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 SoilSoilMoist—non-toxic, MSDS and purchasing info available at www.soilmoist.com. Corn SeedsToothpicksSpoonEye dropper or small syringeFluorescent 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.

Expected Outcomes:

2-3 days

Clear Tubes with SoilMoist and seeds

Planter cells filled with potting soil

3-5 days

Shoot

Root

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.

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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.

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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.

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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.

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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.

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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.

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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:

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 non-random 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:

PhototropismPhototropism (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 one-sided 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