3 right side up, 1/3
upside down, and 1/3 sideways). The closer the seeds are to the side of the tube the better
view you will get; however contact with the tube will restrict the movement of the root
and the shoot. The seeds should be ~1 inch down. Try to arrange the seeds so that they
can all be viewed without blocking one another.
6. Fill any remaining space on top with the expanded SoilMoist.
7. The dropper can be used to add water if the gel seems to be drying out during the growth
period.
8. Using a toothpick and a piece of paper, make a flag with your name on it to identify your
plants.
9. Check on the plants at the beginning of each class to see what is going on.
Clear Tubes with SoilMoist and seeds
Planter cells filled with potting soil
After planting the seeds, respond to the following questions. Confine your answers to the portion
above the line. You will answer these questions again after the plants have begun to grow.
How will the seed’s orientation affect the plant’s growth?
_________________________________________________________________________
If the shoot and root orient themselves correctly, will they do so before emerging from the seed
or after?
_________________________________________________________________________
Will they need to emerge from the ground (i.e. be exposed to light) before they can correct
themselves?
_________________________________________________________________________
What external factors/stimuli (e.g. light, radiation, noises, odors) can the plant sense in order to
control its growth pattern?
_________________________________________________________________________
How does your body assess which way is up?
_________________________________________________________________________
What is happening in the seed before the plant begins to photosynthesize? How will this affect
the seed’s shape, size, or orientation?
_________________________________________________________________________
With no nervous system or specialized sensory organs like our eyes, how do plant cells
communicate? Does each cell operate independently or are there cells that control the other
cells?
_________________________________________________________________________
Draw a picture of what the plants will look like one day after the growth tips have begun to
emerge. Draw one plant of each orientation.
Draw a picture of what the plants actually look like one day after the growth tips have begun to
emerge.
Draw a picture of what the plants will look like one week after they have emerged from the
soil/SoilMoist.
Draw a picture of what the plants actually look like one week after they have emerged from the
soil/SoilMoist.
Background Science:
During this experiment, students will be investigating plant responses to gravity and light
(phototropism and gravitropism) and the physiological responses involved in plant responses to
environmental stimuli.
It is important to note that plant responses to these stimuli are not uniform. Some plants
require light to germinate while others do better in the dark; some are capable of anaerobic
respiration leading up to germination and most are not; many plants with low growing habits
prefer to grow in the direction of gravity rather than in opposition to it. This experiment is
designed to show that plants assess and respond to their environment and to discuss the
physiological means of doing so. It is not designed to teach how these processes are manifested
across various plant taxa. The processes are similar among plants, but the results are very
diverse. Statements such as “All plants grow toward light and against gravity” are
oversimplifications. The take away message from this lesson is that all plants assess light levels
and gravity vectors and respond according to their growth and development patterns. The
following link discusses common misconceptions in more detail:
http://www.actionbioscience.org/education/hershey3.html
Students should be able to distinguish the plant responses to light and gravity and have an
understanding of the basic plant hormones which control these responses. The most relevant
hormone in this experiment is Auxin, which stimulates cell elongation and has an effect on
phototropism and gravitropism.
There are many good resources for this topic online. Here are a few particularly good,
basic reference materials:
http://www.news.wisc.edu/11876
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTHORM.html
http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html
A weird site with a lot of good diagrams and pictures
http://onlinebiologybook.blogspot.com/
A few more detailed articles:
http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1365-3040.1997.d01-124.x
http://www.springerlink.com/content/713wpj9f2p6pnm29/fulltext.pdf
Variations:
Given the size of the planters, the length of the requisite growing time, and the large
number of items to cover in a plant unit, this experimental design can be modified to incorporate
any number of other experiments. While uniformity of design allows for better mathematical
analysis, variation might make the results more visually compelling or memorable. Here are a
few:
Control:
This is just the above design minus the gel and tubes. Rightfully so, a lot of students
question the affect the gel has on the plant growth. Planting a control group also conforms to and
confirms the validity of accepted scientific methods.
Blended Gel:
Follow the same procedure as with the normal gel, but, after the gel has been expanded,
pulse it in a blender to reduce the particle size to about 10-20% of the particle size of the original
gel. The setup looks the same except the blended gel is more transparent. This transparency
comes from the reduction in the number of air bubble trapped in the gel. Without room for air
flow the seeds will not germinate and the plants will not grow. Those that do survive will have to
use their energy much slower, and it will take them much longer to break through the surface.
Interestingly, if you cut a section of drinking straw and place one end on the seed and the other in
open air (imagine a snorkel), the chances of successful germination increase dramatically. This
shows very clearly the need for gas exchange in order to complete cellular respiration. In the trial
run only 5 in 46 students mentioned gas exchange as a vital need for plant life.
No light:
Cover some of either the control or normal gel plantings and maintain in a light free
space. Most students think these plants will never germinate. In fact, they grow much faster than
the plants with light and still grow straight up until they use all the energy in their seed. This
proves that it is gravity, not just light, that is telling the plants how to grow. It also will
demonstrate the yellow, less robust nature of plants that are unable to photosynthesize. Finally, it
really underscores the idea that a seed is energy. It provides all the energy the plant needs for
more than a week with no photosynthesis.
