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1.3 Stimuli and Responses in Plants After a week… Will a plant grow faster Hello plant, please grow faster! Why is the stimulus given...

1.3 Stimuli and Responses in Plants After a week… Will a plant grow faster Hello plant, please grow faster! Why is the stimulus given if we talk to it? not effective? Like humans and animals, plants can also Responses Res onses of Responses sponses of plants plants lants detect stimuli and respond to them. The stimuli that can be detected by plants include light, water, gravity and touch. The responses of plants can be divided into two as shown in Tropism Nastic movement Figure 1.24. Figuree 1.24 Respon Responses nses of plants to stimuli Tropism Tropism is a directional response of plants to stimuli such as light, water, gravity and touch coming from a certain direction. A certain part of a plant will grow towards or move away from the detected stimulus. The part of a plant which grows towards a stimulus is known as positive tropism whereas the part of a plant which grows away from a stimulus is known as negative tropism. The directional response of plants normally occurs slowly and less significantly. Let us carry out Experiment 1.1 to determine the direction of response of plants to light, water, gravity and touch. Experiment 1.1 A Response of plants towards light or phototropism Aim: To study the response of plants to light Problem statement: Which part of plants responds to light? Hypothesis: Shoots of plants grow in the direction of light. Variables (a) manipulated variable : Direction of light towards the shoots of the seedlings (b) responding variable : Direction of growth of the shoots of the seedlings (c) constant variables : Seedlings of the same type and height, volume of water 30 1.3.1 1.3.2 1.3.3  Chapter 1: Stimuli and Responses Materials Green pea seedlings, soil, water and three boxes (one box with an opening at the top and two other boxes with openings at the side) Apparatus Three beakers Procedure Light Green pea seedlings Light Light Beaker Beaker Beaker Box A Box B Box C Figure 1.25 1. Set up the apparatus as shown in Figure 1.25. 2. Observe and sketch the positions of the shoots of the seedlings in boxes A, B and C. 3. Keep all three boxes in the laboratory for five days. Keep the soil moist by watering it with the same amount of water daily. 4. After five days, observe and sketch the positions of the shoots of the seedlings in boxes A, B and C. Conclusion Is the hypothesis accepted? What is the conclusion of this experiment? Questions 1. What is the stimulus used in this experiment? 2. State the part of the plant that responds to the stimulus. 3. Does the part of the plant in question 2 show positive or negative phototropism? Explain your answer. B Response of plants to gravity or geotropism Aim: To study the response of plants to gravity Problem statement: Which part of plants responds to gravity? Hypotheses: (a) Roots of plants grow in the direction of gravity. (b) Shoots of plants grow in the opposite direction of gravity. Variables (a) manipulated variable : Position of the seedlings relative to the direction of gravity (b) responding variable : Direction of growth of the roots and shoots of the seedlings (c) constant variables : Presence of water, absence of light, seedlings with straight roots and shoots Materials Green pea seedlings with straight roots and shoots, moist cotton wool and plasticine 1.3.1 1.3.2 1.3.3 31 Apparatus Petri dish Procedure 1. Set up the apparatus as shown in Figure 1.26. Make sure that the green Petri dish pea seedlings are arranged in different Moist cotton wool positions in the Petri dish. 2. Observe and sketch the position of the Seedlings with shoots and roots of the seedlings in the straight roots and Petri dish. shoots 3. Keep the apparatus in a dark cupboard forr two days. 4. After two days, observe and sketch the Plasticine position of the shoots and roots of the seedlings in the Petri dish. Figure 1.26 Conclusion Are the hypotheses accepted? What is the conclusion of this experiment? Questions 1. Why is the apparatus kept in a dark cupboard? 