Introduction: Plants can’t move like animals do but they respond to certain stimuli, making them change the direction in which they grow. Plants are very sensitive to their environment and have evolved many forms of “tropisms” in order to ensure their survival. A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant. Positive = the plant, or a part of it, grows in the direction from which the stimulus originates. Negative = the plant, or part of it, grows away from the stimulus. Tropisms frequently interact between and among each other, and the final growth form of the plant is influenced by such interactions. Depending on the nature of the stimulus, there are different types of tropisms: Phototropism (light stimulation)
Phototropism is one way a plant can maximize its exposure to sunlight, it is an important survival tactic because plants need sunlight to be able to make food and energy. Seedlings of plants (while they are buried underground in a dark environment) grow straight upward to reach the sunlight above ground. Once they break through the surface, however, they will start bending toward the light (positive phototropism) because the lower, dark side of the plant grows more quickly than the upper, light side, bending the stem upwards.
That is why, when you place a plant beside the window, you will notice after a few days that the shoot is growing towards that window. However, if there is sufficient sunlight in all sides of the plant, such as in the garden outside, it will continue to grow upwards instead of bending. Plant physiologists (scientists who study the working and processors of plants) know that this growth is caused by a plant hormone; however, they still do not fully understand exactly how it works. In 1809, after observing the growth of plant towards a light source, Augustin Pyrame de Candolle, stated that the chance in growth direction is caused by an unequal growth in only one part of the plant.
Seventy years later, English naturalist Charles Darwin found his seedlings influenced by the direction of his light source. He learned that, when tips of these seedlings were covered, they did not grow towards the light. However, when only the stems of these seedlings were covered, they did. It was not until the 1920s that a Dutch botanist named Frits W. Went (1903–1990) proved the connection between phototropism and a plant hormone called auxin. He discovered that plants manufacture a growth stimulant (which he named auxin) in their tips, which they then send to other cells in the plant.
As briefly mentioned above, in phototropism, this growth hormone is distributed unevenly when the light source comes from only one direction. 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. 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. Experiment 3, Plant Labyrinth, will be focused on phototropism. Geotropism (gravity or earth stimulation)
Geotropism, also called gravitropism as plants use the gravitational pull for orientation. The roots of the plant grow into the soil (positive geotropism) in the direction of the gravitational pull of the Earth. The shoots of the plant grow in the opposite direction against gravity towards the light (negative geotropism, positive phototropism). Amyloplasts, which are non-pigmented organelle found in some plant cells, settle at the bottom of the cells of the shoots and roots in response to gravity, causing calcium signalling and the release of indole acetic acid. Indole acetic acid (IAA, the most common, naturally-occurring plant hormone of the auxin class) inhibits cell elongation in the lower side of roots, but stimulates cell expansion in shoots, which causes shoots to grow upward. (As can be seen in figure 1) Experiment Upside Down Mung beans will be mainly focused on geotropism. Hydrotropism (water simulation)
Hydrotropism is a plant’s growth response to water concentrations. For example, roots of plants are positively hydrotropic; they grow towards moist soils to avoid drought stress (positive hydrotropism). Water and other soluble minerals nutrients are taken up by root hairs. Auxins play a key role in bending the plants root towards the water because they cause one side of the root to grow faster than the other and thus the bending of the root. (The same function applies to phototropism and geotropism) Background – Photosynthesis
Plants need sunlight to make energy to survive. Photosynthesis is the process in which plants and algae that contains chlorophyll use sunlight to convert water and carbon dioxide into glucose and oxygen. The input is carbon dioxide (CO2), water (H2O), minerals and light, and the output is carbohydrates (food) and oxygen. The equation is shown below.
Photosynthesis is divided into two parts: Light-dependent reactions (light reactions) Light reactions need light to produce organic energy molecules (ATP and NADPH). They are initiated by colored pigments, mainly green colored chlorophylls. Light-independent reactions (dark reactions)
Dark reactions make use of these organic energy molecules (ATP and NADPH). This reaction cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy, while NADPH provides the electrons required to fix the CO2(carbon dioxide) into carbohydrates. Photosynthesis is essential to life on Earth, it is the foundation of life. One of the raw materials required is carbon dioxide. Carbon dioxide in high concentration is harmful to humans and other aerobic (oxygen breathing) organisms. One of the ‘waste’ products of photosynthesis is oxygen, which, ironically, is vital for most life-forms on Earth, including humans. At night time the plants can no longer photosynthesise, so they have to use up some of the sugar that it has stored during the day. Without continual photosynthesis, the amount of oxygen in the atmosphere would slowly diminish.