Miscellaneous:
If there are any cells left over, you may as well plant something. When planting corn (a
monocot) I planted radishes in the extra cells just to show the difference between the way a
monocot and a dicot grow. Once the plants are there, they should provide a valuable visual for
many of the ideas covered in a plant unit as they are or with slight modification: effects of
rooting hormone, leafs for examination under a microscope, taproot vs. fibrous root, etc.
Articles:
Seeking the Light
Development of Gravity Sensitive Plant Cells in
Microgravity
All living things sense gravity like humans might sense light or sound. The Biological Research
In Canisters (BRIC–14) experiment, explores how moss cells sense and respond to gravity and
light.
This experiment studies how gravity influences the internal structure of moss cells and seeks to
understand the influences of the spaceflight environment on cell growth. This knowledge will
help researchers understand the role of gravity in the evolution of cells and life on earth.
Left: Moss Sample in Petri Dish.
Above: Effects of Phototropism.
Plants respond to gravity (gravitropism) and light (phototropism). Typically, plant shoots will
grow away from the direction of gravity and grow towards a light source. Some plants are
primarily gravitropic while others are primarily phototropic. The moss, Ceratodon, is comprised
of long chains of cells that grow from the filament tips. On earth, heavy particles in these tip
cells fall toward gravity, causing the moss to grow away from the direction of gravity. When
exposed to the microgravity environment of space, gravitropic forces no longer affect the moss.
Due to decreased gravity, heavy particles don’t fall out in the same manner. The resulting
random particle distribution will cause changes in growth characteristics. Light direction is not
altered in microgravity so the plant will still grow phototropically (towards light) just like on
earth.
The scientist’s original
hypothesis was that both
random cell structure and
cell growth would occur in
space. The objectives for
BRIC–14 experiment were
developed from the
knowledge gained during a
previous shuttle flight,
STS–87. Unexpectedly,
moss specimens grown on
STS–87 showed nonrandom subcellular
component distribution and
spiral growth.
Above: Non-random spiral growth after
phototropically induced directional
growth
(from STS–87).
For STS–107, the BRIC–14 experiment expands on the previous results with three major
objectives. 1. Determine the age or developmental stage at which moss grows in a non-random
pattern when exposed to microgravity conditions; 2. Determine the minimum illumination level
required to impose a phototropic response on the growth pattern of the moss in the absence of
gravity; and 3. Understand how microgravity affects the distribution of cell substructures.
Background Information:
To address the first objective of this flight experiment, selected moss colonies will be grown
while exposed to a directional light for six days before launch. Once in space, the lights will be
turned off and the moss will continue to grow in darkness. This moss will be compared to moss
that is grown without any exposure to light but has had similar exposure time to microgravity.
This part of the experiment will help determine the age and developmental growth stage of the
moss at which non-random spiral growth is exhibited.
The second objective of
the experiment is to
determine the illumination
intensity required to
induce a phototropic
response in the absence of
gravity. This part of the
experiment will expose
moss to three different
levels of light and observe
at which light intensity
samples respond. The
moss will grow in the dark
for seven days in space
prior to the lights turning
Above: Moss from STS–87, showing
on. This will allow the
moss time to establish a
random growth pattern
prior to exposure to a
directional light source.
spiral growth patterns developed in the
dark in microgravity.
The third objective is to understand how the nonrandom distribution of cell substructure takes
place in space. Scientists have known for quite some time that fibers inside cells are responsible
for the organization of subcellular components called organelles. An unexpected finding from
STS–87 is that these heavy organelles, which are affected by gravity on earth, form non-random
groups within the cells. The investigators hypothesize that this grouping is organized by these
same fibers, although normally, the fibers don’t cause grouping on earth. To test this theory,
chemicals will be applied that breakdown the fibers. If the fibers play a role, then the organelles
should become randomly distributed inside the cells during spaceflight. This experiment will
provide information about how the positions of heavy organelles are controlled and organized
inside cells on earth.
Above: Cellular substructure distribution (from the STS–87 experiment).
The astronauts will check the temperature and verify that the flight hardware is functioning each
day. They will also switch the growth lights on and off at various locations in the flight hardware
and will use a specialized tool to apply chemicals to the moss. These chemicals, called fixatives,
will stop the growth process of the moss and preserve the specimens for analysis after the
mission has ended.
Science Discipline Supported
This research primarily addresses Fundamental Space Biology, but can also be related to other
disciplines. Similar flight experiments can be conducted on the International Space Station to
increase knowledge of how natural processes react to space and enrich life on Earth through
people living and working in space.
Principal Investigator: Dr. Fred Sack,
Ohio State University
Co-investigator: Dr. Volker Kern,
Ames Research Center
Project Manager: Guy Etheridge,
Kennedy Space Center
Project Engineer: Dave Reed,
Kennedy Space Center Moss
Resources
Visit BRIC-14 for a printable PDF version of this research.