2. Based on your observations, state the direction of growth of the: (a) shoots of the seedlings (b) roots of the seedlings 3. Do plants show positive geotropism or negative geotropism? Explain your answer. C Response of plants to water or hydrotropism Aim: To study the response of plants to water Problem statement: Which part of plants responds to water? Hypothesis: Roots of plants grow in the direction of water. Variables (a) manipulated variable : Presence of a source of water (b) responding variable : Direction of growth of roots of seedlings (c) constant variables : Gravity, absence of light and seedlings with straight roots Materials Green pea seedlings with straight roots, moist cotton wool and anhydrous calcium chloride Apparatus Rough wire gauze and two beakers Procedure 1. Set up the apparatus as shown in Figure 1.27. 2. Observe and sketch the positions of the roots of the seedlings in beakers X and Y. 3. Keep both beakers X and Y in a dark cupboard. 4. After two days, observe and sketch the positions of the roots of the seedlings in beakers X and Y. 32 1.3.1 1.3.2 1.3.3  Chapter 1: Stimuli and Responses Moist cotton wool Seedlings with straight roots Wire gauze Water Anhydrous calcium X chloride Y Figure 1.27 Conclusion Is the hypothesis accepted? What is the conclusion of this experiment? Questions 1. What is the stimulus used in this experiment? 2. State the part of the plant that responds to the stimulus. 3. What is the function of the anhydrous calcium chloride in beaker Y? 4. Does the part of the plant in question 2 show positive or negative hydrotropism? Explain your answer. Plants need to be responsive towards stimuli such as light, gravity and water so that they can respond appropriately to ensure their sustainability and survival. Why do plants need light and water? Name one stimulus that can be detected by plants but not investigated in Experiment 1.1. Phototropism Source of light Phototropism is the response of plants Grow towards light towards light. Shoots of plants show positive phototropism which is growth towards the direction of light (Photograph 1.19). As plants need light to carry out photosynthesis, positive phototropism ensures that shoots and leaves of plants obtain enough sunlight to make food through photosynthesis. Photograph 1.19 Shoots of plants show positive phototropism Hydrotropism Hydrotropism is the response of plants towards water. Roots of plants show Water positive hydrotropism which is growth Moist towards the direction of water (Figure 1.28). soil Positive hydrotropism allows roots Positive of plants to obtain water to carry out hydrotropism photosynthesis and absorb dissolved (Grow towards mineral salts to stay alive. water) Figure 1.28 Roots showing positive hydrotropism 1.3.1 1.3.2 1.3.3 33  Geotropism Positive geotropism Geotropism is the response of plants towards gravity. (Grow in the Roots of plants show positive geotropism which is direction of downward growth towards the direction of gravity. gravity) Negative geotropism Positive geotropism allows the roots of plants to grow (Grow in deep into the ground to grip and stabilise the position the opposite of the plant in the ground. On the other hand, shoots direction of of plants show negative geotropism which is upward gravity) growth in the opposite direction of gravity. Negative geotropism allows the shoots and leaves of plants to Figure 1.29 Roots show positive grow upwards to obtain sunlight for photosynthesis geotropism whereas shoots show (Figure 1.29). negative geotropism Thigmotropism Thigmotropism is the response towards touch. Tendrils or twining stems show positive thigmotropism when they cling onto whatever objects or other plants they come into contact with (Photograph 1.20). This response enables plants to grow upwards to obtain sunlight and grip objects to obtain support. Roots show negative thigmotropism since they avoid any object that obstructs their search for water. (a) Cucumber plant has tendrils that twine (b) Morning glory plant has stems that twine around objects in contact with it around objects in contact with it Photograph 1.20 Tendrils and stems which twine around objects show positive thigmotropism Nastic Movement Nastic movement is the response towards a stimulus such as touch but does not depend on the direction of Nastic movement 5 the stimulus. What are other stimuli that can cause nastic *( of the Mimosa sp. : movement? Why is nastic movement not a type of tropism? Nastic movement occurs more rapidly than tropism. For ,.      7( example, the Mimosa sp. responds to touch by folding its leaves inwards when touched as shown in Photograph 1.21. This nastic movement serves as a defence of the Mimosa sp. against its enemies and strong wind. 34 1.3.1 1.3.2