If plants do not use photosynthesis, they would be unable to food for themselves and oxygen for other life forms. Entire food chains would lose its foundation, herbivores that depend on plant material for food would die off, and without their food source, and carnivores would disappear too. Soon, almost all creatures (including humans) would starve and die off because they ultimately depend on other plants and creatures for food. Furthermore, there will be massive amounts of carbon dioxide and very little oxygen left on Earth, making it difficult to breathe. Experiment 2 – Upside Down Mungbeans
Aim: To determine if parts of the plant respond to tropism and change directions in which they are supposed to grow, and to find out if the growth of plant will be affected if they are turned upside down. Hypothesis:
It is expected that, although the plant is turned upside down, it will still grow its shoot upwards towards the sun and roots downward. Justification: The shoot of the plant will respond to phototropism (light stimulation) and grow towards the light of the sun. The roots of the plant will respond to geotropism (gravity or earth stimulation) and grow towards the ground. As a result, if the plant is turned upside down, its shoot will grow in the opposite direction (to which it is facing), upwards, whereas its roots will also grow in the opposite direction (to which it is facing) downwards. Materials:
• 1 x packet of mungbean seeds • 3 x jiffy pellets/pots • strings • 6 large paper clips • disposable gloves • dust masks • water Diagram:
Method: 1.A small amount of water is poured onto a saucer. 2.The dry jiffy pellets are dunk in and sat for a few minutes with the little hole facing up. 3.A few drops of water were added until the jiffy pots are fully swollen, but not too wet for planting. 4.2 seeds were planted onto each of the jiffy pots (make sure seeds are fully covered with jiffy pot mix) 5.The jiffy pots were placed in a dark cupboard for one or two days. 6.After one or two days, the jiffy pot was removed from the dark cupboard 7.Dried out jiffy pots were watered and left in a cool, well lit spot (e.g near the window) for a couple of days. 8.Once the plants are about 4cm long, observations were recorded in scientific journal and pictures were taken. 9.The jiffy pots were turned upside down and two paper clips were pushed through the fine mesh on both sides of the jiffy pots. 10.Strings were attached to the paper clips and the jiffy pots with the mung beans were hanged upside down in a cool spot near the window. Photos were taken. 11.The plant was checked the next day. Observations were recorded and photos were taken for scientific reports. 12.Over the next few days the mung beans were repeatedly checked and watered to keep them alive and healthy. Variables:
Independent variable: Direction in which the jiffy pots are facing. (downwards) Dependent variable: Direction in which plants are growing. Controlled variables: Location of plants, volume of water given, volume of jiffy pots, relatively the same amount of sunlight, length of time between watering sessions, duration of experiment.
Risk Assessment. Risk Level of RiskPrecaution Disposable GlovesLowMay easily be punctured, allowing entry of liquid. Latex may cause allergic reaction to some people. Check for latex allergies before use. Check for talc allergies, if gloves are powdered with talc. PaperclipLowMay cause puncture wounds if unbent.
Jiffy pelletsLowMay be thrown or used to knock someone or something out. Particles may get in eye. Wear mask. Dust MaskLow Use a P2 rated mask in preference to a cheap dustmask since it is much more effective in preventing inhalation of particles.
Results: Preparation (27th Feb):
Day 4 (3rd March):
Day 6 (March 5th):
Day 7 (March 6th):
Day7 (March 6th)
Day 13 (March 12th):
Discussion: Questions posed in Point 4 of Methods. • Did your plants know where up was and started growing this way? Guess what might happen to the roots of the plant if it is left like this. • What type of tropisms are acting here? Conclusion:
Experiment 3 – Plant Labyrinth Aim: To determine if a plant placed in a dark box will respond to phototropism and grow through the maze, towards the hole and into the light. Hypothesis: It is expected that the plant, if placed in a dark box with only a hole on top, will respond to phototropism and grow towards the hole and into the light. Justification: The plant will grow towards the hole because it is responding to positive phototropism. Plants in a dark room/box needs sufficient amount of sunlight to survive. Auxin that flows within the shoots of the plant will allow the plant to bend towards sunlight. Materials:
• shoebox with lid • scissors • seeds • spare cardboard • tape • potting mix • Styrofoam cup • pencils/skewers Diagram:
Method: 1.The bottom of the Styrofoam cup was punctured using the sharp point of a pencil. 2.The cup was filled with potting mix and wetted using water. It is left to stand for a few minutes to make sure that it’s not overly wet. 3.2 seeds were planted into the pot.
4.The shoe box were halved with scissors, a large hole was cut at one end of the shoebox. 5.Repairs were made to the shoebox to make sure no light (apart from those from the hole) could get in using tape. 6.The spare pieces of cardboard were cut to the size half the width of the shoebox and the same height of the shoebox. 7.A cardboard piece was fitted into the right side of one-third mark using tape; another cardboard piece was fitted into the left side of the two-third mark so that it forms a maze. 8.The cup containing newly-planted seeds were placed inside the box under a saucer, and opposite the hole. 9.The lid was firmly sealed onto the box using tapes. Any remaining holes were covered with tape. 10.The box was placed near a sunny window, with the hole facing towards the window so sunlight could reach the plant. 11.Pictures were taken and observations were made every several days. Variables:
Independent variable: The design of the box (how it was structured, where the hole was placed) Dependent variable: Direction of growth of the plant. Controlled variable: number of seeds, amount of water given, volume of soil, location of box, location of plant, amount of sunlight Risk Assessment: RiskLevel of RiskPrecaution ScissorsMediumBlade edges are sharp and may cause cuts. May be used as a weapon. Avoid use of scissors with sharp points unless necessary. Store securely. Act maturely Pencils/skewersMediumEdges of pencils/skewers are sharp and may cause puncture wounds.
Avoid use of sharp points unless necessary. Store securely. Act maturely. Cardboard & Cardboard boxes. LowFlammable. Keep away from naked flames. Do not store filled boxes above shoulder height. Cellophane tapeLowPossibility of suffocation. Flammable. Keep away from naked flames. Foam cupLowOrganic solvents may dissolve the plastic of the cup, causing it to leak. Use insulating foam cups for hot liquids. Do not use plastic cups for organic solvents. Potting MixLowPossibility of microbial contamination, including Legionella. Wear gloves and face mask. Dampen carefully to reduce dust production. Wash hands thoroughly after use.
Results: Setup:
Discussion: What makes the plant grow through the maze? What type of tropism is at work here? • What colour are the plants? What will happen