Visit http://spaceresearch.nasa.gov/sts-107/overview.html to learn more about the other OBPR
investigations flying on STS-107.
See Florida Today for a news article on this subject.
Article 2:
Phototropism
Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in the direction of its light
source. Plants are very sensitive to their environment and have evolved many forms of
"tropisms" in order to ensure their survival. A tropism is the growth of a plant as a response to a
stimulus, and phototropism occurs when a plant responds to light by bending in the direction of
the light. Although plant physiologists (scientists who study how the processes of a plant actually
work) know that this growth is caused by a plant hormone, they still do not fully understand
exactly how it works.
Bending toward the light
Most of us at some time have noticed a houseplant on a windowsill that seems to have all of its
thin stems leaning in the same direction, as if it were trying to press itself against the glass.
Picking it up and turning the entire pot in the opposite direction so that the plant is pointing away
from the window will only result, about eight hours later, in the plant having reversed itself and
going about its business of pointing its leaves toward the window again. This is not because
plants especially like
Plants respond to the direction and amount of light they receive. The seedlings on the left grew
toward the light it received on only one side. The plant in the center received no light. The plant
on the right was grown in normal, all-around light. (Reproduced by permission of
Photo Researchers, Inc.
)
windows but rather because light is essential to their survival, and they have developed ways of
making sure they get all they need.
We know then that it is the light coming through the window that the plants are striving to get
closer to, but how is a plant, which is rooted in soil, able to "move" toward the light? Actually,
the plant does not so much move toward the light source as it grows in that direction. As already
noted, this growth of a plant that occurs as a response to a stimulus is called a tropism. There are
several forms of tropisms, such as gravitropism or geotropism, in which a plant reacts to the
force of gravity; hydrotropism, in which the presence of water causes a response;
galvanotropism, in which a plant reacts to a direct electrical current; thigmotropism, in which a
plant responds to being touched or some form of contact; and chemotropism, in which a plant
reacts to a chemical stimulus. Since the prefix "photo" refers to light, phototropism involves a
plant responding to light. In all of these tropisms, the plant's response involves some form of
growth. Finally, all tropisms are either positive or negative, although these words are not always
used. So when a plant's leaves grow toward the light (stimulus), it is technically called positive
phototropism. When its roots normally grow away from the light, it is called negative
phototropism.
Words to Know
Auxin: Any of various hormones or similar synthetic substances that regulate the growth and
development of plants.
Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to
manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing
oxygen as a by-product.
Tropism: The growth or movement of a plant toward or away from a stimulus.
How phototropism works
It is known that as long ago as 1809, Swiss botanist Augustin Pyrame de Candolle (1778–1841)
observed the growth of a plant toward the light and stated that it was caused by an unequal
growth on only one part of the plant. However, he could not understand how this was happening.
Some seventy years later, English naturalist Charles Darwin (1809–1882) began to grow canary
grass in order to feed the birds he kept, and he eventually discovered that it was the tips of the
sprouting seedlings that were influenced by the direction of their light source. He and his son
Francis learned this when they covered the tips of some seedlings and found that they did not
move toward the light. When only the seedlings' stems were covered, however, they still moved
toward the light.
It was not until the 1920s that Dutch botanist Frits W. Went (1903–1990) proved the connection
between phototropism and a plant hormone called auxin. Went discovered that plants
manufacture a growth stimulant (which he named auxin) in their tips, which they then send to
other cells in the plant. In phototropism, however, this growth hormone is distributed unevenly
when the light source comes from only one direction. Specifically, more auxin flows down the
dark side, meaning that it grows faster than the exposed side of the plant. This unequal or onesided growth (also called differential growth) brings about the curving or bending of the plant
toward the light source. Went named this growth hormone after the Greek word auxein, which
means "to increase." Although it was isolated and named, auxin was not understood chemically
until twenty years later when it was finally identified chemically as indole-3-acetic acid.
Plants can react and adjust
Understanding what plant tropism is and, specifically, what happens during phototropism makes
us realize that plants, as living things, necessarily demonstrate the several characteristics of life.
Specifically, this includes growth, response to stimuli, and adaptation. It is because of its
hormones that a plant's stem always grows upwards and its roots always grown downward. Since
plants must make their own food to survive (by changing light energy into chemical energy—a
process called photosynthesis), the ability to capture as much of this light energy as possible is
crucial to its survival. Thus, plants have developed a chemical response to light or the lack of it
that causes their stems to bend toward the stronger light.
Today, we know that a certain minimal amount of light (whether natural or artificial) has to be
present for the plant to react chemically. This is called its threshold value. Despite our
understanding of the basic stages and phases of phototropism, we are only now beginning to
obtain the most basic knowledge of what goes on at the genetic and molecular level. We do
realize however that plants are living, sensitive things that can adjust to their environment and
actually seek out the light they need if they are not getting enough.
Citation:
The inspiration for parts of this assignment came from an article in “The American Biology
Teacher” Volume 62 Issue 4 pp. 297–302